Selenium Bioaccessibility and Speciation in Selenium-Enriched Lettuce

Mar 27, 2017 - Lettuce: Investigation of the Selenocompounds Liberated after in. Vitro Simulated Human Digestion Using Two-Dimensional HPLC-ICP-. MS...
0 downloads 0 Views 1MB Size
Subscriber access provided by UQ Library

Article

Selenium bioaccessibility and speciation in selenium-enriched lettuce: investigation of the selenocompounds liberated after in vitro simulated human digestion using two-dimensional HPLC-ICP-MS Emanueli DO NASCIMENTO DA SILVA, Federica AURELI, Marilena D'AMATO, Andrea RAGGI, Solange CADORE, and Francesco CUBADDA J. Agric. Food Chem., Just Accepted Manuscript • Publication Date (Web): 27 Mar 2017 Downloaded from http://pubs.acs.org on March 27, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

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.

Page 1 of 31

Journal of Agricultural and Food Chemistry

1

Title

2

Selenium bioaccessibility and speciation in selenium-enriched lettuce:

3

investigation of the selenocompounds liberated after in vitro simulated human

4

digestion using two-dimensional HPLC-ICP-MS

5

Running title

6

Selenium bioaccessibility and speciation in selenium-enriched lettuce

7

Authors

8

Emanueli DO NASCIMENTO DA SILVAa, Federica AURELIb, Marilena D’AMATOb, Andrea RAGGIb,

9

Solange CADOREa, Francesco CUBADDAb*

10

Affiliations

11

a

Institute of Chemistry, University of Campinas, CEP 6154, 13083-970, Campinas, SP, Brazil

12

b

Department of Food Safety, Nutrition and Veterinary Public Health, Istituto Superiore di Sanità-

13

14

Italian National Institute of Health, Viale Regina Elena 299, 00161 Rome, Italy.

* Corresponding author

15

Viale Regina Elena 299, 00161 Rome, Italy

16

e-mail: [email protected]

17

phone +39 06 49906024

18

fax +39 06 49902540

19

1 ACS Paragon Plus Environment

Journal of Agricultural and Food Chemistry

20

Abstract

21

The evaluation of selenium-enriched vegetables as potential dietary sources of selenium, an

22

essential element for humans, requires an assessment of the plant accumulation ability as well as of

23

the bioaccessibility and speciation of the accumulated selenium, which influence its biological

24

effects in humans. Lettuce hydroponically grown at three selenite (SeVI)/selenate (SeIV)

25

amendment levels was characterized accordingly. Selenium accumulation in lettuce leaves was

26

greatest with Se(VI) amendment, whereas bioaccessibility was 70% on average in both cases.

27

Selenium speciation in gastrointestinal hydrolysates, characterized by anion and cation exchange

28

HPLC-ICP-MS, showed that Se(IV) was largely biotransformed into organoselenium metabolites,

29

with selenomethionine accounting for 1/3 of the total detected species, whereas Se(VI) was

30

incorporated as such in the edible portion of the plant, with only a small fraction (~20%) converted

31

into organic species. Taking into account both nutritional quality and safety, the Se(IV)-enriched

32

lettuce appeared more favourable as potential selenium source for human consumption.

33

Keywords: selenium, food, speciation, in vitro simulated digestion, biofortification, human health

34

2 ACS Paragon Plus Environment

Page 2 of 31

Page 3 of 31

35

Journal of Agricultural and Food Chemistry

1 Introduction

36

Selenium is an essential element in humans, being needed as L-selenocysteine (SeCys) for the

37

synthesis of selenoproteins and found as such in the active centre of a number of selenoprotein

38

enzymes.1 Iodothyronine deiodinases, glutathione peroxidases (GPxs), thioredoxin reductases, and

39

selenoprotein P (SEPP1) are important selenoproteins that have a variety of functions, including

40

antioxidant effects, T-cell immunity, thyroid hormone metabolism, selenium homeostasis and

41

transport, and skeletal and cardiac muscle metabolism.2 Insufficient or sup-optimal selenium intakes

42

may cause a range of detrimental effects on human health and the relationships between selenium

