Evaluation of Iodine Bioavailability in Seaweed Using in Vitro

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EVALUATION OF IODINE BIOAVAILABILITY IN SEAWEED USING IN VITRO METHODS Raquel Domínguez González, Gabriela M Chiocchetti, Paloma Herbello-Hermelo, D. Velez, V. Devesa, and Pilar Bermejo-Barrera J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b02151 • Publication Date (Web): 30 Aug 2017 Downloaded from http://pubs.acs.org on August 30, 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

EVALUATION OF IODINE BIOAVAILABILITY IN SEAWEED USING IN VITRO METHODS

M. Raquel Domínguez-González#, Gabriela M. Chiocchetti&, Paloma Herbello-Hermelo#, Dinoraz Vélez&, Vicenta Devesa&, Pilar Bermejo*#

#

Department of Analytical Chemistry, Nutrition and Bromatology, Faculty of Chemistry,

Health Research Institute of Santiago de Compostela (IDIS), Universidade de Santiago de Compostela, 15782 – Santiago de Compostela, Spain &

Instituto de Agroquímica y Tecnología de Alimentos (IATA-CSIC), Av. Agustín Escardino

7, 46980 Paterna (Valencia), Spain.

Corresponding Author: ORCID iD: 0000-0001-5864-6144 *

e-mail adress: [email protected] (Pilar Bermejo-Barrera)

Telephone Number: 34600942346 Fax Number: 34981547141

E-mail adresses: M. Raquel Domínguez-González: [email protected] Gabriela M. Chiocchetti: [email protected] Paloma Herbello-Hermelo: [email protected] Dinoraz Vélez: [email protected] Vicenta Devesa: [email protected]

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ABSTRACT

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Due to the high levels of iodine presents in seaweed the ingestion of large amount of this type

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of food can produce excessive intake of iodine. However, the food after ingestion suffers

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different chemistry and physical processes that can modify the amount of iodine that reaches

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the systemic circulation (bioavailability).Studies on the bioavailability of iodine from food are

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scarce and indicate that the bioavailable amount is generally lower than ingested. Iodine in

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vitro bioavailability estimation from different commercialized seaweed has been studied using

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different in vitro approaches (solubility, dialyzability and transport and uptake by intestinal

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cells). Results indicate that iodine is available after gastrointestinal digestion for absorption

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(bioaccessibility: 49-82%), being kombu the seaweed with the highest bioaccessibility. The

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incorporation of dialysis cell cultures to elucidate bioavailability modifies the estimation of

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the amount of iodine that may reach the systemic circulation (dialysis: 5-28%; cell culture: ≤

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3%). The paper discusses advantages and drawbacks of these methodologies for iodine

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bioavailability in seaweed.

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Keywords: seaweed, iodine, solubility, dialyzability, bioavailability, Caco-2 cells, HT29-

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MTX cells.

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INTRODUCTION

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Iodine is an essential element, necessary for synthesis of the thyroid hormones, thyroxine (T4)

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and triiodothyronine (T3). It is found in nature in various chemical forms: inorganic salts

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(iodides and iodates), inorganic diatomic iodine and organic mono or diatomic iodine.

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Deficiencies in iodine, which affects about two thousand million people worldwide, cause

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endemic goitre, the most visible sign, and central nervous system damage, causing mental

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retardation in children1.

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For the general population the major source of iodine is food, being seafood products

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the type of food with the highest content. Concentration of iodine in fish may reach up to 15

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mg/kg fresh weight (fw)2; whereas in shellfish vary between 0.32 and 135 mg/kg fw3,4.

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Seaweeds have an inherent biologic capacity to concentrate iodine from the sea, consequently,

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concentrations up to 6138 mg/kg (dry weight, dw) have been found in commercialized

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samples5,6,7. Milk and dairy products may also contain relevant concentrations of iodine

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(milk: 0.017-0.365 mg/kg; dairy products: 0.044-1.36 mg/kg)8. In fact, due to their high

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consumption, they are the most important source of iodine in many countries9,10. In addition

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to dietary sources, various mineral supplements and medical preparations can further increase

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iodine intake.

