SUSCEPTIBILITY OF BETA-LACTOGLOBULIN AND SODIUM CASEINATE TO PROTEOLYSIS BY PEPSIN AND TRYPSIN

The susceptibility of beta-LG and sodium caseinate to proteolysis by p epsin and trypsin was investigated using SDS or urea-PAGE. The effects were studied of heat, urea, and 2-mercaptoethanol on proteolysis. Nat ive beta-LG was resistant to hydrolysis by pepsin or trypsin because o f its compact globular structure. Heat treatment of beta-LG solutions at 90 to 100 degrees C for 5 or 10 min caused changes in the structure or conformation of the protein that rendered it accessible to pepsin and enhanced the extent of proteolysis by trypsin. The susceptibility of beta-LG to proteolysis by pepsin was markedly increased in the pres ence of urea (3 to 6 M), and the effect was reversible after removal o f urea by dialysis. Proteolysis by trypsin was also increased by the p resence of 2% 2-mercaptoethanol. Sodium caseinate was very accessible to pepsin without pretreatment and was extensively hydrolyzed at pH 1 to 5 in the presence of 5 M urea (which prevented the protein from pre cipitation in the isoelectric region); optimal pH was about 2. The act ivity of pepsin on sodium caseinate at pH 2 was not significantly affe cted by urea concentration up to about 8 M. The results indicated that the changes in conformation and structure of beta-LG that were induce d by heating, reduction, or urea rendered the protein susceptible to p eptic hydrolysis.