A study on subunit groups of soybean protein extracts under SDS-PAGE

Protein extracts of 640 soybean cultivars and landraces, mainly from China and a few from the US, were analyzed for their components and subunits based on distribution patterns of bands with varying molecular weights (MW) under SDS-PAGE (sodium dodecyl sulfate-polyacrylamide gel electrophoresis). The number and molecular weight of the bands in SDS-PAGE varied among materials and showed a tendency of continuous distribution. Accordingly, the SDS-PAGE patterns of the soybean protein extracts were divided into two regions: the region of bands with MW < 44 KDa and that with MW >= 44 KDa. The first region containing mainly 11S proteins was divided into four parts, called subunit groups, i.e. 11S-1 (14.4-22 KDa), 11S-2 (22-26 KDa), 11S-3 (26-34 KDa) and 11S-4 (34-44 KDa). The second region containing mainly 7S protein was divided into six subunit groups, i.e. 7S-1 (44-49 KDa), 7S-2 (49-55 KDa), 7S-3 (55-67 KDa), 7S-4 (67-73 KDa), 7S-5 (73-82 KDa) and 7S-6 (82-91 KDa). The sum of relative contents of 11S-1-11S-4 was obtained as the relative content of 11S protein, those of 7S-1-7S-6 as that of 7S protein, and therefore, the 11S/7S ratio obtained. The proposed criteria were demonstrated to be simple, stable and feasible. Among the 640 tested materials, 39 lacked 11S-1 but none lacked the other 11S subunit groups, while deficiencies existed in all the six subunit groups of 7S, indicating a great potential for the genetic variation of protein components and subunits for breeding for the improvement of protein qualities.