galloprovincialis collected from different locations

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1 Selenium and its distribution in edible mussel Mytilus galloprovincialis collected from different locations Urška Kristan 1,2, Vekoslava Stibilj 1 1 Department of Environmental Sciences, Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana 2 Jožef Stefan International Postgraduate School, Ljubljana, Slovenia vekoslava.stibilj@ijs.si Abstract. Mussels Mytilus galloprovincialis collected and bought from different locations (Slovenian coastline, Italy and NE Pacific) were analysed by techniques of hydride generation atomic fluorescence spectrometry (HG-AFS) and liquid chromatography coupled to inductively coupled plasma mass spectrometry (HPLC-ICP-MS) in order to assess selenium (Se), its distribution and Se species in raw and cooked soft tissue. Total Se concentration in raw mussels ranged between 3.15 to 8.27 µg/g, while in cooked ones, Se concentration were half lower. Selenium species identified were selenomethione (SeMet) and selenocystine (SeCys2). Keywords: Selenium, mussel, ICP-MS, HG-AFS, speciation 1. Introduction Selenium (Se) is an essential trace element with a very complex impact on human and animals. Furthermore, the line between essentiality and toxicity is very narrow; less than 0.1 µg Se/g can cause Se deficiency, while 0.5 µg Se/g toxicity. The principal source of trace elements for humans is diet. Some special nutrients, such as Se is known as antagonist to some toxic elements especially to Hg, protect organism against cancer. Furthermore, it has a very important role as part of the active site in selenoproteins such as glutathione peroxidase [1, 2]. The minimum selenium daily requirements for Slovenia have been summarized from the values accepted in Austria, Germany and Switzerland from the year of The Dietary Reference Intake (DRI) recommendations for Se intake were set between 30 to 70 µg/day for men and women. [3].

2 Speciation of Se is of particular interest since bioavailability of the element is important and mostly depends on its chemical form [4]. Selenium intake generally occurs via plant food and seafood, where most of Se has been associated with organic forms, such as selenomethionine (SeMet) and trimethylselenonium ion (TMSe + ) both found in mussel, while inorganic Se was not found in mussel tissue [5]. Mediterranean mussels Mytilus galloprovincialis lives attached on hard substrata, being filter feeders exposed to ambient seawater. The mussels are primarily used as food and also as indicators of environmental pollution, due to their ability to accumulate high levels of different contaminants (heavy metals, hydrocarbons and pesticides) [6]. The aim of this study was to determine total concentration of selenium and its species in marine mussel Mytilus Galloprovincialis from different locations (Slovenia, Italy and NE Pacific). Furthermore, we compared Se concentration in fresh and cooked mussel tissue in order to see the change in amount of bioavailable Se. 2. Materials and methods 2.1 Samples Mussel samples used in this analysis were brought from Italy and also collected from Slovenian mussel breeder. Mussels were cleaned by removing soft tissue from shell, except for the mussels which were used in cooking procedure. These mussels were first cleaned under the tap water and furthermore cooked in white vine (we followed typical cooking procedure of mussel in Slovenia) in order to see the change between Se and its species in raw and cooked mussel. Another set of samples represented the mussels which were bought in the supermarket (fishing area FAO87, NE Pacific). These mussels were bought frozen and were previously removed from shell and cleaned by the producer, that means that the mussels were subjected to higher temperatures. Soft tissue of fresh and cooked mussel were dried in a freeze dryer at -46 C for 172h to constant weight, homogenized firstly in agate mill and later on with the mill Pulverisete 7 (Fritsch) at a rotational speed of 18,000 rpm. Samples were stored in polyethylene containers at 18 C. 2.2 Procedures and methods Determination of total Se concentration by hydride generation atomic fluorescence spectrometry (HG-AFS): Approximately 0.2 g of dry mussel sample was weighed in a Teflon tube, in which mineralisation of the sample was performed using 0.5 ml of