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Most studies identify deficiencies in iodine intake11, however, in some cases high

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intakes above the recommended values have also been described. In certain susceptible

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population groups (individuals with pre-existing thyroid disease, the elderly, fetuses and

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neonates or patients with other risk factors), ingestion of iodine above the recommended

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levels might increase the risk of developing iodine-induced thyroid dysfunction12. Moreover,

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it has been shown that chronic exposure to high levels of iodine also affects healthy adult and

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child population13.The ingestion of large amounts of seaweed, marine fish, ground beef

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containing thyroid tissue, iodized water, bread, salt and iodide-containing dietary supplements 3

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can produce excessive intake of iodine14. Daily iodine intake from nori, wakame and kelp in

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Japanese population can be estimated at 1.2 to 1.3 mg/day15, much higher than the daily

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intake recommended by the Scientific Committee on Food of the European Commission(600

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µg/day14). Yeh et al.6 also show that a regular consumption of brown seaweed kombu (0.76-

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0.96 g) by Taiwanese population entails high iodine intakes (up to 4.8mg/day).

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Assessment of exposure to iodine is carried out in most cases by performing an

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evaluation of the contents of the element in foodstuffs. However, the food after ingestion

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suffers different processes that can modify the quantity of iodine that finally reaches the

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systemic circulation. The digestion process allows the release of iodine from food in the

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lumen where it may interact with other dietary compounds forming complexes that could

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affect its subsequent intestinal absorption. Furthermore, the absorption may be modified by

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competition of iodine and other elements or compounds in the food matrix or diet for the

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same transport mechanisms. Studies on the amount of iodine from food that may reach the

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systemic circulation (bioavailability),16,17are scarce and indicate that the bioavailable amount

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is generally lower than that ingested. It is therefore necessary to consider this variable in the

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exposure assessment of iodine through food products.

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The aim of this study is to estimate the bioavailability of iodine present in different

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types of seaweed. For this purpose, the iodine solubility and dialyzability values after a

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simulated gastrointestinal digestion, and the iodine transport and uptake by intestinal

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epithelial cells (co-cultures Caco-2/HT29-MTX) have been evaluated.

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MATERIALSANDMETHODS

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Instrumentation. Perkin Elmer Nex-IonTM 300X Inductively Coupled Plasma Mass

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Spectrometer (ICP-MS) (Massachusetts, USA) equipped with a SC2 DX autosampler from

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Elemental Scientific (Omaha, USA) and Ethos Plus microwave laboratory station from

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Milestone(Cinisello Balsamo, MI, Italy) with 100 mL closed Teflon vessels and Teflon 4

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covers, HTC adapter plate and HTC safety springs (Milestone,Cinisello Balsamo, MI, Italy)

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were used for iodine determination. Cellu Sep H1 high grade regenerated cellulose tubular

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membranes (molecular weight cut-off 10 kDa, 50 cm length, diameter dry 25.5 mm and a

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volume to length ratio of 5.10 mL/cm) from Membrane Filtration Products Inc. (Seguin, TX,

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USA)were used for dialyzability assays. Six-well plates with polyester membrane inserts (24

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mm diameter, pore size 0.4 µm, Transwell®, Corning, Cultex,NY, USA) and Millicell®-ERS

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voltohmmeter (Millipore Corporation, Massachusetts, USA) were employed for transport and

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uptake cell assay. A freezing point osmometer, Automatic Micro-Osmometer Type 15 Löser,

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Löser Messtechnik, Germany) was used for adjusting osmolarity of the bioaccesible fractions.

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Other equipment included: Boxcult incubator situated on a Rotabit orbital-rocking platform

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shaker (Selecta, Barcelona, Spain); ORION 720A plus pH-meter with a glass–calomel

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electrode (ORION, Cambridge, UK);cellulose acetate syringe filters (0.45 µm) (Millipore,

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Massachusetts, USA);fluorescence microplate reader (PolarSTAR OPTIMA, BMG-Labtech,

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Germany); PowerWave HT microplate scanning spectrophotometer (BioTek Instruments,

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Vermont, USA), MilliQ water-purification system (Millipore, Massachusetts,USA) for ultra-

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pure water (resistivity 18MΩcm) collection.

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Chemicals. For the alkaline solubilisation of seaweed tetramethylammonium

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hydroxide (TMAH)25% (m/m) from Merck (Germany) in water was used. Digestive enzymes

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(porcine pepsin and pancreatin), bile salts (approx. 50% sodium cholate and 50%sodium

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deoxycholate) and piperazine-N,N’-bis(2-ethanesulfonicacid) disodium salt (PIPES) were

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obtained from Sigma Chemicals(Madrid, Spain). HCl and NaCl were acquired from Panreac

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(Barcelona, Spain) and ammonium hydrogen carbonate from Merck (Darmstad, Germany).

