PLASMIN IN MILK AND DAIRY-PRODUCTS - AN UPDATE

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 preferentiall y cleaves Lys-X and Arg-X bonds. Bovine plasminogen is different from human plasminogen because it is not activated by streptokinase. Plasmi n purified from bovine milk is identical to that from blood when compa red 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 degrees C. Methods for determining plasmin a ctivity in milk have been developed. Synthetic chromogenic substrates are often used because they are sensitive and require little sample pr eparation. Plasmin is associated with casein micelles in milk and degr ades beta-, alpha(s1)- and alpha(s2)-caseins to gamma-caseins, proteos e-peptones and possibly lambda-casein. Peptide bonds in beta-, alpha(s 1-) and alpha(s2)-caseins that are sensitive to plasmin in buffered mo del systems have been identified. Some of these bonds are hydrolysed i n milk and cheese but more work is required to link work in model syst ems to milk and dairy products. Enzyme activity increases in milk with stage of lactation, severity of mastitic infection and lactation numb er. 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 ro le in the gelation of UHT-treated milk is not fully understood. (C) 19 96 Elsevier Science Limited