Plasmin-mediated proteolysis of casein in bovine milk.

Abstract

Plasminogen was found to be present in bovine milk by crossreactivity between rabbit antiserum to plasminogen and casein prepared from milk by acid precipitation. This result was further supported by recovery of intact 125I-labeled plasminogen from rabbit milk after its i.v. injection. Freshly isolated whole bovine casein was observed to undergo slow autoproteolysis at 37.degree. C. Polyacrylamide gel electrophoresis revealed gradual disappearance of major caseins accompanied by appearance and increase in intensity of numerous electrophoretic bands. This autoproteolysis was inhibited by low concentrations of .epsilon.-aminocaproic acid (0.1 mM) and DFP (1 mM); Catalytic amounts of urokinase accelerated the process. Autoproteolysis of isolated bovine .beta.-casein was shown by urea and sodium dodecyl sulfate gel electrophoresis to result in formation of .gamma.1-and .gamma.2-caseins. Similar electrophoretic bands were formed when .beta.-casein was degraded by plasmin prepared from bovine blood serum. Bovine plasmin apparently occurs in milk and is identical to alkaline milk protease.