Identification of the chemical forms of selenium in soy protein

Rosemary Rodibaugh, Purdue University

Abstract

Soybeans (Glycine max. L. Merr., 'Century') were grown hydroponically and intrinsically radiolabeled with $\sp{75}$Se, an isotope of selenium (Se). The isotope was provided as $\sp{75}$Se-Na$\sb2$SeO$\sb3$ during the reproductive stage of growth until onset of senescence. Harvested seeds were processed into defatted soy meal. Soluble proteins were extracted in 20mM Tris-HCl buffer (pH 8.5) and fractionated into 11S, 7S, and 2S protein fractions by isoelectric precipitation. The 11S and 7S globulins, containing the glycinin and conglycinin storage proteins respectively, constitute the majority of extractable soy proteins. These storage proteins are the predominant proteins in soy protein isolate frequently used in foods for human consumption. Approximately 24% of the defatted meal was soluble protein and accounted for 64% of the radioactivity associated with the soybean meal. The 11S fraction contained approximately 31% of the extracted protein and 27% of the extracted radioactivity. The 7S fraction contained approximately 32% and 35% of the extractable protein and radioactivity, respectively. The 2S fraction, containing the sulfur (S)-rich trypsin inhibitors, accounted for 17% of the protein and 27% of the radioactivity extracted from the defatted soy meal. Purification of the storage proteins by gel filtration and affinity chromatography showed higher levels of radioactivity associated with glycinin than conglycinin. Purified 11S proteins contained 1.09 ng Se per mg protein while 7S proteins contained 0.36 ng Se per mg protein. Acid hydrolysates of carboxymethylated 11S protein were analyzed by ion-exchange chromatography on a Beckman Model 119Cl amino acid analyzer. Radioactivity eluted from the column in four main regions with approximately 70% of recovered radioactivity eluting in the position of selenomethionine. Radioactive fractions from the amino acid analyzer were analyzed by thin layer chromatography on cellulose TLC plates. Radioactivity was recovered in zones with R$\sb{\rm f}$ values identical to selenomethionine and selenocystine standards. These results indicate the majority of $\sp{75}$Se incorporated into soybeans is in the form of selenomethionine and mainly associated with the 11S storage proteins. Selenocystine is also present in the 11S protein in low concentration compared to selenomethionine.

Degree

Ph.D.

Advisors

Mason, Purdue University.

Subject Area

Food science

COinS