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Binding of Sodium Cholate in-Vitro by Cationic Microfibrillated Cellulose Xuhai Zhu, Yangbing Wen, Lijuan Wang, Changmo Li, Dong Cheng, Hongjie Zhang, and Yonghao Ni Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie503909g • Publication Date (Web): 12 Nov 2014 Downloaded from http://pubs.acs.org on November 17, 2014

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Industrial & Engineering Chemistry Research

Binding of Sodium Cholate in- Vitro by Cationic Microfibrillated Cellulose Xuhai Zhua, Yangbing Wen*a,b, Lijuan Wanga, Changmo Lic, Dong Chenga, Hongjie Zhanga, and Yonghao Ni*a,b a. Tianjin Key Laboratory of Pulp and Paper, Tianjin University of Science & Technology, Tianjin 300457, China b. Limerick Pulp & Paper Centre & Department of Chemical Engineering, University of New Brunswick, Fredericton, Canada NB E3B 5A3 c. Key Laboratory of Food Nutrition and Safety, Ministry of Education of China, Tianjin University of Science and Technology, Tianjin 300457, China

ABSTRACT In this work, a series of cationic Microfibrillated Cellulose (CMFC) were prepared by introducing quaternary amine groups, and the sorption of a model compound for bile salts, sodium cholate, in-vitro was determined. Various variables, contact time, initial concentration of sodium cholate and the presence of salts, were investigated. Experimental results showed that the in-vitro binding of sodium cholate of the CMFC was 57.95 mg/g, which was about 70% lower than that of cholestyramine. The isotherm data were analyzed according to Langmuir, Freundlich, Tempkin models. Characteristic parameters of each model were determined. Results also showed that the Langmuir isotherm performed the best correlation for the sorption of sodium cholate onto the CMFC, and the maximum capacity, Qmax was 416.67 mg of sodium cholate per gram of CMFC. The sorption kinetics underwent a pseudo-second order equation, suggesting a chemisorptions mechanism. Additionally, the presence of salts in the reaction system greatly prevented the CMFC sorption towards cholate, implying the interaction between sodium cholate and CMFC was electrostatic in nature. All these results support the great potential for using the CMFC as a sorbent to decrease cholesterol. Keywords: Microfibrillated Cellulose (MFC), Cationization, Sodium cholate, Sorption, CMFC

1. INTRODUCTION In the past several decades, increasing cases of obesity, diabetes, hypertension, dyslipidemia and cardiovascular diseases have presented, particularly in the developed economies. 1

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Consequently, lowering the concentration of blood lipids, mainly cholesterol, is requested in these cases.1 Bile acid sorbents are a kind of polymers that can be used as cholesterol depressing agents. For example, cholestyramine is quaternized styrene-divinilbenzene copolymer;2 colestipol is synthesized from tetraethylenpentamine and epichlorohydrin;3 Colestimide is a type of anion-exchange resin with an imidazolium salt on an epoxide polymer skelton.4 When such insoluble polymers are ingested, bile acids can be adsorbed onto them. This sorption has been theorized to be the cause of increased fecal bile acid excretion and, in turn, reduce the concentration of serum and/or tissue cholesterol.5 However, the efficiency of these sorbents is rather low due largely to poor dispersion of the resins in the intestine because of their insolubility in water and inaccessibility of bile acids to binding sites on the resins on account of their bulky steroid nucleus.4 Moreover, the mentioned sorbents are based on synthetic polymers. In the clinical practice, these sorbents cause a lots of adverse effects such as constipation, nausea and meteorism.6 Therefore, some polysaccharides, such as dextran, have also been used as bile acid sorbents depending on their biocompatibility, biodegradability, hydrophilicity and absence of toxicity. For example, DEAE-dextran, a derivative of crosslinked dextran, has been reported as bile acid sorbent.7 Moreover, cationic hydrogels based on chitosan were considered as potential delivery systems for protein therapeutics and antigens.8 The application of micro based polysaccharides as a sorbent is according to the ionic and / or hydrophobic mechanism.9 Cellulose- based materials, Cellulose Nanocrystal (CNC) and Microfibrillated cellulose (MFC), are novel biomaterials with large surface area.10,