43

intake/status and various health outcomes, e.g. gastrointestinal and prostate cancer, cardiovascular

44

disease, diabetes, male fertility, have been intensively investigated in the last years. Evidence to

45

date suggests that selenium undernutrition may play an important role in some of these conditions

46

but since the range of intake separating deficiency and toxicity is narrow, care has to be taken in

47

identifying the optimal range for health and avoiding selenium overexposure.1,2

48

Dietary reference intakes set on the basis of the optimization of plasma GPx3 activity are

49

typically around 55 µg/day,3 whereas higher dietary intakes are necessary for the levelling off of

50

plasma SEPP1 concentration and an adequate intake of 70 µg/day for adults was set by EFSA using

51

this criterion.4 The selenium content of staple foods such as grains and vegetables depends on the

52

selenium content of the soil as well as on its geochemical characteristics, which modulate selenium

53

phytoavailability. The amount of selenium in the diet largely depends on where crops are cultivated,

54

the soil/fodder to which animals are exposed, and the actual foods consumed. Low-selenium areas

55

are present in a number of countries worldwide and therefore a large proportion of the human

56

population is thought to have sub-optimal to insufficient selenium intakes.3,5

57

L-selenomethionine (SeMet) is the predominant selenium species in almost all food sources

58

whereas SeCys is the main form of selenium in mammalian proteins and is typically found in foods

59

of animal origin.6 SeMet may unspecifically replace methionine residues in proteins, with the 3 ACS Paragon Plus Environment

Journal of Agricultural and Food Chemistry

60

resulting proteins being referred to as selenium-containing proteins, whereas SeCys constitutes a

61

specific amino acid residue in selenoproteins.1,3 Selenites (SeO32–, Se(IV)) and selenates (SeO42–,

62

Se(VI)), the most common selenium inorganic compounds, are found in water and, as minor

63

species, in most foods.1,7 In plants of the Brassica genus (e.g. broccoli) and the Allium genus (e.g.

64

onion, garlic), the non-protein selenoaminoacids Se-methylselenocysteine (MeSeCys) and γ-

65

glutamyl-Se-methylselenocysteine (γ-Glu-MeSeCys) are present and become the major species

66

when these vegetables incorporate large amount of selenium.8,9

67

Dietary selenium is generally well absorbed, but retention is higher with organic compounds

68

compared to inorganic ones.6 Se(VI) is better absorbed than Se(IV), but a significant fraction is lost

69

in the urine, whereas Se(IV) is better retained than Se(VI).10 Upon absorption, SeCys, Se(IV) and

70

Se(VI) are available for the synthesis of selenoproteins whereas SeMet can substitute for

71

methionine in proteins where it can act as a selenium store, entering the functional selenium body

72

pool to be converted to SeCys upon turnover of tissue proteins.4 At dietary levels leading to

73

excessive intakes, toxic effects of selenium are also species-related, with inorganic selenium – and

74

especially Se(IV) – being more toxic than SeMet and most organic selenocompounds.6,11 In

75

summary, the beneficial or toxic effects of selenium are not only dose-dependent, but also related to

76

the chemical form of the element and its bioavailability. Therefore selenium speciation is important

77

to have an insight into the bioavailability of dietary selenium and the relationships between intake,

78

selenium status and health outcomes.

79

Even though no selenium requirement has been shown and selenium essentiality is still

80

controversial for higher plants, plants readily take up and assimilate selenium using sulfur

81

transporters and biochemical pathways. Importantly, plant foods are the main source of dietary

82

selenium and thus plant selenium metabolism is key for selenium nutrition of humans.12-18 A

83

number of studies investigated the extent of selenium uptake and assimilation when different plant

84

species are selenium-enriched, focusing on the conversion of inorganic selenium in soil/growth

4 ACS Paragon Plus Environment

Page 4 of 31

Page 5 of 31

Journal of Agricultural and Food Chemistry

85

medium into organoselenium compounds biosynthesized in plant tissues. Although Se-enrichment

86

of lettuce (Lactuca sativa L.) has been widely studied in either soil or hydroponic growing

87

conditions,19-28 limited and conflicting information is available on the selenium species found in the

