Mljekarstvo (Jun 2002)
Plasmin: indigenous milk proteinase
Abstract
The most important characteristic of plasmin, as significant indigenous milk proteinase, its concentration, concentration measuring procedure and activity of plasmin are described. The most important factors, which have an influence on concentration and plasmin activity in milk, are stage of lactation and mastitis (high somatic cell count – SCC). In high SCC milk indigenous proteinase activity increased, especially in plasmin and plasminogen system.Specific hydrolytic activity of plasmin during primary proteolysis of some casein fractions is described. ß-CN is most susceptible fraction, but αs1-CN and αs2-Cn are less susceptible to degradation by plasmin. Almost all fractions of κ-CN are resistant to degradation by plasmin. Activation of plasminogen to plasmin is very complex biochemical process influenced by activators and inhibitors in milk, and can be increased in high SCC milk. There are many various types of inhibitors in milk serum and ßlactoglobulin is the most important after its thermal denaturation. Addition of aprotinin and soybean tripsin inhibitors in milk inhibits plasmin activity. Most important characteristic of plasmin is its thermostability onpasteurisation and even sterilisation. Mechanism of thermal inactivation of plasmin with developing covalent disulphide interaction between molecule of plasmin and serum proteins (mostly ß-laktoglobulin) is described. Thermosensitive inhibitors of plasminogen activators and inhibitors of plasmin are inactivated by short pasteurisation and therefore increase plasmin activity,while higher temperature and longer treatment time inactivate plasmin activity.
