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Cellular Transport of Esculin and its Acylated Derivatives in Caco-2 Cell Monolayers and their Antioxidant Properties in Vitro Mengmeng Zhang, Xuan Xin, Furao Lai, Xiaoyuan Zhang, Xiao-Feng Li, and Hui Wu J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b02525 • Publication Date (Web): 14 Aug 2017 Downloaded from http://pubs.acs.org on August 17, 2017

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Journal of Agricultural and Food Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Journal of Agricultural and Food Chemistry

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Cellular Transport of Esculin and its Acylated Derivatives in Caco-2

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Cell Monolayers and their Antioxidant Properties in Vitro

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Mengmeng Zhang†, Xuan Xin†, Furao Lai†, Xiaoyuan Zhang§, Xiaofeng Li*#, and Hui Wu*†

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Affiliation

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†College of Food Science and Engineering, South China University of Technology,

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Guangzhou, Guangdong, 510640, China

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§Research Institude of Shaoguan Huagong High-tech Industry, South China

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University of Technology, Guangzhou, Guangdong, 510640, China

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#State Key Laboratory of Pulp and Paper Engineering, South China University of

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Technology, Guangzhou, Guangdong , 510640, China

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Short title: Bioavailability and Antioxidant Properties of Esculin and its Acylated

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Derivatives

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Corresponding authors:

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*Xiaofeng Li, State Key Laboratory of Pulp and Paper Engineering, South China

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University of Technology, Wushan Road 381, Guangzhou, Guangdong, China.

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Tel: (+86)20-22236819; E-mail: [email protected] ; Fax: (+86)20-87112853

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*Hui Wu, College of Food Science and Engineering, South China University of

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Technology, Wushan Road 381, Guangzhou, Guangdong, China. Tel: (+86)20-87112853; E-mail: [email protected]; Fax: (+86)20-87112853 1

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ABSTRACT: Esculin has many pharmacological effects, but these are difficult to

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observe after oral administration owing to poor lipid solubility. In our previous study,

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five acylated derivatives with different acyl chain lengths (EA, EP, EO, EL, and EM)

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were synthesized to improve the lipophilicity of esculin. In this study, the

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bioavailability and antioxidant activity of the five derivatives were investigated. The

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logP of esculin, EA, EP, EO, EL, and EM were -1.1±0.1, -0.3±0.14, 0.1±0.17,

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1.6±0.09, 2.4±0.11 and 2.8±0.18, and their Papp were 0.71±0.02, 1.24±0.18,

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1.74±0.11, 11.6±3.6, 4.11±1.03 and 2.64±0.97 ×10−6 cm/s, respectively. Besides, the

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bioavailability of EO, EL, and EM were seriously affected by carboxylesterase. The

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results of ABTS, ORAC, and DPPH assays indicated that the antiradical ability of

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the five derivatives did not exceed that of esculin. However, EA, EP, and EO showed

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more effective inhibition of AAPH-induced oxidative hemolysis than esculin did

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(p

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0.05) before and after the experiment, which indicated that the monolayers retained

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good integrity, and the used exposure times and concentrations of all samples were

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appropriate for the experiment. Table 2 showed the results of the transport experiment.

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It was found that for all samples, the Papp of AP to BL transport was close to that of

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BL to AP transport. This indicated the efficiency of transport from AP and BL sides

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were similar for each compound. Uptake ratio (UR) was defined as the quotient of the

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absorptive permeability and the secretory permeability (Papp AP−BL/Papp BL−AP). A

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value of UR or ER (efflux ratio, Papp BL−AP/Papp AP−BL) higher than 1.5 suggests

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the participation of an active transport mechanism; if the values are close to 1.0, this is

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a passive transport.20 All UR were approximately 1.0 in this result, which indicated

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that passive transport occurred for all samples.

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Propranolol is a high permeability-high solubility lipophilic drug, which can be

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absorbed almost completely after oral consumption. Furosemide is a low

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permeability-high solubility hydrophilic compound with very low bioavailability.19

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On the basis of these two extremes for the estimation of bioavailability, it was

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determined that esculin has a much lower bioavailability than furosemide. The

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bioavailability of EA and EP was better than that of esculin but similar to that of 11

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furosemide. EO had a much better bioavailability than EA and EP did, but it was still

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lower than that of propranolol. These results agreed with the prediction of logP values.

