Susceptibility of beta-lactoglobulin and sodium caseinate to proteolysis by pepsin and trypsin.
The susceptibility of beta-LG and sodium caseinate to proteolysis by pepsin and trypsin was investigated using SDS or urea-PAGE. The effects were studied of heat, urea, and 2-mercaptoethanol on proteolysis. Native beta-LG was resistant to hydrolysis by pepsin or trypsin because of 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 presence of urea (3 to 6 M), and the effect was reversible after removal of urea by dialysis. Proteolysis by trypsin was also increased by the presence 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 precipitation in the isoelectric region); optimal pH was about 2. The activity of pepsin on sodium caseinate at pH 2 was not significantly affected by urea concentration up to about 8 M. The results indicated that the changes in conformation and structure of beta-LG that were induced by heating, reduction, or urea rendered the protein susceptible to peptic hydrolysis.
Guo
M R
MR
Food Chemistry, University College, Cork, Ireland.
Fox
P F
PF
Flynn
A
A
Kindstedt
P S
PS
D000818:Animals; D002364:Caseins / Q000378:metabolism*; D004591:Electrophoresis, Polyacrylamide Gel; D006863:Hydrogen-Ion Concentration; D007782:Lactoglobulins / Q000378:metabolism*; D008892:Milk / Q000737:chemistry*; D010434:Pepsin A / Q000378:metabolism*; D014357:Trypsin / Q000378:metabolism*
D002364:Caseins; D007782:Lactoglobulins; D014357:Trypsin; D010434:Pepsin A