88

edible portion of the plant.29-30 Furthermore, no information exists on selenium bioaccessibility in

89

selenium-enriched lettuce, i.e. on the selenium fraction in lettuce leaves that is solubilized after

90

human gastrointestinal digestion and becomes available for intestinal absorption.11-31

91

The aim of this study was to investigate the extent of selenium accumulation and selenium

92

bioaccessibility in lettuce hydroponically grown on Se(VI) and Se(VI) enriched substrates, and

93

characterize the selenium species released after in vitro simulated human digestion by HPLC-ICP-

94

MS. Since the use of a single chromatographic principle in HPLC-ICP-MS determination of the

95

released selenocompounds is at risk of misidentification due to the possible coelution of some of

96

them, we applied a two-dimensional approach using both anion and cation exchange HPLC-ICP-

97

MS.

98

2 Material and methods

99

2.1 Instrumentation

100

Chromatographic separations were performed using an HPLC system consisting of a Perkin-

101

Elmer Series 200 LC binary pump, an autosampler, and a column thermostat. The outlet of the

102

HPLC column was directly connected via PEEK capillary tubing to the nebulizer of the ICP-MS

103

(Nexion 350D, Perkin-Elmer, USA), which served as the selenium specific detector for both total

104

and speciation analysis. The ICP-MS instrument was equipped with a quartz concentric nebulizer

105

and a quartz cyclonic spray chamber. Chromatographic data were collected, stored, and processed

106

using the Perkin-Elmer software Chromera version 4.1.0.

5 ACS Paragon Plus Environment

Journal of Agricultural and Food Chemistry

107

Page 6 of 31

2.2 Reagents and standards

108

Deionized water obtained by a Milli-Q Element System (Millipore, France) was used

109

throughout the work. Nitric acid 68% v/v (Carlo Erba Reagenti, Italy) and hydrogen peroxide 31%

110

v/v (Merck KGaA, Germany), both ultrapure grade, were used for oxidative digestion of samples

111

and gastrointestinal hydrolysates. Calibrants and the internal standard solutions used for total

112

selenium analysis were obtained from standard certified solutions with a content of 1 mg mL-1

113

(High Purity Standard, USA), by dilution with acidified (HNO3) deionized water as necessary. For

114

speciation analysis, 1 mg mL-1 stock solutions, expressed as selenium, were prepared by dissolving

115

in water adequate amounts of selenious acid [Se(IV)], selenic acid [Se(VI)], SeMet, L-selenocystine

116

(SeCys2), and MeSeCys (all from Sigma–Aldrich, USA), methyl-selenomethionine (MeSeMet), and

117

γ-glutamyl-Se-methylselenocysteine

118

Selenohomolanthionine (SeHLan) was kindly provided by Prof. Yasumitsu Ogra. Selenomethionine

119

selenoxide (SeOMet) was prepared according to Michalska-Kacymirow et al.32 Standard stock

120

solutions were stored at 4 ºC and the exact concentrations were ascertained by ICP-MS analysis.

121

The purity of the standards was checked by HPLC-ICP-MS and no species interconversion was

122

found. Analytical-grade ammonium acetate, pyridine, formic acid 98% (Merck KgaA, Darmstadt,

123

Germany), and methanol (J.T. Baker, Deventer, The Netherlands) were used for the preparation of

124

the chromatographic mobile phases. Mobile phases were filtered through a Millipore Express Plus

125

0.22 µm membrane. Porcine enzymes (pepsin, pancreatin), α-amylase from Bacillus subtilis, bile

126

salts, KCl, and MgCl2(H2O)6, (all from Sigma–Aldrich, USA), CaCl2, (Baker Instra Analyzed,

127

Avantor Material, The Netherlands), NaCl, (NH4)CO3, KH2PO4, NaHCO3 (all from Merck KGaA,

128

Germany) were used in simulated gastrointestinal digestion. Ultrapure grade 37% v/v hydrochloric

129

acid (Carlo Erba Reagenti, Italy) and sodium hydroxide (Sigma–Aldrich, USA) were used to adjust

130

pH.