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However, EL and EM had a much lower bioavailability than EO did, which was

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similar to that of EA and EP. It is very different from the prediction of logP values.

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This difference was not caused by the molecular weight. Although the molecular

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weights of EL and EM were greater than 500 g/mol, they were only 56 and 85 g/mol

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more than that of than EO (Mw = 466 g/mol), respectively. Thus, there must be other

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explanations for the huge difference.

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The results in Table 2 showed that the recovery of esculin, EA, and EP was

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greater than 90%. However, EO, EL, and EM had a very low recovery. A recovery

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of >80% gives an acceptable approximation of the Papp value. A lower recovery will

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result in the underestimation of the Papp values;20 thus, the practical Papp values of EO,

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EL, and EM were greater than the measured values. The reason may be that the

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compound was retained inside the cells or that the compound was metabolized.20 Then,

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the cells were lysed and the cellular concentrations of these esters were determined.

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As shown in Table 3, these esters were detected in the lysate. However, compared

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with the initial concentrations (100 nmol), the concentration of the esters in cells was

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so low that the loss could not be offset. This means that EO, EL, and EM were

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metabolized. In fact, vast amounts of esculin were detected in the samples of the AP

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and BL sides of EO, EL, and EM (Table 3). This indicated that the ester bonds of EO,

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EL, and EM were broken, and EO, EL, and EM were metabolized back to esculin. In

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addition, almost no esculin was detected in the samples of EA and EP. This may have 12

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occurred because the enzyme breaking the ester bond was mainly present in the

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cytoplasm. The poor permeability of EA and VP would make it hard for these

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compounds to enter the cytoplasm. Even if EA and VP were hydrolyzed, the

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concentration of the product was too low to be detected. Interestingly, for EO, EL,

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and EM, the amount of esculin molecules on the AP side was 2-3 times higher than

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that on the BL side, which appeared to indicate that the enzyme was mainly situated

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in the cytomembrane of the AP side.

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Transport of Esculin and its Esters Across Caco-2 Cell Monolayers without

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CES-mediated Hydrolysis CES are members of the α/β hydrolase fold family and

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show ubiquitous tissue expression profiles with high levels in the liver, small intestine

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and lung. They are the most important hydrolases for ester-containing drugs.21 Caco-2

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cells were reported to have sufficient CES.21, 33 Thus, it was speculated that CES

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hydrolyzed EO, EL, and EM. To demonstrate this speculation, BNPP, a specific CES

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inhibitor was used to inhibit CES-mediated hydrolysis. After pretreatment with BNPP,

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the Papp of EA and EP was unchanged, whereas the Papp of EO, EL, and EM increased

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significantly (Table 4); especially, the Papp of EO was close to that of propranolol.

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Additionally, the recoveries of EO, EL, and EM were greater than 90% (Table 4).

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These results demonstrated that CES led to the main hydrolysis of EO, EL, and EM.

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However, a small amount of esculin was detected (data not shown), which indicated

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that other esterases existed.17 CES is predominantly located in the endoplasmic

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reticulum membrane,34 which explained the low levels of hydrolysis of EA and EP.

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However, it is difficult to determine why the mole number of esculin in the AP side 13

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was much greater than that in the BL side. In fact, the efflux of drugs was dependent

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on the relative size of the AP and BL membranes in Caco-2 cells. It is already

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accepted that the brush border membrane has a larger surface area than the BL

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membrane owing to the presence of microvilli.21 Thus, after the hydrolysis, the

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produced esculin would be apt to flow to the AP side. The process of transport is

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shown in Figure 3, and the amounts of each part are shown in Table 3. In addition, the

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Papp of EO was close to that of EL and EM after the treatment with BNPP, but the Papp

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of EO was higher than that of EL and EM without BNPP treatment. These results

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suggested that the esters with a longer acyl chain were more easily hydrolyzed by

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CES; this may be another explanation for the low levels of hydrolysis of EA and EP.

