SOME PROPERTIES OF A SELENIUM-INCORPORATING SULFUR-RICH PROTEIN IN SOYBEANS (GLYCINE-MAX L)

Sulfur-rich protein (SRP) incorporated selenium when soybeans were hydroponically grown and intrinsically radioactively labeled with Se-75. SRP polypeptides with estimated molecular weights 17 000-18 000 contained twice the radioactivity compared to polypeptides with estimated molecular weights of 28 000--29 000, accounted for more (Se) than half of the radioactivity in the SRP, and had the highest in vitro resistance to trypsin hydrolysis. The amount of selenium in SRP was 14.86 mumol/mol of SRP. The chemical form of Se-75 in SRP was selenomethionine. Nonradioactive SRP had an absorption maximum at 280 nm, A280nm1% = 7.65 +/- 0.95 (1 M NaCl), no free sulfhydryl group, no carbohydrates, a Stokes radius of 41.64 +/- 0.96 angstrom, and a fluorescence maximum at 332-334 nm. Among the proteases tested, papain was most efficient in hydrolyzing SRP in vitro. Disulfide bonds conferred stability to SRP against proteolysis by trypsin and chymotrypsin in vitro.