11

Previous studies have already

demonstrated that CNC or MFC or other modified cellulose materials can adsorb various organic compounds.12-15 The highly porous nano-cellulose-based composites have been studied as oral drug delivery systems.16, 17 Since the cellulose micro- and nano-fibrils is not hydrolyzed by human digestive enzymes but promise great potential as bile acid sorbents in the physiological condition of gut. It is an attractive material for food supplement recommended to people with high cholesterol. In this study, cationically modified MFC was produced as a bile acid sorbent. A cationic modifier, N-(2-3-epoxypropyl) trimethylammonium chloride (EPTMAC), was grafted onto the cellulose before homogenization. The cationic character of CMFC was expected to increase 2


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colloidal stability and binding capacity. A biocompatible cellulose-based material was prepared by modifying MFC with trimethylammonium chloride group. By combining the unique MFC properties, such as large specific surface area, with the cationic charge of the trimethylammonium chloride group, we prepared a biocompatible cellulose-based material. The equilibrium and kinetics data of the sorption, along with the sorption behavior of the material was studied using a bile salt model compound, sodium cholate, in the absence and presence of electrolytes.5

2. MATERIALS AND METHODS 2.1. Materials. Dissolving pulp, produced with hardwood using sulfite process and provided by a Chinese pulp mill in the Shandong Province, was used as the raw material for the preparation of Microfibrillated cellulose. The pulp was never-dried. The enzyme, monocomponent endoglucanase (Novozym 476, Novozym 435), was used as obtained. N-(2-3-epoxypropyl) trimethylammonium chloride (EPTMAC), porcine pancreatin and cholestyramine were obtained from Sigma-Aldrich Co., while sodium cholate was supplied by TCI (Tokyo, Japan). All the other reagents were from Tianjin Chemical Reagent Co. Ltd., China. Distilled and deionized water was used in all the experiments unless otherwise specified. 2.2. Preparation of MFC. The cationic Microfibrillated cellulose (CMFC) was prepared from the enzyme-treated hardwood sulfite-based dissolving pulp. Then the treated pulp was subjected to a cationic modification process to add cationic property on the cellulose. The cellulose, obtained from the cationization step, was subquently homogenized under high pressure. The enzyme pretreatment and cationic modification process were followed a procedure reported in the literatures.18-20 7.5g EPTMAC and 2.25g NaOH dissolved in 15g distilled water were added to 60g with 25% aqueous enzyme-treated hardwood sulfite dissolving pulp dispersion. Then, the slurry was diluted with 180g isopropanol and was allowed to react at 50℃for 3h; thereafter, distilled water was used to wash the cationic pulp. The obtained suspension was dialyzed (molecular weight cut-off 14000) against distilled water for 3 days. 5g of the dialyzed cationic pulp was diluted to 1% w/w of 500g mixtures and then homogenized to produce CMFC. For un-modified MFC, 5g of enzyme-treated dissolving pulp was straightly diluted to 1% w/w of 500g mixtures and homogenized to produce the un-modified MFC. 2.3. Charge density measurement. The charge densities of samples were determined using a 3



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Particle

Charge

Detector,

Mütek

PCD

04

titrator

(Herrsching,

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Germany).