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There are five subfamilies of CES: CES1–CES5. The major CES isozyme in

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human intestine is human CES2 isozyme (hCE2), whereas the human CES1 isozyme

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(hCE1) is predominant in Caco-2 cells. The substrate specificities of hCE1 and hCE2

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are different, with hCE2 mainly hydrolyzing drugs in which an alcohol group of the

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pharmacologically active parent drug has been modified with a simple acyl group. In

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contrast, drugs in which the carboxyl group of the pharmacologically active drug is

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modified with small alcohol groups are preferentially hydrolyzed by hCE1. hCE1 is

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also capable of hydrolyzing substrates of hCE2 at low levels of activity, whereas

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hCE2 is unable to hydrolyze substrates of hCE1.21 Esculin esters appeared to be

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substrate of hCE2, which suggested that they may undergo more extensive hydrolysis

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in the human intestine. Besides CES, there are other enzymes or proteins affecting the

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bioavailability of esters in the human intestine, such as, UGT, cytochrome P450, 14

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multidrug resistance-associated protein 2, breast cancer resistance protein.35 The

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colonic microflora also can hydrolyze the ester drugs.36 Even food microstructure

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affects the bioavailability.37 Thus, for the bioavailability of esculin and its esters in

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body, more research is needed.

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Antiradical Ability of Esculin and its Esters Scavenging free radicals is an

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essential ability for most antioxidants. The antioxidant activities of esculin and its

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esters were measured by the ABTS, ORAC, and DPPH assays. Like most of the

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acylated derivatives and their parent compounds,14,

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esculin esters was similar to or lower than that of esculin (Table 5 and Figure 4).

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However, one report indicates that the antiradical ability of acylated derivatives was

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higher than that of parent compounds.40 That study may be explained by the location

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at which the acylation occurred, which was the phenolic hydroxyl group. The

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phenolic hydroxyl group of polyphenols is closely related to their antioxidant

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activity.41 The incorporated fatty acid chains caused electronic and steric effects in the

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benzene ring and improved the antioxidant activity.40 In our study, the acylation

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occurred at the stable glycosidic moiety and had little effect on the active site of

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esculin. Thus, the antiradical ability of the acylated derivatives could not exceed that

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of esculin. As shown in Table 5 and Figure 4, in the ABTS and ORAC assays, the

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antiradical ability of esculin esters decreased with an increase in the acyl chain length.

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The values for EA and EP were close to those for esculin, but those of EO, EL, and

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EM were much lower than that of esculin. The cause of these differences is shown in

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Figure 5. At the same concentration, the distribution of antioxidants in Figure 5A

38, 39

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provided more contact with free radicals than that in Figure 5B. Therefore, the

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antioxidants in Figure 5A would exhibit more effective antiradical ability than those

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in Figure 5B. The logP values of EA and EP indicated a relatively high hydrophilicity.

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The ABTS and ORAC assays occur in a hydrophilic environment. Thus, EA and EP

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could be evenly distributed in the system, whereas EO, EL, and EM had higher

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lipophilicity and would form micelles. This structure will decrease the contact

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between the active site of esculin and free radicals. In the DPPH assay, the antioxidant

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activity of EO was similar to that of esculin, because that the solvent in the reaction

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system was methanol. EO is more soluble in methanol and a homogeneous

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distribution can be achieved. This result was in agreement with some previous

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studies.39, 42 However, EL and EM had a lower DPPH free radical scavenging ability.

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This may be explained by the hydrophobicity of EL and EM, which was much higher

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than that of methanol and still led to a low solubility in methanol.43

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Attenuation of Erythrocyte Hemolysis by Esculin and its Esters Although

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many reports have indicated that the antiradical ability of acylated derivatives was

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lower than that of their parent compounds, these new compounds exhibited more

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remarkable antioxidant capacities when tested in food matrices, such as oils and

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oil-in-water emulsions.25 This phenomenon can be explained by the polar paradox

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theory, whcih can be applied to cells.9, 25 However, there are only a few studies that

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use cells to evaluate the antioxidant activity of these acylated derivatives.39, 42 The cell

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lines used in those studies may possess esterases. As previously mentioned, the esters

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may be hydrolyzed by certain esterases in cells. Thus, to exclude the effect of 16

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esterases on these esters, the erythrocytes were used as a model to explore the

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antioxidant activities of esculin and its esters, because the metabolic networks in

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erythrocytes are relatively simple. The esterases are not included in metabolic