(γ-Glu-MeSeCys)

(PharmaSe,

6 ACS Paragon Plus Environment

Lubbock,

TX).

Page 7 of 31

Journal of Agricultural and Food Chemistry

131

2.3 Procedures

132

2.3.1 Selenium-enrichment of lettuce

133

The greenhouse experiments were conducted at the School of Agricultural Engineering of the

134

University of Campinas. Seedlings of red leaf lettuce (Lactuca sativa L., cv. "Veneza Roxa") were

135

grown hydroponically. Each hydroponic system had four channels, each connected to 10-L vessels

136

of nutrient solution, and comprised 3-4 control or Se-enriched plants. The composition of the

137

nutrient solution (Hidrogood, Brazil) is shown in Table S1 (Supporting information). Nutrient

138

solutions were prepared so as to contain 0, 10, 25 and 40 µmol Se L-1 of either sodium selenate

139

(Na2SeO4) or sodium selenite (Na2SeO3) (Sigma-Aldrich, USA), according to the design shown in

140

Table S2 (Supporting information). The solutions were constantly aerated and the pH was

141

monitored daily and adjusted between 5.5-6.5 with 6 mol L-1 NaOH or 6 mol L-1 HCl, as necessary.

142

The conductivity was maintained in the range 2.5-3.5 mS cm-1; when the value was out of this range

143

the solution was replaced. After 28 days, the plants were harvested, washed with tap and deionized

144

water, and dried in an oven at 60 °C for 72 h.

145

2.3.2 In vitro simulated gastrointestinal digestion

146

Selenium bioaccessibility was assessed using a standardised static in vitro method simulating

147

human gastric and gastrointestinal digestion.33 Table S3 (Supporting information) summarizes the

148

composition of the gastrointestinal fluids used in this study and Figure S1 (Supporting information)

149

shows the overall scheme of the in vitro digestion experiments. Briefly, each batch of lettuce

150

samples was incubated for 2 minutes at 37 ºC with saliva in a mixing water bath (GFL 1083,

151

Gesellschaft für Labortechnik mbH, Burgwedel, Germany), then the gastric juice was added and the

152

samples were kept in the mixing water bath for 2 h. Another batch of samples was submitted in

153

parallel to the same procedure, but after the gastric digestion the intestinal juice was added and the

154

samples were incubated for other 2 h. The gastric and the gastrointestinal hydrolysates were

155

centrifuged at 10,000 rpm for 25 min at 4 ºC. The supernatant was then collected, filtered through

7 ACS Paragon Plus Environment

Journal of Agricultural and Food Chemistry

Page 8 of 31

156

0.45 µm membranes, divided into aliquots and stored at -80 ºC until analysis. Procedural blanks

157

were run in parallel in order to check the presence of selenium in the reagents. The in vitro

158

enzymolysis procedure was carried out in triplicate for each lettuce sample.

159

2.3.3 Total selenium analysis

160

For total selenium analysis, samples were submitted to microwave-assisted oxidative

161

digestion by means of an Ultrawave single reaction chamber (Milestone, Sorisole, Italy). For lettuce

162

samples, approximately 0.15 g of dried sample weighed into a Teflon® flask were added with 3 mL

163

of HNO3 and 1 mL of deionized water. Total selenium solubilized by gastric and gastrointestinal

164

digestion was also determined by ICP-MS. Aliquots of extracts (2 mL) were added with 1 mL of

165

HNO3, and submitted to the same digestion procedure as above. The digested samples were made

166

up to 20 mL with deionized water prior analysis. Measurements were carried out using

167

82

168

quantification. The limits of detection and quantification were 0.06 and 0.21 µg L-1, respectively.

169

The accuracy of total selenium determinations, as assessed through analysis of the reference

170

material in cabbage matrix IAEA 359 (International Atomic Energy Agency, Vienna, Austria), was

171

satisfactory with no statistical difference between found values (0.12 µg g-1, 95% C.I. = 0.113-0.133

172

µg g-1) and information values (0.12 µg g-1, 95% C.I. = 0.109-0.131 µg g-1).