Poly

(diallyldimethylammonium chloride) (PDADMAC) solution (0.1mN) or polyelectrolyte potassium polyvinyl sulfate (PVSK) solution (0.1mN) was used as titrant.21 2.4. Morphology analysis of sorbents. The morphology of the freeze dried dissolving pulp as well as of the freeze dried CMFC were taken with a Hitachi SU-1510 field emission SEM operating at 3.0kV. The sample was mounted onto a substrate with double-sided adhesive tape and coated with gold.22 2.5. In-vitro sodium cholate sorption. In-vitro sodium cholate binding of CMFC was determined according to a previously published procedure.23 Cholestyramine was used as a positive control, and hardwood sulfite-based dissolving pulp was used as a negative control. 50 milligrams (dry weight) of cholestyramine, hardwood dissolving pulp, un-modified MFC and CMFC was mixed with 5mL of 0.01 N hydrochloric acid and incubated in a shaking water bath at 37 ℃ for 1 h, which simulated gastric digestion. The pH of the materials was then adjusted to 6.8 with 0.1 N sodium hydroxide. 20 ml of sodium cholate working solution (1.4 µmol/ml) in a 0.1M phosphate buffer, pH 6.8 and 25 ml of porcine pancreatin (10 mg/ml in a 0.1M phosphate buffer, pH 6.8, providing amylase, protease and lipase for digestion of samples ) were added and incubated in a shaking water bath at 37℃ for 2 h. The mixtures were transferred to 10ml centrifuge tubes and centrifuged at 99,000g for 30min at 6℃. Then clear filtrates were collected for analysis. 2.6. Sorption of sodium cholate on CMFC. To investigate the sorption kinetics of sodium cholate on CMFC (charge density: 0.69 meq/g), the sodium cholate solution of known concentrations (10ml) was added to 80mg of the CMFC in a 80 ml Elenmeyer flask and shaken at 120 rpm and 37℃ for various time intervals. To determine the sorption isotherms of sodium cholate on CMFC (charge density: 0.69 meq/g) and the effect of electrolytes on the sorption, various sodium cholate solutions of known concentrations (10ml) were added to 80mg of the CMFC in a 80 ml Elenmeyer flask and shaken at 120 rpm and 37℃ for 300min. Water and 0.05 M bicarbonate buffer containing 0.15 M NaCl were used as the incubation media, separately. Samples were transferred to 10ml centrifuge tubes and centrifuged at 99,000g for 30min at 6℃ in an ultracentrifuge. Then the clear filtrate was collected for analysis. 2.7. Sodium cholate analysis. sodium cholate in the original solution and clear filtrate (1ml) were pretreated using 9ml of 42 % w/w sulfuric acid at 70℃ for 20 min, then their concentrations 4


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were determined using a UV spectrophotometer at 378nm, according to a method described in an earlier study.5 A calibration curve was prepared for quantitative analysis. 6 replicates were carried out and the average and the standard error (SEM) were reported.

3. RESULTS AND DISCUSSION

3.1. In-vitro sodium cholate sorption. Table 1 shows the sorption results of sodium cholate with hardwood dissolving pulp, unmodified MFC and cationic MFC (CMFC). The sorption of cholestyramine was also carried out under the same condition as those three sorbents. Results showed that bile acid binding has a significantly higher sorption of cholestyramine than all cellulose materials tested based on the equal dry matter (DM) basis (Table 1). As shown in the table, dissolving pulp fiber has the lowest binding capacity (8.79 µmol/100 mg); although the charge density of the two materials is the same, unmodified MFC (made from the dissolving pulp) has a much higher binding capacity for cholate (20.66 µmol/100 mg), which can be attributed to the increased fiber surface area.12,

24

By rending the cationic MFC at a charge density of 0.69

meq/g, evidently, the binding capacity for cholate increased to 57.95 µmol/100 mg under the studied conditions. The increased binding capacity of CMFC in comparison with that of dissolving pulp can be explained as follows: a) According to the previous literature,25 the specific surface area of modified MFC is around 50 m2/g, which is about 10 times greater than that of the fiber, this also can be seen from the Figure 1 that the specific surface area increased, which means MFC has easier accessibility to bile acids and thus more efficient sorption of bile acid in the gut, b) the cationic character due to the EPTMAC cationic modification. It can be seen that with the same amount of sorbent, CMFC has about 30% capacity of that of cholestyramine for binding sodium cholate.

5



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(a)

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(b)

Figure 1. The SEM images of hardwood dissolving pulp fibers (a), and CMFC (b).

Table 1. In-vitro sodium cholate binding by hardwood dissolving pulp fibers, un-modified MFC, CMFC, and cholestyramine on equal weight (dry matter (DM) basis, an average of 6 replicates).

Sorbent

sodium cholate binding (µmol/100 mg DM)

sodium cholate binding relative to cholestyramine,%

Charge density (meq/g)

Hardwood dissolving pulp fibers

8.79±0.50

4.51

-0.06

Un-modified MFC

20.66±0.65

10.60

-0.06

CMFC

57.95±0.30

29.74

0.69

Cholestyramine

194.86±0.50

100.00

3.71*

*The charge density of cholestyramine was adapted from an earlier study.

