Plasmin in milk and dairy products: an update

Abstract

Abstract Plasmin is secreted as plasminogen that is activated in blood and milk. Its role in blood is to proteolytically break down blood clots. The enzyme has affinity for lysine and arginine residues and preferentially cleaves Lys-X and Arg-X bonds. Bovine plasminogen is different from human plasminogen because it is not activated by streptokinase. Plasmin purified from bovine milk is identical to that from blood when compared kinetically, immunologically and by partial sequencing (15% of the total amino acids sequenced). Plasmin purified from milk has optimum activity at pH 7.5 and 37 °C. Methods for determining plasmin activity in milk have been developed. Synthetic chromogenic substrates are often used because they are sensitive and require little sample preparation. Plasmin is associated with casein micelles in milk and degrades β-, α s1 - and α s2 -caseins to γ-caseins, proteose-peptones and possibly λ-casein. Peptide bonds in β-, α s1 and α s2 -caseins that are sensitive to plasmin in buffered model systems have been identified. Some of these bonds are hydrolysed in milk and cheese but more work is required to link work in model systems to milk and dairy products. Enzyme activity increases in milk with stage of lactation, severity of mastitic infection and lactation number. Plasmin contributes to proteolysis during ripening of some cheese varieties depending on cooking temperature and pH during ripening. The enzyme is heat resistant and survives many UHT treatments, but its role in the gelation of UHT-treated milk is not fully understood.