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networks of erythrocytes.16 The hemolysis of erythrocytes has been used extensively

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as an ex vivo model for the study of the antioxidant activity. In the erythrocyte

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hemolysis assay, free radicals induced by AAPH can cause lipid peroxidation and loss

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of erythrocyte membrane integrity, which can ultimately lead to hemolysis.16

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As shown in Figure 6A, erythrocyte hemolysis was effectively attenuated by

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esculin and its esters. Their activity was in the order EO > EP ≈ EA > esculin > EL ≈

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EM. The samples without AAPH supplementation did not induce hemolysis (data not

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shown). AAPH-induced ROS generation can cause membrane lipid peroxidation and

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result in the release of MDA. As seen in Figure 6B, the level of MDA in erythrocytes

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was in accordance with the result of the hemolysis.

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The structure of an erythrocyte is similar to a micro-balloon with a lipid bilayer

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surface. Therefore, its interaction with water is similar to an oil-in-water emulsion. An

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oil-in-water emulsion generally consists of three essential parts: lipid droplets, the

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continuous aqueous phase, and the oil-water interface. According to the polar paradox

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theory, nonpolar antioxidants are more effective than their polar homologs in

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oil-in-water emulsions.44 Nonpolar antioxidants are mainly concentrated at the

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oil-water interface, the optimum location for shielding the oil droplets from oxidation;

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therefore, they would inhibit lipid oxidation more efficiently.25 In fact, lipophilicity

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has been viewed as an important factor with respect to the effectiveness of the 17

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anti-hemolytic substances in hemolytic studies.16,

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hydrophilicity of esculin was high and it was mainly distributed in the aqueous phase.

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The mechanism of the inhibitory effect on hemolysis is only dependent on the ability

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of scavenging free radicals produced by AAPH. For EA, EP, and EO which have a

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relatively higher lipophilicity; the mechanism is not only dependent on the antiradical

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ability, but also the location on the “oil-water interface” (Figure 7B). The increased

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lipophilicity of the esters made it easier to form a shield around the cells; thus, the

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inhibition of hemolysis occurred in the order: EO > EP ≈ EA > esculin

As shown in Figure 7A, the

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However, the polar paradox theory has faced some challenges in recent years. One

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such challenge is the “cutoff effect”, which means that a nonlinear relationship

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existed between polarity and the antioxidant efficacy in emulsions for antioxidants.

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There is a threshold for the antioxidant activity conferred by the increased alkyl

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chain lengths; with further chain length extension, the activity will drastically

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decrease. Several studies have revealed that short-medium-chain lipophilic esters

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were able to improve the efficacy of antioxidants in emulsions better than long-chain

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esters.9 This phenomenon also was observed in this study. The reason may be that if

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the lipophilicity of the antioxidants was too high, they would most likely be placed at

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the interior of the emulsion dissolved in the oil droplet, or aggregate readily in the

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aqueous phase rather than orienting themselves at the interfacial layer.9,

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indicated that the low inhibitory effects of EL and EM on hemolysis may be a result

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of the ease of entering the cell and forming micelles (Figure 7C).

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This

Some reports have deemed that a good antioxidant for oil-in-water emulsions 18

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should be an effective surfactant. At certain hydrophilic-lipophilic balance (HLB)

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values (between 8 and 11), the lipophilic antioxidants can be located in the oil-water

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interface. The cutoff effect appears when HLB values are lower than 8.25 However, all

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HLB values of esculin esters were greater than 11 (EA, 16.9; EP, 16.3; EO, 13.9; EL,

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12.3; EM, 11.7), and the cutoff effect appeared at 12.3. The explanation for the

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different cutoff chain lengths may be that an effective surfactant depends on different

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parameters that may vary for each specific series of phenolipids, such as the optimum

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HLB value or the specific polarity and geometry of the polar head.25 Although

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erythrocytes could be approximately regarded as the emulsion or oil droplets, the

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structure of the cytomembrane is quite different from that of oil droplets.