26

3.2. Sorption isotherms. Figure 2 shows the sorption isotherm of sodium cholate on the CMFC. The amount of sorption sodium cholate increases with increasing cholate concentration, finally reaches a plateau state at the high concentration range. The sorption of sodium cholate onto other sorbents, like activated carbon, cationic dextran and cholestyramine, showed similar trends at various temperatures.5, 27 Considering the fact that the molecule of sodium cholate is large, the sodium cholate is unlikely to be diffused into the internal structure of the CMFC, which is consistent with the results for surface modified cellulose fibers as an adsorbent for dissolved organic pollutants, including several aromatic compounds and herbicides.15, 24, 28 Therefore, the sorption of cholate was mainly based on the electrostatic interaction between the negatively charged carboxyl groups of the cholate anions and the positively charged groups on the external surface of the CMFC. 6


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400

Sodium cholate remained in solution (Ce), mg/l

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


350 300 250 200 150 100 50 0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Adsorbed sodium cholate (qe), mg/g

Figure 2. Sorption isotherm of sodium cholate on CMFC (at 37 , 120rpm for 300min)

The experimental sorption data plotted in Figure 2 was employed to determine the sorption isotherm of sodium cholate on CMFC. Three sorption models, i.e., Langmuir, Freundlich and Tempkin isotherm, were adapted. Langmuir isotherm models the monolayer coverage of the sorption surface. It assumes that the sorption occurs at specific homogeneous sorption sites of the adsorbent and intermolecular force decreases rapidly with increasing the distance from the sorption surface. The Langmuir equation can be expressed as:29

Ce 1 C = + e qe Qmax a L Qmax

(3)

where qe is the amount of substrate adsorbed on CMFC at equilibrium (mg/g), Ce is the concentration of adsorbate in solutions at equilibrium (g/l), Qmax is the maximum possible sorption amount (mg/g), and aL is the Langmuir constant (l/mg). Freundlich equation is about the sorption on the heterogeneous surface or supporting sites for varied affinities. This equation is also used to assess the sorption intensity of the adsorbent towards the adsorbate and is expressed as:30

ln qe = ln K F +

1 ln Ce n

(4)

where KF is the Freundlich constant (mg/g) indicating the sorption capacity and strength of the adsorptive bond and n is the heterogeneity factor which represents the bond distribution.26 Its value indicates the degree of non-linearity between solution concentration and sorption are as follows: if the value of 1/n is between 0.1～1.0, the sorption is easy; if the value is above 1, it is 7



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difficult to proceed.31 The Tempkin isotherm equation considers the presence of indirect adsorbate-adsorbate interactions and is represented as:32

θ=

qe RT = ln K 0Ce qmax ∆Q

(5)

where θ is fractional coverage, R is universal gas constant (J/mol K), T is temperature (K), ∆Q=(-∆H) is variation of the sorption energy (kJ/mol), and K0 is Tempkin equilibrium constant (l/mg).

Table 2. Constant parameters derived from fitting the experimental data of Fig. 3 into isotherm models presented in Eqs. (3)-(5).

Cationic-MFC Langmuir model Qmax(mg/g) R2

416.67 0.01 0.992

RL

0.07-0.32

aL(l/mg)

Freundlich model KF(ml/g) n R2

47.47 3.50 0.977

Tempkin model ΔH (kJ/mol) K0 (ml/mg) R2

13.03 0.12 0.964

Results of the isotherm analysis are listed in Table 2. Generally, on the basis of correlation coefficient, the data are well fitted into all the investigated models. However, the most fixable one is the Langmuir model, implying both the multi-layer sorption and the sorption interaction on CMFC were marginal. The maximum surface sorption capacity Qmax and the Langmuir constant aL are listed in Table 3, which are 416.67 mg/g and 0.01 l/mg, respectively. Both the two values are approximately the same as those obtained for Cholestyramine in the literature,2 but the binding capacity of this CMFC was still lower than Cholestyramine under similar physiological condition to those in the gut. Presumably, the reason is that CMFC has less charge density and is less hydrophobic, which implies that CMFC has low selectivity towards cholate and is easily 8