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Standard solutions of I-and IO3-were prepared from potassium iodide (99.5%) and potassium

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iodate (99.7-100.4%), both from Merck (Darmstad, Germany). Standard stock solutions of

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1000µg/mL of iodinated amino acids 3-iodo-L-tyrosine (MIT) and 3,5-diiodo-L-tyrosine 5

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dehydrate (DIT) (both from Sigma-Aldrich-Fluka, Madrid, Spain) were prepared dissolving

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0.1g of each compounds in 100mL of 0.01M NaOH/MeOH (1:1). The NIES-09, Sargasso, a

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certified reference material of brown seaweed was supplied by the National Institute of

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Environmental Studies (Ibaraki, Japan).

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The Caco-2 cell line (human colon carcinoma) was acquired from the European Collection of

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Cell Cultures (ECACC, number 86010202, Salisbury, UK). The HT29-MTX cell line was

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kindly provided by Dr. Técla Lesuffleur (Institut National de la Santé et de la Recherche

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Médicale, INSERM UMR S 938, Paris, France).All reagents for maintaining the cell culture

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and their transport assays were provided by Hyclone (Fisher, Madrid, Spain).Dulbecco’s

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modified Eagle’s medium (DMEM) containing 4.5 g/L glucose and 0.87 g/L glutamine; fetal

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bovine serum (FBS); non-essential amino acids (NEAA); sodium pyruvate; N-2-

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hydroxyethylpiperazine-N′-2-ethanesulfonic

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solution;amphotericin B;tripsyn/EDTA (ethylene diamine tetraacetic acid) solution;minimum

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essential medium (MEM); Hanks’ balanced salt solution (HBSS);PBS free of Ca2+ and Mg2+;

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HBSS free of Ca2+ and Mg2+. Sodium resazurin (7-hydroxy-3H-phenoxazin-3-one-10-oxide

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sodium salt), bovine serum albumin (BSA) and lucifer yellow(LY) were acquired from Sigma

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(Madrid, Spain).Bio-Rad Protein Assay kit for protein determination was obtained from

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Biorad (California, USA).

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acid

(HEPES);

penicillin/streptomycin

Seaweed samples. Three species of brown algae were purchased in food stores of included species of brown algae: Undaria pinnatifida (wakame), Hizikia

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Valencia(Spain),they

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fusiforme (hijiki) and Laminaria japonica (kombu). The samples were cookedin MilliQ

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water;20g of seaweed sample (dried weigh (dw)) and 500g of MilliQ water were boiled

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according to the instructions indicated in the food packaging. Samples were ground and store

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at -20ºC until analysis.

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Microwave assisted alkaline digestion (MAE) procedure. This procedure was

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developed previously by our research group 5 .The ICP-MS instrumental operating conditions

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are show in table 1.

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In vitro digestion procedures

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Solubility assays. The simulated in vitrodigestion usedwas a modification of the

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procedure described by Laparra et al.18. Cooked seaweed sample (0.25 g) were weighed in

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Erlenmeyer flask, 20 g of ultrapure water was added and 6 mol/L HCl was added until to

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obtain at pH 2. Gastric solution (10% m/v pepsin in 0.1 mol/L HCl) was added to provide 1

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mg of pepsin/g sample. Afterwards ultrapure water was added sample up to 25 g, and the

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sample was incubated in a shaking water bath (120 strokes/min) at 37 °C for 2 h.

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Subsequent the pH was increases to pH 6.5 using 1 mol/LNH4HCO3. The intestinal solution

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(0.4% m/v pancreatin and 2.5% m/v bile extract in 0.1 mol/L NH4HCO3) was added to

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obtain0.25 mg of pancreatin and 1.5 mg of bile extract per gramof seaweed, and again the

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sample was incubated at 37 °C for 2 h. After the intestinal step, the pH was adjusted to 7.2

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using 0.5mol/LNH4OH. In order to separate the soluble (bioaccessible) phase and the

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residue,the digest was transferred to a polypropylene centrifuge tube and centrifuged at 10000

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rpm for 30 minutes at 4°C.Both fractions,soluble and residual, were kept at -20 ºC before

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measurements. In vitro digestion assays were performed by triplicate. Reagent blanks were

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also obtainedto control possible contamination.

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Iodine solubility was determined using the following equation (Equation1):

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Solubility = [A/B] × 100(Equation1)

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where A is the concentration of iodine in the soluble fraction after application of the in vitro

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digestion, and B is the concentration of iodine in the cooked seaweed.