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The Role of “Oil-Water Interface” in Inhibiting Hemolysis A “shield” around

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the cells was supposed to exist according to the polar paradox theory. The existence of

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the “shield” was dependent on the “oil-water interface”. To further demonstrate the

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role of “oil-water interface”, erythrocyte hemolysis was performed in cold ultrapure

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water. The water was then treated with AAPH and esculin or esculin esters according

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to the method described in the assay for erythrocyte hemolysis. The level of MDA

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showed that esculin was a potent inhibitor of membrane lipid peroxidation. The

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activity of the esters was in the order, EA ≈ EP > EO > EL ≈ EM (Figure 8), which

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was similar to the order of the antiradical abilities. This indicated that the inhibitory

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effect of the esters on membrane lipid peroxidation was dependent on the intact closed

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structure of the cytomembrane. The structure of cytomembrane was destroyed and, as

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there was not the “oil-water interface”, the esters could not aggregate. Thus the 19

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“shield” cannot be formed, and the esters cannot show an improved inhibition of lipid

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peroxidation.

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In conclusion, this study investigated the bioavailability and antioxidant activity of

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esculin and its acylated derivatives. It was found that all derivatives had a better

417

bioavailability than esculin. However, the bioavailability of derivatives with short or

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long alkyl chain still was low. The derivative with medium alkyl chain had a much

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better bioavailability. Besides, derivatives with medium and long alkyl chain were

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hydrolyzed by CES, which significantly decreased the bioavailability of esters.

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These results hinted that the acylation was an effectual method to improve the

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bioavailability of polyphenols with poor lipophilicity. However, certain factors should

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be considered in choosing acyl donors, such as the suitability between the lipotropism

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of the acyl donor and hydrophilicity of these polyphenols and relationship between

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the structure of the acyl donor and the substrate specificity of CES. The antioxidant

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activity analysis showed that although the antiradical ability of the acylated esculin

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was not improved, the protective effect on cells under the oxidative stress conditions

428

was enhanced. These results indicated that the antioxidant activity of these acylated

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derivatives was dependent on not only the antioxidant properties of their parent

430

compound, but also their distribution in the reaction system of the antioxidant activity

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assays. Compared with other research, our study proposed that acyl donors should be

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chosen according to the purpose of acylation. For example, if the acylated derivatives

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will be used as drug or prodrug, the acyl donor should help the derivatives cross the

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intestinal wall and vascular wall. If the derivatives were used to protect the fatty or 20

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oily foods from oxidation, the acyl donor should help the derivatives locate in the

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oil-water interface; thus, this study provides the guidance for the acylation of

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polyphenols with poor lipophilicity.

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Funding This work was financially supported by the National Natural Science

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Foundation of China (Nos. 31270636, 21676105), Self Determined Research Fund

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of SCUT from the College Basic Research and Operation of MOE (No. 2015ZZ111),

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and Science and Technology Planning Project of Guangdong Province (Nos.

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2016A040402020, 2016B010121014).

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Supporting Information Standard curves, limit of quantification and limit of

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detection; Intra- and inter-day accuracy and precision; HPLC chromatograms of

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blank HBSS and HBSS spiked with standard compounds.

