Plasmin Activity in Milk products

Plasmin (fibrinolysin; EC 3.4.21.7) is one of the most important indigenous milk proteinases that can have a great effect on the quality of milk. It is a heat-stable alkaline serine proteinase and it is optimally active at about pH 7.5 and 37°C. Milk contains the complete plasmin system: plasmin, plasminogen, plasminogen activators (PAs), which fall into two principal classes, urokinase-type (u-PA) and tissue-type (t-PA), and inhibitors of PAs and plasmin .Both tPA and uPA convert bovine PG to PL by hydrolyzing the Arg557–Ile558 bond.

The principal substrate for plasmin is B-CN, from which it produces Y₁- , Y₂- and Y₃-CNs and proteose peptone (PP) 5, PP8 slow and PP8 fast. Alfa s₂,-Casein in solution is also hydrolysed very rapidly by plasmin at bonds Lys-X and Arg-X. Plasmin contributes to proteolysis of Alfa s₁-casein, but it does not appear to be hydrolysed to a significant extent in milk. Although k-casein contains several Lys and Arg residues, it appears to be quite resistant to plasmin, presumably due to a relatively high level of secondary and tertiary structure.