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Dialyzability assays. To evaluate the iodine dialyzability, Cellu Sep® H1 high grade

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regenerated cellulose tubular membranes were used. At the beginning of the intestinal stage of

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seaweed, the dialysis membranes filled with 20 mL of a 0.15 N PIPES solution (pH 7.5

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adjusted with HCl) were placed inside the flasks. After intestinal digestion, membranes were

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removed and their outer surface was rinsed with ultra-pure water and the membrane

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containing solution (dialyzate) and the residual (non-dialyzable fraction) were transferred to

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polyethylene vials and separately weighted.Both, dialyzate and residual fractions, were kept at

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-20 ºC until measurements.

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Dialyzability of iodine species standards (species found in the seaweeds under study),I-

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30µg/mL, MIT 0.4 µg/mL and DIT 6µg/L,was also evaluated. Firstly, dialysis membranes

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filled with PIPES solution were placed inside aqueous solutions of iodine species (25 mL).

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After 2 h, membranes were removed and dialyzatefractions were analyzed. Afterwards, the

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assay was performed subjecting the standards of the iodine species to the gastrointestinal

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digestion in presence of the membranes, following the protocol described above for seaweeds.

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Iodine bioaccessibility expressed as dialyzability (D) was calculated using the following

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equation(Equation2): 157

D (%) =

[ I ] dialyzate [ I ]total

x100 ( Eq.2) 158

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where [I]dialyzateand [I]total are the iodine concentrations in the dialyzate fraction.

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Cell transport and uptake assays

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Cell culture maintenance and seeding. Caco-2 cells were cultured in DMEM

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supplemented with 10% (v/v) FBS, 1% (v/v) NEAA, 1 mM sodium pyruvate, 10 mM HEPES,

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100 U/mL of penicillin, 0.1 mg/mL of streptomycin and 0.0025 mg/L of amphotericin B

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(DMEMc). The maintenance of HT29-MTX cells was done in DMEM supplemented with

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10% (v/v) FBS, 1 mM sodium pyruvate, 10 mM HEPES, 100 U/mL of penicillin, 0.1 mg/mL

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of streptomycin and 0.0025 mg/L of amphotericin B (HT-DMEMc).

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The cells were incubated at 37 °C at 95% relative humidity and a CO2 flow of 5%. The

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medium was changed every 3 days. When the cell monolayer reached 80% confluence, the

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cells were detached with a solution of trypsin (0.5 g/L) and EDTA (0.22 g/L) and reseeded at

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a density of 5–6 × 104 cells/cm².

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All the transport assays were carried out in 6-well plates with polyester membrane inserts

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(Transwell). The cells resuspended in HT-DMEMc were seeded (5.5 × 104 cells/cm², 1.5

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mL) on the apical side to produce co-cultures of Caco-2/HT29-MTX (80/20). Then 2 mL of

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HT-DMEMc was added to the basolateral chamber and cells were maintained in conditions

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described before (11–12 days post-seeding). Throughout cell differentiation, transepithelial

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electrical resistance (TEER) was measured with a Millicell®-ERS voltohmmeter to evaluate

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the progress of the monolayers.Caco-2 cells wereused between passages 11 and 20 and HT29-

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MTX cells between passages 15 and 24.

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Cell viability assays. In order to work under sublethal conditions, the effect of the

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iodine species present in seaweed on the viability of Caco-2 and HT29-MTX cells was

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evaluated using sodium resazurin (Sigma). Cells were seeded in 24-well plates at a density of

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2.5×104 cells/cm2with 1 mL of DMEMc or HT-DMEMc, depending on the cell type. After

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differentiation took place, the cells were exposed for 24 h to various concentrations of I-(1,

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10, 100 and 1000 mg/L), IO3- (1, 10, 100 and 1000 mg/L), MIT(0.1, 1, 5, 10 mg/L) and

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DIT(0.1, 1, 5, 10 mg/L) prepared in MEM supplemented with 1 mM sodium pyruvate, 10 mM

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HEPES, 100 U/mL of penicillin, 0.1 mg/mL of streptomycin and 0.0025 mg/L of

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amphotericin B. After exposure, cells were incubated with resazurin solution following the

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protocol described by Rocha et al. (2011)19.

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Determination of apparent permeability coefficients (Papp) of iodine species.

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Transport of standards of I- (20 mg/L), IO3- (20 mg/L), MIT (10 mg/L) and DIT (10 mg/L)

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was studied. Iodine standards prepared in HBSSwith 10 mM HEPES were added to the apical

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compartment(1.5 mL) and HBSS-10 mM HEPES to the basolateral compartment (2 mL). At

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the various assay times (30, 60, 90 and 120 min), aliquots (600 µL) were obtained from the

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basolateral compartment and replaced with an equal volume of HBSS-10 mM HEPES. Iodine

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concentration was determined in the aliquots to evaluate apparent permeability coefficients

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(Papp) using Equation 3.