3 concentrated H 2 SO 4 and 1.5 ml of concentrated HNO 3 by heating the tube on aluminium block for 24 h at 80 C, then increasing the temperature to 130 C and maintaining for 60 min. After cooling the solution to room temperature, 2 ml of 30 % H 2 O 2 was added and the solution reheated for 10 min at 115 C. This step was repeated. After cooling, addition of ml V 2 O 5 in H 2 SO 4 followed and the solution was heated at 115 C for approximately 20 min until the solution became blue in colour (in order to eliminate the surplus of H 2 O 2 ). Reduction of Se 6+ to Se 4+ was made by addition of concentrated HCl to a final concentration around 6M, and heating for 10 min at 90 C. Samples were then diluted with MiliQ water, depending on the foreseen Se concentration in the samples. HG-AFS was used for Se detection [7]. Enzymatic extraction: All samples of fresh and cooked soft mussel tissue were extracted in duplicate as described by Mazej et al. [8]. The supernatant was filtered successively through 0.45 and 0.22 µm filters and then subjected to selenium speciation analysis by liquid chromatography coupled to inductively coupled plasma mass spectrometry (HPLC-ICP-MS). Supernatants and sediments were stored at - 20 C until analysis for total Se in sediments and supernatants by HG-AFS was carried out. Sediments and supernatants: Selenium in the sediments (residue) after enzymatic extraction was determined by the same procedure as that for total selenium determination described above. The Se content in supernatant was digested by HNO 3. 1 ml of concentrated HNO 3 was added to 0.5 of supernatant in a 50 ml Teflon tube and heated for 30 min at 80 C and then for 15 min at 160 C. After cooling, addition of 0.5 ml of H 2 O 2 followed and the solution was evaporated at 120 C to 0.5 g. This step was repeated twice. For reduction of Se 6+ to Se 4+ concentrated HCl was added. After dilution, selenium was determined by continuous HG-AFS. Separation and detection of Se species: For selenium species determination in supernatants, an ion exchange HPLC system coupled directly to an ICP-MS set-up was used. For Se species separation, a Hamilton PRP-X 100 anion-exchange column and a Zorbax 300-SCX cation-exchange column were used. The flow rate was 0.5 ml/min, and the volume of the sample injected was 100 µl. The method

4 and operating conditions are described in details elsewhere [9]. Selenium species in supernatants were confirmed by the standard addition method. 3. Results and discussion Selenium concentrations determined in soft mussel tissue were variable between different locations from 3.15 to 8.27 µg/g dry matter (DM) (Table 1). The highest concentrations obtained were in mussels collected from Italy, where the amount of Se ranged between 7.7 and 8.8 µg/g, while in cooked mussel Se concentration was around half the amount; from 4.1 to 4.4 µg/g, so we could conclude that the rest of Se remained in liquid where mussels were cooked. Mussels bought from Slovenian Table 1: Se and its distribution in raw and cooked mussel from different locations Mussel tissue DM a Sample (n) Total Se µg g-1 Soluble Se µg g-1 Average solubility (%) Selenium species identified SeCys2 (µg Se/g) SeMet (µg Se/g) Se as TMSe + Mussel from Slovenian breeders (4) Mussel from Italy (4) Mussel FAO87 (5) raw 5.81 ± ± ± 0.03 (4.7) 0.09 ± 0.01 (1.9) 0.65 ± 0.08 (13.7) cooked 3.52 ± ± ± 0.09 (24.9) 0.27 ± 0.02 (9.8) 0.83 ± 0.04 (28.7) raw 8.27 ± ± ± 0.03 (7.1) 0.33 ± 0.03 (6.5) 0.86 ± 0.06 (16.6) cooked 4.24 ± ± ± 0.09 (13.1) 0.37 ± 0.03 (14.1) 0.48 ± 0.14 (17.8) raw 3.15 ± ± ± 0.04 (22.8) 0.30 ± 0.02 (14.9) 0.47 ± 0.01 (23.4) SRM 2976 (3) 1.74 ± ± ± 0.03 (14.8) 0.07 ± 0.01 (6.9) 0.13 ± 0.01 (15.1) (n) Number of samples analysed a Results are given as the average ± standard deviation on dry matter basis (DM) b (% of identified soluble Se) breeders contained from 5.5 to 6.1 µg/g DM of Se, whilst in cooked ones Se ranged between 3.3 to 3.6 µg/g DM. The lowest concentration obtained was in mussels from NE Pacific, where average concentration was around 3.2 µg/g DM. The accuracy of Se determination was checked by analysing the certified reference material SRM 2976 and good agreement was found between the total value obtained of 1.74 ± 0.07 µg/g and certified of 1.80 ± 0.15 µg/g, while there is no data about Se species. After enzymatic extraction the proportion of soluble Se was similar in all samples, around 63, 67 and 76 % for mussel from NE Pacific, Slovenia and Italy, respectively. As is it seen in the table cooking indeed have the