173

2.3.4 Selenium speciation analysis

Se as analytical masses with

74

77

Se and

Ge as internal standard, and the method of standard additions for

174

The selenium species liberated by simulated gastrointestinal digestion were characterized by

175

anion exchange and cation exchange HPLC-ICP-MS. Selenocompounds in extracts were identified

176

by retention time matching with the standard substances spiked to the sample extracts. Quantitative

177

calculations were based on peak areas using external calibration or the method of standard additions

178

as appropriate, depending on sample dilution. For anion exchange separation a PRP-X100 column

179

(4.6·x 250 mm, 10 µm) equipped with a guard-column was used; the eluents were 5 mM acetate

180

buffer (pH 4.7) (A) and 150 mM acetate buffer (pH 4.7) (B), and gradient elution at 1.0 mL min-1 8 ACS Paragon Plus Environment

Page 9 of 31

Journal of Agricultural and Food Chemistry

181

(0-4 min-100% A, 4-6 min-from 100% A to 15% A, 6-30 min-85% B and 15% A) was used. Fifty

182

µL of the filtered hydrolisates were injected. For cation exchange separation a Chrompack

183

IonoSpher-5C column (3.0·x 100 mm, 5 µm) equipped with guard-column was used; eluent A, i.e.

184

3% (v/v) MeOH at pH 3.0, and eluent B, i.e. 10 mM pyridinium formate with 3% (v/v) MeOH at

185

pH 3.0 - were used at 1 mL/min according to the gradient programme 0.1-3.5 min-92.5% A/7.5% B,

186

3.5-13.5 min-72% A/28% B, 13.5-16.5 min-72% A/28% B, 16.5-28 min-92.5% A/7.5% B. Twenty

187

µL of the filtered hydrolysates were injected for measurements. The chromatographic purity of the

188

anion exchange peak eluting at 2.7 min was assessed by orthogonal chromatography, i.e. collecting

189

the fraction eluting from 1.5 to 4.5 minutes and analysing it by cation exchange HPLC-ICP-MS

190

with the same conditions as above.

191

3 Results and discussion

192

3.1 Selenium accumulation in lettuce

193

There was no observable toxic effect of selenium application on lettuce plants in the

194

conditions of this study. In particular, there was no statistically significant influence of the selenium

195

form and applied concentration on plant biomass (ANOVA, p > 0.05).

196

Selenium accumulation in lettuce leaves as a function of applied selenium was quite different

197

when the latter was provided in the form of Se(IV) compared to Se(VI) (Figure 1). The selenium

198

concentrations in the edible portion of the plant resulting from the treatment with Se(VI) were much

199

higher (Table 1). The dissimilar selenium deposition observed in lettuce leaf tissue is consistent

200

with the different absorption and assimilation of Se(IV) and Se(VI) in higher plants. Se(IV) is

201

rapidly converted to organic forms, which have low mobility in the xylem and are incorporated as

202

selenoaminoacids into root proteins; in contrast, Se(VI) in roots is not easily converted to organic

203

forms and being much more mobile in the xylem it is easily transported to the aerial part of the

9 ACS Paragon Plus Environment

Journal of Agricultural and Food Chemistry

204

plant.13,15-18 This is confirmed by the high Se(VI) to total selenium ratio found in leaves of Se(VI)-

205

enriched plants (vide infra).

206

Several studies observed that lettuce leaves readily accumulate selenium - and at a greater

207

degree when supplied as Se(VI) compared to Se(IV) - as found in the present study.21,23-26 In

208

comparative studies, lettuce was found to achieve higher selenium concentrations in the edible

209

portion than other plants (e.g. radish, tomato, strawberry) under the same growing and

210

biofortification conditions; for other species (e.g. chicory and cucumber) the selenium levels were

211

comparable.19-20,22 An evaluation of 30 diverse accessions of lettuce for their capacity to accumulate

212

selenium showed an over twofold change in total selenium levels between cultivars, highlighting

213

useful variability in lettuce germplasm for optimal biofortification.25 Selenium accumulation, for

214

Se(VI) treatment, appeared to be associated with an altered expression of genes involved in

215

selenium/sulphur uptake and assimilation, whereas the stimulating effect on plant growth found in

