Use of sodium dodecyl sulfate polyacrylamide gel electrophoresis to measure degradation of soluble soybean proteins by Prevotella ruminicola GA33 or mixed ruminal microbes in vitro.

Degradation of soluble soybean meal proteins by Prevotella ruminicola GA33 and mixed ruminal bacteria was studied using SDS-PAGE. This technique facilitated the quantification of peptides (molecular weight > or = kDa) released during the partial hydrolysis of soybean proteins. In P. ruminicola incubations, conglycinin alpha and alpha' subunits were degraded completely within 4 h, whereas the beta subunit of beta-conglycinin as well as the basic and acidic polypeptide components of glycinin were more resistant to degradation. Several peptides derived from digested soybean proteins appeared during the incubation. The N-terminal amino acid sequences of these peptides were similar to amino acid sequences in soybean proteins. The N-terminus of a 23-kDa peptide was identical to the N-terminus of the beta subunit of beta-conglycinin; the N-terminus of a 59-kDa peptide differed from amino acids 101 to 115 of the alpha subunit of beta-conglycinin by two of 15 amino acids; the N-terminus of a 64-kDa peptide differed from amino acids 67 to 76 of the alpha' subunit of beta-conglycinin by three of 10 amino acids. The transient accumulation of these peptides when soluble soybean proteins were incubated with mixed ruminal microorganisms suggests a role for P. ruminicola in the early stages of protein digestion in the rumen. Approximately 30% of the soluble soybean protein incubated with mixed ruminal microorganisms was not degraded after 9 h, and SDS-PAGE indicated that the basic subunit of glycinin and several peptides (molecular weights less than 23 kDa) make an appreciable contribution to metabolizable protein when soybean protein is fed to ruminants.