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14. Ma, X.; Yan, R.; Yu, S.; Lu, Y.; Li, Z.; Lu, H., Enzymatic acylation of isoorientin and isovitexin from bamboo-leaf extracts with fatty acids and antiradical activity of the acylated derivatives. J Agr Food Chem 2012, 60, 10844-10849. 15. Mbatia, B.; Kaki, S. S.; Mattiasson, B.; Mulaa, F.; Adlercreutz, P., Enzymatic synthesis of lipophilic rutin and vanillyl esters from fish byproducts. J Agr Food Chem 2011, 59, 7021-7027. 16. Zhang, M.; Zhang, H.; Li, H.; Lai, F.; Li, X.; Tang, Y.; Min, T.; Wu, H., Antioxidant Mechanism of Betaine without Free Radical Scavenging Ability. J Agr Food Chem 2016, 64, 7921-7930. 17. Ohura, K.; Sakamoto, H.; Ninomiya, S.-i.; Imai, T., Development of a Novel System for Estimating Human Intestinal Absorption Using Caco-2 Cells in the Absence of Esterase Activity. Drug Metab Dispos 2010, 38, 323-331. 18. Xu, J.; Qian, J.; Li, S., Enzymatic acylation of isoorientin isolated from antioxidant of bamboo leaves with palmitic acid and antiradical activity of the acylated derivatives. Eur Food Res Techn 2014, 239, 661-667. 19. Chen, X.-M.; Dai, Y.; Kitts, D. D., Detection of Maillard Reaction Product [5-(5, 6-Dihydro-4 H-pyridin-3-ylidenemethyl) furan-2-yl] methanol (F3-A) in Breads and Demonstration of Bioavailability in Caco-2 Intestinal Cells. J Agr Food Chem 2016, 64, 9072-9077. 20. Hubatsch, I.; Ragnarsson, E. G.; Artursson, P., Determination of drug permeability and prediction of drug absorption in Caco-2 monolayers. Nature Protoc 2007, 2, 2111-2119. 21. Ohura, K.; Nozawa, T.; Murakami, K.; Imai, T., Evaluation of transport mechanism of prodrugs and parent drugs formed by intracellular metabolism in Caco‐2 cells with modified carboxylesterase activity: Temocapril as a model case. J Pharm Sci-US 2011, 100, 3985-3994. 22. Chen, P. X.; Tang, Y.; Zhang, B.; Liu, R.; Marcone, M. F.; Li, X.; Tsao, R., 5-Hydroxymethyl-2-furfural and derivatives formed during acid hydrolysis of conjugated and bound phenolics in plant foods and the effects on phenolic content and antioxidant capacity. J Agr Food Chem 2014, 62, 4754-4761. 23. Ma, Q.; Xie, H.; Li, S.; Zhang, R.; Zhang, M.; Wei, X., Flavonoids from the Pericarps of Litchi chinensis. J Agr Food Chem 2014, 62, 1073-1078. 24. Su, C.; Xia, X.; Shi, Q.; Song, X.; Fu, J.; Xiao, C.; Chen, H.; Lu, B.; Sun, Z.; Wu, S., Neohesperidin dihydrochalcone versus CCl4-induced hepatic injury through different mechanisms: the implication of free radical scavenging and Nrf2 Activation. J Agr Food Chem 2015, 63, 5468-5475. 25. Lucas, R.; Comelles, F.; Alcantara, D.; Maldonado, O. S.; Curcuroze, M.; Parra, J. L.; Morales, J. C., Surface-Active Properties of Lipophilic Antioxidants Tyrosol and Hydroxytyrosol Fatty Acid Esters: A Potential Explanation for the Nonlinear Hypothesis of the Antioxidant Activity in Oil-in-Water Emulsions. J Agr Food Chem 2010, 58, 8021-8026. 26. Zhu, S.; Li, Y.; Li, Z.; Ma, C.; Lou, Z.; Yokoyama, W.; Wang, H., Lipase-catalyzed synthesis of acetylated EGCG and antioxidant properties of the acetylated derivatives. Food Res Int 2014, 56, 279-286. 23

