Partial purification and characterization of glycinin subunits from Thai soybean Glycine max. variety Sojo5

Abstract

Glycinin (11S globulins) from Thai varieties of soybean [Glycine max (L.) Merr] Sojo4, Sojo5, Chiang Mai60, and Chiang Mai2 were separated by chromatography on DEAE-Sephadex A-50. It has been found that Sojo5 cultivar glycinin contained 0.80% yield, the highest yield among all four. Partially purified glycinin from all four cultivars were analyzed by sodium dodecyl sulfate as well as isoelectric focusing polyacrylamide gel electrophoreses. The results demonstrated that the proteins contain two sizes of subunits with different isoelectric points, the acidic subunits (MW about 37,000-42,000) and the basic subunits (MW about 20,000). The intermediary subunits of glycinin from Sojo5 cultivar were partially purified by chromatography on DEAE-Sephadex A-50 in the presence of urea and characterized by sodium dodecyl sulfate polyacrylamide gel electrophoresis. It was found that the partially purified intermediary subunits (peak III from ion exchange chromatography) consist of one acidic (MW about 37,000-38,000) and one basic subunit (MW about 20,000-21,000) held together by disulfide bonding. When the partially purified intermediary subunits were dialyzed against the 3.5mM potassium phosphate buffer (pH 7.6) and centrifuged in sucrose density gradient, four protein peaks were observed. When the dialysis step was omitted, two major peaks with sizes corresponding to the acidic and basic subunits were obtained. Electron microscopy experiments confirmed the reported 3-dimensional structure of glycinin and that of the intermediary subunits. The results are consistent with the model of glycinin with two annular-hexagonal structures packed one on top of another, each composed of 6 subunits. Each pair of the intermediary subunits consists of one acidic and one basic subunits linked by disulfide bonding. This study emphasizes the characterization of intermediary subunits as well as confirms published results on the quarternary structure and the solubility behavior of glycinin. When the ionic strength was lowered from 0.5 to 0.2 or 0.03, the addition of salt to the solutions led to increased solubility of both partially purified glycinin and partially purified intermediate subunits.