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influenced by other anionic or amphiphilic molecules. The equilibrium parameter, RL of the Langmuir isotherm and defined as RL=1/(1+aLC0), where C0 is the initial sodium cholate concentration (mg/l) and aL is the Langmuir constant related to the energy of sorption (l/mg), was also determined (Table 2). As shown in Table 2, RL is found to be in the range of 0.07-0.32, it can be concluded that the sorption of sodium cholate onto CMFC is a favorable process.33

500 0.5 mM 450


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1.0 mM 1.5 mM

400

2.0 mM 350 300 250 200 150 100 50 0 0

50

100

150

200

250

300

Time, min

Figure 3. Sorption of sodium cholate on CMFC as a function of time (initial sodium cholate concentrations of 0.5, 1.0, 1.5 and 2.0 mM, at 37℃, and 120 rpm in the absence of electrolytes).

3.3. Sorption kinetic analysis. The sorption data of sodium cholate versus contact time are presented in Figure 3. As it illustrated, the sorption of sodium cholate onto CMFC was fast. Three kinetic models, namely, the pseudo-first order, pseudo-second order and Elovich’s equation,34 were applied to the kinetics data shown in Figure 3. Results demonstrated that neither pseudo first-order kinetic nor Elovich’s kinetic was adequately able to fit the data (results not shown). On the other hand, the pseudo-second order kinetic model was in a good agreement with the experimental data, as shown below. Table 3. Determined pseudo-second order kinetic parameters for the sorption of sodium cholate on CMFC based on the experimental data of Figure 3.

C0 (mM) Pseudo-second order kinetics qe (mg/g) k2 (×10-3g/(mg min)) R2

0.5

1.0

1.5

2.0

161.29 16.71 1.000

227.27 1.21 1.000

285.71 0.72 0.999

344.83 0.76 1.000

The pseudo-second order equation is shown as Eq. (6):34

9



t 1 1 = + t 2 qt k 2 qe q e

(6)

where qe and qt are the amount of sodium cholate adsorbed (mg/ml) at equilibrium and at time t (min), respectively and k2 (g/(mg min)) is the rate constant of pseudo-second-order sorption. The parameters determined by fitting the data of Figure 3 in the pseudo-second order model are listed in Table 3. Evidently, the pseudo-second order model adequately predicted the present sorption data, as shown in Figure 4, and the sorption process was controlled by chemisorptions.

1.4 0.5 mM 1.0 mM 1.5 mM 2.0 mM

1.2 1.0

t/qt

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0.8 0.6 0.4 0.2 0.0 0

50

100

150

200

250

300

Time, min

Figure 4. Pseudo-second order kinetics for the sorption of sodium cholate onto CMFC (initial sodium cholate concentrations of 0.5, 1.0, 1.5 and 2.0 mM, at 37℃, and 120rpm in the absence of electrolytes).

3.4. Effect of electrolytes. Cationic polymers, used as bile acid sorbents, have an anion-exchange nature and their binding ability can be affected by the presence of salts, naturally found in gastrointestinal fluids. Figure 5 shows the results in a 0.05 M bicarbonate buffer containing 0.15 M NaCl, which is similar to physiological condition in the gut.4 It can be seen that the sorption of sodium cholate onto the CMFC decreased significantly with the presence of electrolytes. As discussed, the electrostatic interactions between the dissociated sodium cholate and cationic groups of the sorbent is the dominant mechanism, the presence of salts screened the charge interaction, thereby causing the decreased sorption of sodium cholate. In the literature, the sorption of sodium cholate onto the bile acid sorbents which was primarily based on charge interaction also reduced in the presence of electrolytes. For example, the sorption of sodium cholate by dextran hydrogels with amino groups was affected by the nature of the competing anion and decreased in the order Cl->H2PO42->HCO32-; the binding of cholate to both colestimide and cholestyramine decreased markedly with the increasing concentration of electrolyte.4, 5

10


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400

in water in bicarbonate buffer

350


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300 250 200 150 100 50 0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Sodium cholate remained in solution (Ce), g/l

Figure 5. Effect of electrolytes on the sodium cholate sorption onto CMFC (0.05 M bicarbonate buffer containing 0.15 M NaCl at 37℃, 120rpm for 300min).