5 effect on Se concentration since the amount of Se was almost half lower in cooked mussels, nevertheless the percentage of Se solubility stayed the same. Liquid where mussels from Italia were cooked was also analysed; the amount of Se was around 3.6 µg /g DM. Although a lot of Se stayed in liquid where mussels are cooked, we need to take into account that by typical preparation of mussels liquid is also included in the meal. In order to determine whether losses of selenium occur during sample preparation (extraction and separation), a mass balance was drawn up between the total selenium in the sample and the amount of selenium in the supernatant and residue. The mass balance obtained for all mussel samples was around 100 %, showing that there was no loss of Se. After enzymatic extraction, chromatographic analysis of the supernatant was performed. Two selenium species were identified and confirmed, SeCys 2 and SeMet. In all chromatograms we obtained one peak, which could not be identified due to the lack of Se standards. We compared the peak with literature data, where authors identified TMSe +, and found that the retention time on cationic column (with the same conditions as ours) are the same, so we can conclude that the unknown Se species could be TMSe + [10]. References: [1] M. Plessi, D. Bertelli, A. Monzani. Mercury and selenium content in selected seafood. Journal of Food Composition and Analysis, 14: , 2001 [2] M. Angeles Quijano, P. Moreno, A. M. Gutierrez, M. Concepcion Perez-Conde, C. Camara. Selenium speciation in animal tissues after enzymatic digestion by high-performance liquid chromatography coupled to inductively coupled plasma mass spectrometry. Journal of Mass Spectrometry, 35: , 2000 [3] German Nutrition Society (DGE), Austrian Nutrition Society (ÖGE), Swiss Society for Nutrition Research (SGE), Swiss Nutrition Association (SVE). Reference Values for Nutrient Intake, 1 st ed. Druckerei V+V, Bonn, [4] C. Thiry, A. Ruttens, L. De Temmerman, Y. J. Scheinder, L. Pussemier. Current knowledge in species-related bioavailability of selenium in food. Food Chemistry, 130: , 2012 [5] L. Hinojosa Reyes, J. L. Guzman Mar, G. M. Mizanur Rahman, B. Seybert, T. Fahrenholtz, H.M. Skip Kingston. Simultaneous determination of arsenic and selenium species in fish tissues using microwave-assisted enzymatic extraction and ion chromatography-inductively coupled plasma mass spectrometry. Talanta, 78: , 2009 [6] A. Osterc, T. Kanduč, Z. Šlejkovec, V. Stibilj, A. Ramšak. Mytilus Galloprovincialis as an indicator of environmental pollution along NE coast of Adriatic. In Proceedings of the tenth international conference on the Mediterranean coastal environment, MEDCOAST 11, Rhodes, Greece, 2011 [7] P. Smrkolj, V. Stibilj. Determination of selenium in vegetables by hydride generation atomic fluorescence spectrometry. Analytica Chimica Acta, 512: 11-17, 2004 [8] D. Mazej, I. Falnoga, M. Veber, V. Stibilj. Determination of selenium species in plant leaves by HPLC-UV-HG- AFS. Talanta, 68: , 2006 [9] P. Cuderman, I. Kreft, M. Germ, M. Kovačecič, V. Stibilj. Selenium species in selenium-enriched and drought exposed potatoes. Jornal of Agricultural and Food Chemistry, 56: , 2008 [10] P. Moreno, M. A. Quijano, A. M. Gutierrez, M. C. Perez-Conde, C. Camara. Fractionation studies of selenium compounds from oysters, and their determination by high-performance liquid chromatography coupled to inductively coupled plasma mass spectrometry. Journal of Analytical Atomic Spectrometry, 16: , 2001

6 For wider interest Selenium (Se) is a complex, essential trace element for animal and human. It has numerous important biological functions that depend on the activity of certain Secontaining proteins. It is essential for the body because it forms seleno-enzymes that carry out redox reactions such as glutathione peroxidase (GPx), thioredoxin reductase, and thyroid hormone deiodinase families. However, Se is also considered to be a toxic element at high concentrations. Function and bioavailability of this element are strongly correlated with its chemical form, so it is necessary to control the selenium intake to avoid deficiency diseases and toxicity problems. Therefore it is important to determine the selenium species in foods, especially in seafood, because of its known accumulation capacity. Our aim in this work was to investigate selenium and its species with different analytical techniques in edible mussel Mytilus galloprovincialis collected from different locations (Slovenian coastline, Italy, Udine and NE Pacific). Furthermore we wanted to see if cooking of mussel has any effect on selenium concentration and its distribution. In this experiment we followed typical cooking procedure which is mostly used in Slovenia. To determine total concentration of Se, hydride generation atomic fluorescence spectroscopy (HG-AFS) was used. Total Se concentrations in mussels differ between the locations where mussels were bred. The lowest concentrations obtained were in mussel that comes from NE Pacific, but here we need to take into account that mussels were already cleaned and removed from shell when we bought them from the supermarket, while the mussels from Slovenia and Italy were cleaned in our laboratory. Selenium speciation was performed by a liquid chromatography as separation system and coupled to mass spectrometer as detector system (HPLC- ICP-MS). Two selenium species were determined, while future work will involve further species identification.