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27. Purser, S.; Moore, P. R.; Swallow, S.; Gouverneur, V., Fluorine in medicinal chemistry. Chem Soc Rev 2008, 37, 320-330. 28. Rothwell, J. A.; Day, A. J.; Morgan, M. R., Experimental determination of octanol− water partition coefficients of quercetin and related flavonoids. J Agr Food Chem 2005, 53, 4355-4360. 29. Wang, Z.-Y.; Bi, Y.-H.; Yang, R.-L.; Zhao, X.-J.; Jiang, L.; Ding, C.-X.; Zheng, S.-Y., Highly efficient enzymatic synthesis of novel polydatin prodrugs with potential anticancer activity. Process Biochem 2017, 52, 209-213. 30. Rubas, W.; Jezyk, N.; Grass, G. M., Comparison of the permeability characteristics of a human colonic epithelial (Caco-2) cell line to colon of rabbit, monkey, and dog intestine and human drug absorption. Pharm Res-Dordr 1993, 10, 113-118. 31. Hou, T.; Wang, J.; Zhang, W.; Xu, X., ADME evaluation in drug discovery. 6. Can oral bioavailability in humans be effectively predicted by simple molecular property-based rules? J Chem Inf Model 2007, 47, 460-463. 32. Lipinski, C. A.; Lombardo, F.; Dominy, B. W.; Feeney, P. J., Experimental and computational approaches to estimate solubility and permeability in drug discovery and development settings. Adv Drug Deliver Rev 1997, 23, 3-25. 33. Lu, Y.; Bao, N.; Borjihan, G.; Ma, Y.; Hu, M.; Yu, C.; Li, S.; Jia, J.; Yang, D.; Wang, Y., Contribution of Carboxylesterase in Hamster to the Intestinal First-Pass Loss and Low Bioavailability of Ethyl Piperate, an Effective Lipid-Lowering Drug Candidate. Drug Metab Dispos 2011, 39, 796-802. 34. Hakamata, W.; Tamura, S.; Hirano, T.; Nishio, T., Multicolor Imaging of Endoplasmic Reticulum-Located Esterase As a Prodrug Activation Enzyme. Acs Med Chem Lett 2014, 5, 321-325. 35. Brodie, B. B.; Gillette, J. R.; La Du, B. N., Enzymatic metabolism of drugs and other foreign compounds. Annu Rev Biochem 1958, 27, 427-454. 36. Manach, C.; Scalbert, A.; Morand, C.; Rémésy, C.; Jiménez, L., Polyphenols: food sources and bioavailability. Am J Clin Nutrt 2004, 79, 727-747. 37. Parada, J.; Aguilera, J., Food microstructure affects the bioavailability of several nutrients. J Food Sci 2007, 72. 38. Trujillo, M.; Gallardo, E.; Madrona, A.; Bravo, L.; Sarria, B.; Gonzalez-Correa, J. A.; Mateos, R.; Luis Espartero, J., Synthesis and Antioxidant Activity of Nitrohydroxytyrosol and Its Acyl Derivatives. J Agr Food Chem 2014, 62, 10297-10303. 39. Bernini, R.; Crisante, F.; Barontini, M.; Tofani, D.; Balducci, V.; Gambacorta, A., Synthesis and Structure/Antioxidant Activity Relationship of Novel Catecholic Antioxidant Structural Analogues to Hydroxytyrosol and Its Lipophilic Esters. J Agr Food Chem 2012, 60, 7408-7416. 40. Zhong, Y.; Shahidi, F., Lipophilized Epigallocatechin Gal late (EGCG) Derivatives as Novel Antioxidants. J Agr Food Chem 2011, 59, 6526-6533. 41. Kurek-Górecka, A.; Rzepecka-Stojko, A.; Górecki, M.; Stojko, J.; Sosada, M.; Świerczek-Zięba, G., Structure and antioxidant activity of polyphenols derived from propolis. Molecules 2013, 19, 78-101. 24

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42. Tofani, D.; Balducci, V.; Gasperi, T.; Incerpi, S.; Gambacorta, A., Fatty Acid Hydroxytyrosy Esters: Structure/Antioxidant Activity Relationship by ABTS and in Cell-Culture DCF Assays. J Agr Food Chem 2010, 58, 5292-5299. 43. Medina, I.; Alcantara, D.; Gonzalez, M. J.; Torres, P.; Lucas, R.; Roque, J.; Plou, F. J.; Morales, J. C., Antioxidant Activity of Resveratrol in Several Fish Lipid Matrices: Effect of Acylation and Glucosylation. J Agr Food Chem 2010, 58, 9778-9786. 44. Panya, A.; Laguerre, M.; Bayrasy, C.; Lecomte, J.; Villeneuve, P.; McClements, D. J.; Decker, E. A., An Investigation of the Versatile Antioxidant Mechanisms of Action of Rosmarinate Alkyl Esters in Oil-in-Water Emulsions. J Agr Food Chem 2012, 60, 2692-2700. 45. Ximenes, V. F.; Lopes, M. G.; Petronio, M. S.; Regasini, L. O.; Siqueira Silva, D. H.; da Fonseca, L. M., Inhibitory Effect of Gallic Acid and Its Esters on 2,2 '-Azobis(2-amidinopropane)hydrochloride (AAPH)-Induced Hemolysis and Depletion of Intracellular Glutathione in Erythrocytes. J Agr Food Chem 2010, 58, 5355-5362.

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FIGURE CAPTIONS

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Figure 1 Chemical structure of esculin and its esters.

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Figure 2 LogP values of esculin and its esters. Different letters indicate significant

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differences (p