Plasmin-Mediated Proteolysis of Casein in Bovine Milk

• Published in: Proceedings of the National Academy of Sciences - PNAS 1979-05, Vol.76 (5), p.2244-2248
• Main Authors: Eigel, W. N., Hofmann, C. J., Chibber, B. A. K., Tomich, J. M., Keenan, T. W., Mertz, E. T.
• Format: Article
• Language: English
• Subjects: Animals; Biochemistry; Caseins - metabolism; Cattle; Electrophoresis; Female; Fibrinolysin - antagonists & inhibitors; Fibrinolysin - metabolism; Gels; Hydrolysis; Milk - enzymology; Molecular Weight; Plasminogen - metabolism; Plows; Protease Inhibitors - pharmacology; Rabbits; Skim milk; Sodium; Sulfates; Ungulates
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.76.5.2244

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 intact125I-labeled plasminogen from rabbit milk after its intravenous injection. Freshly isolated whole bovine casein was observed to undergo slow autoproteolysis at 37 degrees 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 ε -aminocaproic acid (0.1 mM) and diisopropyl fluorophosphate (1 mM); catalytic amounts of urokinase accelerated the process. Autoproteolysis of isolated bovine β -casein was shown by both urea and sodium dodecyl sulfate gel elee-trophoresis to result in formation of γ1- and γ2-caseins. Similar electrophoretic bands were formed when β -casein was degraded by plasmin prepared from bovine blood serum. These results support the hypothesis that bovine plasmin occurs in milk and is identical to alkaline milk protease.