216

some cases correlated with the activities of antioxidant enzymes.25

217

3.2 Selenium bioaccessibility in lettuce

218

The fraction of leaf selenium that was solubilized after incubation with simulated salivary

219

fluid and simulated gastric fluid (succession of the oral and gastric phases of human digestion) is

220

referred to as bioaccessible selenium after gastric digestion in this work. Se(IV)-enriched lettuce

221

had a mean gastric bioaccessibility of 32% (s.d. 2%, min-max 31-34%) whereas for Se(VI)-

222

enriched lettuce the fraction solubilized was 50% on average (s.d. 8%, min-max 43-58%), and it

223

was positively associated (r=0.978) with the magnitude of Se(VI) application (Table 1). In the non-

224

enriched lettuce, the fraction of leaf selenium solubilized amounts to an intermediate value of 42%

225

(s.d. 9%).

226

After completion of the simulated human digestion with the intestinal phase, Se(IV)-enriched

227

lettuce had a gastrointestinal bioaccessibility of 68% on average (s.d. 3%, min-max 65-71%), which

10 ACS Paragon Plus Environment

Page 10 of 31

Page 11 of 31

Journal of Agricultural and Food Chemistry

228

was negatively associated (r=-0.997) with the magnitude of Se(IV) application, whereas for Se(VI)-

229

enriched lettuce the fraction solubilized is 72% on average (s.d. 6%, min-max 66-76%), and it is

230

greater (76%) for the two higher levels of Se(VI) application (Table 1).

231

The higher gastric bioaccessibility of selenium in Se(VI)-enriched plants is consistent with a

232

selenium speciation dominated by Se(VI), which is easily solubilized in the conditions of human

233

stomach digestion. On the other hand, if a substantial proportion of the total selenium is present as

234

protein-bound SeMet, the initial proteolytic hydrolysis promoted by pepsin in the gastric phase is

235

not sufficient for the release of the selenoaminoacid that requires the cleavage of peptide bonds

236

occurring during intestinal proteolysis. The latter is the case of Se(IV)-enriched plants (vide infra).

237

Irrespectively of the type of supplementation, the supplementation level and the resulting leaf

238

selenium concentration and speciation, the gastrointestinal bioaccessibility of selenium in Se-

239

enriched lettuce was satisfactory, i.e. 70% on average compared to 62% of the non-enriched lettuce.

240

This value is in the range of the levels commonly observed for selenium bioavailability (in vivo) or

241

bioaccessibility (in vitro) in plant sources.10-11,31

242

3.3 Speciation of bioaccessible selenium in lettuce

243

Figure 2 shows the anion exchange HPLC separation with ICP-MS detection of a mixture of 9

244

selenium standards. Cation exchange HPLC separation with ICP-MS detection was used as an

245

alternative chromatographic system and Figure 3 shows the HPLC-ICP-MS chromatogram of 9

246

selenium compounds of interest.

247

Analysis of the gastrointestinal hydrolysates by HPLC-ICP-MS was carried out to

248

characterize the bioaccessible selenium chemical species, i.e. the actual selenium compounds that

249

are released after simulated human gastrointestinal digestion and become available for intestinal

250

absorption. Since the retention time of some species was affected by the sample matrix, correct

251

assignment of identity as well as quantification were based on coelution with spiked authentic

11 ACS Paragon Plus Environment

Journal of Agricultural and Food Chemistry

252

standards. In addition, the samples were analysed at different dilutions and in all cases species

253

identification was substantiated by spiking the sample with the authentic standard substance.

254

Anion exchange HPLC-ICP-MS revealed that at least twelve chemical species were present in

255

the case of Se(IV)-enriched lettuce (Figure 4). The inorganic species (Se(IV) and Se(VI)) are a very

256

minor fraction of the total chromatographed selenium and they always account for ≤7% of the sum

257

of the eluting species (Table 2). More than 90% of the selenium eluted from the column is thus

258

represented by organic compounds, with two of them featuring as major species and constituting

259

~60% of the sum of the species. One of these two species (retention time 6.9 min) was identified as

260

SeMet. Apart from SeMet, none of the other organic compounds in Figure 2 matched any of the

261

unknown peaks in Figure 4. It has to be noticed that nearly 40% of the selenium solubilized by in

262

vitro gastrointestinal digestion does not elute from the column, which highlights the presence of

263

strongly retained compounds most likely of organic nature.

