Proteinases in normal bovine milk and their action on caseins

Native proteolytic enzymes in good quality normal bovine milk readily hydrolysed the caseins during incubation or storage, producing the gamma-caseins, proteose-peptone components 5 (PP5) and 8-fast (PP8F) and a considerable number of other unidentified fragments, many of which were also subsequently found in the proteose-peptone fraction. The rate of casein hydrolysis was greater in pasteurized than in raw milk, with beta-casein being slightly more susceptible to attack than alpha S1-casein. Measurements of gamma-casein and proteose-peptone formation have been made and it was found that PP5 was an intermediate product that was subject to further proteolysis while PP8F was a stable end-product. With the exception of component 3 (PP3), virtually all constituents of the proteose-peptone fraction increased during storage and appeared to be products of the action of proteolytic enzymes. Further evidence was obtained from the effects of various inhibitors that the principal proteinase of normal milk is plasmin, but slight differences were apparent between the protein breakdown patterns induced by storage and by added plasmin, which was consistent with the presence of more than one proteinase. Incubations in the presence of soya bean trypsin inhibitor to prevent plasmin action clearly revealed that another enzyme(s) was also involved.