Bovine milk procathepsin D and cathepsin D: Coagulation and milk protein degradation

Cathepsin D is an indigenous aspartic proteinase in bovine milk. By competitive enzyme-linked immunosorbent assay the amount of immunoreactive cathepsin D and procathepsin D in bovine skim milk mas estimated to be 0.4 mu g/ml. Immunoreactive cathepsin D purified from whey consisted of a small fraction of mature cathepsin D, but the major form was the proenzyme procathepsin D. A preparation of bovine milk procathepsin D was, like mature cathepsin D, able to degrade purified alpha(s1)- alpha(s2)-, beta- and kappa-casein and alpha-lactalbumin, while beta-lactoglobulin was resistant to cleavage. The cleavage sites in these proteins were determined and compared with those of chymosin. Cathepsin D was capable of generating the alpha(s1)-I, beta-I, beta-II and beta-III fragments originally described from the action of chymosin on the respective caseins: and these fragments were subjected to further proteolysis. Cathepsin D was also able to liberate the caseinomacropeptide from purified kappa-casein; and to coagulate bovine skim milk. This demonstrated that milk contains an indigenous coagulation enzyme present mainly in the whey fraction.