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A hard nut cracked: Brazil nuts’ selenium compounds identified



Brazil nuts are a rich source of selenium, an essential trace element that is believed to offer protection against heart disease and to help prevent cancer particularly of the prostrate. It is also a powerful antioxidant – protecting cells from damage by free radicals. Brazil nuts are one of the best natural sources of selenium containing about 2500 times as much as any other nut so that one single Brazil nut may exceed the U.S. Recommended Daily Allowance of selenium.

The proteins found in Brazil nuts are very high in sulfur-containing amino acids like cysteine (8%) and methionine (18%) and are also extremely rich in glutamine, glutamic acid, and arginine. The reason that Brazil nut concentrates selenium is probably that this element is very, similar chemically to sulfur. Sulfur is frequently deficient in Amazonian soils, especially after decades or centuries of Brazil nut harvesting and export from the casthanais. If the soil contains significant amounts of selenium, this is used by the plant instead of sulfur.

In order to investigate questions of bioavailability and health effects of the selenium from brazil nuts, the selenium speciation, that means the selenium compounds being present in the nut should be known. While the selenium content in Brazil nuts is much higher than in any other fruit or vegetable the expected concentration of Se-containing peptides is ca. two orders of magnitude lower than that of sulfur-containing ones, making their identification a difficult task.

The new study:
The approach developed by the researchers from the University of Pau is based on the selection of a narrow retention-time range for data mining by detection of selenium with a Se-specific, sensitive and matrix independent technique (ICP MS). The advantage of ICP MS is that it provides an unambiguous indication of Se-containing peptides located at given retention times and thus narrows the search range, ideally down to a few mass spectra and the relevant MS/MS acquisitions.  The relevant fractions could then be analysed by MS/MS and the peptides be identified by there characteristic fragmentation patterns.

Sample preparation and clean-up was based on breaking down the Brazil nut proteins using the digestive enzyme trypsin followed by two stages of size exclusion chromatography coupled with ICP (inductively coupled plasma) mass spectrometry (MS) to purify and concentrate the selenium-containing compounds.

According to the researchers opinion, this is the first successful selenoproteomic attempt in a natural food sample at the total level of Se below 100 mg kg–1.

The original study:

M. Dernovics, Pierre Giusti, Ryszard Lobinski, ICP-MS-assisted nanoHPLC-electrospray Q/time-of-flight MS/MS selenopeptide mapping in Brazil nuts, J. Anal. At. Spectrom., 2006. DOI: 10.1039/b608041c

Related studies:

C.L. Secor, D.J. Lisk, Variation in the selenium content of individual Brazil nuts, J. Food Saf., 9/4 (1990) 279-281.

J.C. Chang, W.H. Gutenmann, C.M. Reid, D.J. Lisk, Selenium content of Brazil nuts from two geographic locations in Brazil, Chemosphere, 30 (1995) 801-802.  doi:10.1016/0045-6535(94)00409-N

Sasi S. Kannamkumarath, Kazimierz Wrobel, Katarzyna Wrobel, Anne Vonderheide, Joseph A. Caruso, HPLC-ICP-MS determination of selenium distribution and speciation in different types of nut, Anal. Bioanal. Chem., 373/6 (2002) 454-460. DOI: 10.1007/s00216-002-1354-3

Anne P. Vonderheide, Kazimierz Wrobel, Sasi S. Kannamkumarath, Clayton B'Hymer, María Montes-Bayón, Claudia Ponce de León, and Joseph A. Caruso, Characterization of Selenium Species in Brazil Nuts by HPLC-ICP-MS and ES-MS, J. Agric. Food Chem.,50/20 (2002) 5722 - 5728. DOI: 10.1021/jf0256541

Katarzyna Wróbel, Sasi S. Kannamkumarath, Kazimierz Wróbel, Joseph A. Caruso, Hydrolysis of proteins with methanesulfonic acid for improved HPLC-ICP-MS determination of seleno-methionine in yeast and nuts, Anal. Bioanal. Chem., 375/1 (2003) 133-138.

Thavarith Chunhieng, Konstantinos Pétritis, Claire Elfakir, José Brochier, Thierry Goli, Didier Montet, Study of Selenium Distribution in the Protein Fractions of the Brazil Nut, Bertholletia excelsa, J. Agric. Food Chem., 52/13 (2004) 4318 - 4322. DOI: 10.1021/jf049643e

Sasi S. Kannamkumarath, Rodolfo G. Wuilloud, Joseph A. Caruso, Studies of various elements of nutritional and toxicological interest associated with different molecular weight fractions in Brazil nuts, J. Agri. Food Chem., 52/19 (2004) 5773-5780. doi: 10.1021/jf0496649

Sasi S. Kannamkumarath, K. Wrobel, Rodolfo G. Wuilloud, Studying the distribution pattern of selenium in nut proteins with information obtained from SEC-UV-ICP-MS and CE-ICP-MS, Talanta, 66/1 (2005) 153-159. doi:10.1016/j.talanta.2004.10.010
see corrigendum: ibid., 67/1 (2005) 259

Heidi Goenaga-Infante, Ruth Hearn, Tim Catterick, Current mass spectrometry strategies for selenium speciation in dietary sources of high-selenium, Anal. Bioanal. Chem., 382/4 (2005) 957-967. DOI: 10.1007/s00216-005-3177-5

Emmie Dumont, Liesbet De Pauw, Frank Vanhaecke, Rita Cornelis, Speciation of Se in Bertholletia excelsa (Brazil nut): A hard nut to crack?, Food Chem., 95/4 (2006) 684-692. doi:10.1016/j.foodchem.2005.04.004

Emmie Dumont, Frank Vanhaecke, Rita Cornelis, Selenium speciation from food source to metabolites: a critical review, Anal. Bioanal. Chem., 385/7 (2006) 1304. DOI: 10.1007/s00216-006-0529-8

Related information:

FAO: Information about Brazil nuts
Wikipedia: Brazil nuts
Fitness & Nutrition: Selenium
Amazon Conservation association: Brazil Nuts

Related News:

EVISA News, October 6, 2005: Selenomethionine shows promising results as a protective agent against Esophageal Cancer
EVISA News, January 25, 2006: A new concentric nebulizer for ICP sample introduction at nL-sample flow rates

last time modified: October 25, 2006

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