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Papp = (dC/dt) (Vr/AC0) (Equation3)

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where dC/dt is the flow of iodine species (mg/s); Vr is the volume of the basolateral

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compartment (2 mL); A is the surface of the cell monolayer (4.67 cm2); C0 is the initial iodine

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concentration in the apical compartment (mg/L).

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Paracellular transport.The participation of the paracellular pathway in transport of

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iodine species was evaluated by a modulation of the cell junctions, incubating the cell

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monolayer for 5 min with 5 mM EDTA in PBS without Ca2+ and Mg2+ in the apical side and

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HBSS free of Ca2+ and Mg2+ in the basolateral side. Then the standard of iodine species [I- (20

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mg/L), IO3- (20 mg/L), MIT (10 mg/L) and DIT (10 mg/L)], prepared in medium consisting of

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50% of HBSS without Ca2+ and Mg2+ and 50% of HBSS with Ca2+ and Mg2+, both

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supplemented with 10 mM HEPES, was added to the apical compartment. The acceptor

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medium was collected at various times (15, 30 and 45 min) and the concentration of iodine

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was determined in order to evaluate the Papp (Equation 3). The efficiency of the EDTA in

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modulating the cell junctions was monitored by determining the Papp of Lucifer Yellow (LY),

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which was added at a concentration of 100 µM to the apical compartment. The transport of

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LY to the basolateral side was measured with a fluorescence microplate reader Polar STAR

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OPTIMA at excitation/emission wavelengths of 485/520 nm. 10

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Iodine transport and uptake from seaweed samples by intestinal cells. The

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bioaccessible fractions of the seaweeds obtained previously were inactivated by heating for 5

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min at 100 ºC. Glucose (final concentration 1 g/L, Sigma) was added to facilitate cell viability

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and NaCl (5 mM) was used to adjust the osmolarity to 290±25 mOsm/kg using a freezing

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point osmometer (Löser, Germany).

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Treated bioaccessible fraction (1.5 mL) was added to the apical chamber and 2 mL of HBSS-

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10 mM HEPES to the basolateral compartment. After 2 h, the medium was collected from the

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basolateral compartment to determine the quantity of iodine transported, which was corrected

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per mg of protein. Protein content was determined by Bio-Rad Protein Assay kit, following

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the manufacturer’s instructions and using a standard curve of BSA (0.2–1.0 mg/mL).

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Percentage of transport across the monolayer was calculated using Equation 4.

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Cellular transport (%) =

[ I ]transporte d [ I ]added

x100 ( Equation 4)

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where [I]transported is the concentration of iodine present in the basolateral compartment at the

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end of the assay, and [I]added is the concentration of iodine added to the apical side at the

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beginning of the experiment.

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Evaluation of the integrity of the cell monolayer. Cell monolayer integrity was

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evaluated during the assay by measuring (a) TEER during the transport and (b) LY passage to

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the basolateral compartment. The transport assays were only considered valid if, a) the TEER

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values did not vary by more than 25% from those values observed at the beginning of the

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experiment, and b) the transport of LY did not exceed 2% of the total amount added to the

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apical compartment.

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Iodine determination. Iodinedetermination was performed by ICP-MS using

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conditions listed in Table 1. TMAH extracts were diluted before ICP-MS measurement.

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Tellurium (2 mg/L) was used as an internal standard. He was used in the collision cell at a 11

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flow rate of 80 mL/min in order to obtain the best sensitivity and linear ranges. The

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calibration was perform in the concentration range of 0 and 500µg/L.

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Limit of detection (LOD) and a limit of quantification (LOQ) of ICP-MS determinations,

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were calculated based on the 3.SD/10.SD criterion (SD standard deviation of eleven

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measurements of reagent blank). Accuracy of the method was assessed by analysing by

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triplicate the CRM NIES-09.

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Statistical analysis. The statistical analysis by one-factor analysis of variance

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(ANOVA) with the Tukey HSD post hoc multiple comparison test or using the Student t-test

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(SigmaPlot version 12.0) was used. All analysis was performed by triplicate. Statistical

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significance was accepted for p 10×10-6 cm/s. The permeability values obtained

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in the present study for inorganic iodine species and MIT indicate that these species undergo

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moderate absorption (20-70%); whereas DIT is poorly absorbed (