4. CONCLUSIONS Cationic MFC was studied as a potential sorbent for bile acid. At a cationic density of 0.69 meq/g, the sorption capacity of CMFC was 57.95 mg/g in the physiological condition of gut, which was about 30% of that of Chelestyramine under the same conditions. The sorption isotherms were determined using Langmuir, Freundlich, Tempkin models. Results showed that the Langmuir model represented the best fit of experimental data. The maximum capacity Qmax of CMFC (charge density: 0.69 meq/g) was 416.67 mg/g according on the Langmuir model. The pseudo-second order kinetics model fitted well with the results obtained at different initial sorbent concentrations. Experimental results indicated a chemisorption process due to the charge interactions of the quaternary amine groups of CMFC and carboxylate of sodium cholate. The presence of salts had a strong negative effect on the sorption of Sodium cholate on CMFC.

AUTHOR INFORMATION Corresponding Author *Phone: (86) 138 2075 1954. E-mail: [email protected].

ACKNOWLEDGMENT The authors wish to acknowledge the financial support from the Tianjin Municipal Science a nd Technology Commission (Grant No.12ZCZDGX01100). 11



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REFERENCES (1) Collins, H. M.; Burton, R. A.; Topping, D. L.; Liao, M. L.; Bacic, A.; Fincher, G. B. Review: Variability in fine structures of noncellulosic cell wall polysaccharides from cereal grains: Potential importance in human health and nutrition. Cereal. chem. 2010, 87, 272-282. (2) Ast, M.; Frishman, W. H. Bile acid sequestrants. J. Clin. Pharmacol. 1990, 30, 99-106. (3) K. A.; Kinosian, B.; Jacobson, T. A.; Glick, H.; Willian, M. K.; Koffer, H.; Eisenberg, J. M..Reducing high blood cholesterol level with drugs: cost-effectiveness of pharmacologic management. J. Am. Med. Assoc. 1990, 264, 3025-3033. (4) Honda, Y.;Nakano, M. Studies on adsorption characteristics of bile acids and methotrexate to a new type of anion-exchange resin, colestimide. Chem. Pharm. Bull. 2000, 48, 978-981. (5) Nichifor, M.; Cristea, D.; Carpov, A. Sodium cholate sorption on cationic dextran hydrogel microspheres. 1. Influence of the chemical structure of functional groups. Int. J. Biol. Macromol. 2000, 28, 15-21. (6) Scaldaferri, F.; Pizzoferrato, M., Ponziani; F. R., Gasbarrini, G.; Gasbarrini, A. Use and indications of cholestyramine and bile acid sequestrants. Intern. Emerg. Med. 2013. 8, 205-210. (7) Nechifor, M; Filip, C; Paduraru, I; Nichifor, M; Carpov, A. In: Ottenbrite, R. M.; Chiellini, E. editors. Polymers in Medicine: Biomedical and Pharmaceutical Applications. Lancaster: Technomic Publishing Co.; 1992, 145. (8) Amidi, M.; Mastrobattista, E.; Jiskoot, W.; Hennink, W. E. Chitosan-based delivery systems for protein therapeutics and antigens. Adv. Drug. Deliver. Rev. 2010, 62, 59-82. (9) Mocanu, G.; Nichifor, M. Cationic amphiphilic dextran hydrogels with potential biomedical applications. Carbohyd. Polym. 2014, 99, 235-241. (10) Moon, R. J.; Martini, A.; Nairn, J.; Simonsen, J.; Youngblood, J. Cellulose nanomaterials review: structure, properties and nanocomposites. Chem. Soc. Rev. 2011, 40, 3941-3994. (11) Alila, S.; Besbes, I.; Vilar, M. R.; Mutjé, P.; Boufi, S. Non-woody plants as raw materials for production of microfibrillated cellulose (MFC): A comparative study. Ind. Crop Prod. 2013, 41, 250-259. (12) Morandi, G.; Heath, L.; Thielemans, W. Cellulose nanocrystals grafted with polystyrene chains through surface-initiated atom transfer radical polymerization (SI-ATRP). Langmuir. 2009, 12


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Binding of Sodium Cholate In Vitro by Cationic ... - ACS Publications

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