264

In order to characterize more closely the different organoselenium species present, cation

265

exchange HPLC-ICP-MS was used as an additional, complementary analytical approach (Figure 5).

266

An excellent agreement was found for SeMet, which again was found to account for one third of the

267

selenium species eluted (Table 3). Spiking experiments with SeCys2 showed coelution with the

268

peak at 10.5 min, thus providing evidence of the presence of this species in sample extracts.

269

A specific attention was paid to the possible presence of MeSeCys, which was reported as the

270

major organic species in lettuce in a previous study.30 However, in spiking experiments, none of the

271

several organic compounds present coeluted with the MeSeCys standard, thus demonstrating that

272

this species was absent. The occurrence of SeHLan, a non-protein selenoaminoacid produced by

273

plants via the sulfur assimilation pathway and identified for the first time in selenium-enriched

274

Japanese pungent radish,34 was also excluded based on spiking experiments. Co-chromatography of

275

the γ-Glu-MeSeCys standard spiked in the sample extracts showed that this species was absent as

276

well. 12 ACS Paragon Plus Environment

Page 12 of 31

Page 13 of 31

Journal of Agricultural and Food Chemistry

277

An attempt was then made to identify the organoselenium compounds present in the anion

278

exchange fraction eluting between 1.5-4.5 min, which was collected and injected in cation exchange

279

HPLC-ICP-MS. The fraction contained a species eluting in the void and one at ~8 min, which

280

however did not coelute with any of the available standards and, in particular, SeOMet. This species

281

was a potential candidate for the identity of the anion exchange peak at 2.7 min as it is easily

282

formed by SeMet oxidation and thus it has been widely found in enzymatic extracts of plant

283

materials and selenized yeast.35-38

284

Unfortunately, post-column recovery did not improve with cation exchange chromatography,

285

confirming that reactive compounds that are retained by the column are largely present in the

286

gastrointestinal hydrolysate of Se(IV)-enriched lettuce. In summary, SeMet was confirmed to

287

account for about one third of the sum of the species; taking into account post-column recovery, this

288

translates in not less than 20% of the bioaccessible selenium after gastrointestinal digestion.

289

In gastrointestinal extracts of Se(VI)-enriched lettuce, Se(VI) was found to be the

290

predominant species (Figure 6), accounting for 64-80% of the eluting selenium in anion exchange

291

chromatography (Table 2). Cation exchange chromatography, where Se(VI) elutes in the void

292

(along with some of the early-eluting species in anion exchange chromatography), showed good

293

quantitative agreement (Table 3). Both chromatographic approaches gave consistent results for

294

SeMet, which account for 11-12% of the sum of the species in lettuce grown at the lowest applied

295

dose of Se(VI) and decreases to 4-5% at the highest dose. No MeSeCys was found to be present in

296

extracts of Se(VI)-enriched lettuce, and the same hold true for the other organic species in Fig. 3.

297

Taken together, these results confirms that lettuce plants are able to metabolize the supplied

298

Se(IV) to SeMet and other organic compounds. Upon Se(VI) amendment, the compound is

299

efficiently translocated to the aerial part of the plant mostly unchanged. The more extensive

300

biotransformation to organic forms of Se(IV) compared to Se(VI) is in line with the available

301

evidence.13,15-18 13 ACS Paragon Plus Environment

Journal of Agricultural and Food Chemistry

302

From the standpoint of both nutritional quality and safety, the Se(IV)-enriched lettuce

303

produced at the lowest fortification levels investigated in this study appears more favourable as

304

potential selenium source for human consumption. Taking the 25 µM sample as an example, it

305

provides 1.6 µg g-1 fresh weight of bioaccessible selenium (dry to fresh weight conversion factor is

306

19.8), equivalent to 135 µg for a 100 g portion of lettuce, consisting of