Indigenous Proteinases in Milk

Abstract

The presence of indigenous proteolytic activity in milk has been recognized since the work of Babcock and Russel in 1897. Some early researchers attributed this activity to bacterial enzymes, but later work proved conclusively the presence of indigenous proteinases in milk. More recent research has indicated two major categories of indigenous proteolytic enzymes in milk, both originating from the animal’s blood. The major enzyme system contains plasmin, which is produced by activation of its inactive precursor, plasminogen, an event which is under the control of a complex system of inhibitors and activators. The presence of plasmin in milk and its significance to the quality of dairy products has been recognized for many decades (see Bastain and Brown, 1996, for review) and has thus been the subject of much research. However, the other indigenous proteolytic enzymes in milk, which originate from somatic cells, have been studied in detail only during the last decade. Somatic cells, the principal physiological function of which is the defence of the udder against bacterial infection, have lysosomes which contain active proteolytic enzymes, including elastase, collagenase and cathepsins B, D, G, H and L. The acid proteinase originating from somatic cells, cathepsin D, has been studied most thoroughly in milk. However, it is highly likely that the other proteinases known to be present in lysosomes are also present in milk, as has been demonstrated by the recent identification of immunoreactive cathepsin B in milk (Magboul et al., 2001) and observations of the activity of other indigenous thiol proteinases in milk. This chapter will review recent research on plasmin and its role in the quality of dairy products and will focus also on other indigenous proteinases, which have not been reviewed thoroughly elsewhere.