BOVINE-MILK PROCATHEPSIN-D AND CATHEPSIN-D - COAGULATION AND MILK PROTEIN-DEGRADATION

Cathepsin D is an indigenous aspartic proteinase in bovine milk. By co mpetitive enzyme-linked immunosorbent assay the amount of immunoreacti ve cathepsin D and procathepsin D in bovine skim milk mas estimated to be 0.4 mu g/ml. Immunoreactive cathepsin D purified from whey consist ed of a small fraction of mature cathepsin D, but the major form was t he proenzyme procathepsin D. A preparation of bovine milk procathepsin D was, like mature cathepsin D, able to degrade purified alpha(s1)- a lpha(s2)-, beta- and kappa-casein and alpha-lactalbumin, while beta-la ctoglobulin was resistant to cleavage. The cleavage sites in these pro teins were determined and compared with those of chymosin. Cathepsin D was capable of generating the alpha(s1)-I, beta-I, beta-II and beta-I II fragments originally described from the action of chymosin on the r espective caseins: and these fragments were subjected to further prote olysis. Cathepsin D was also able to liberate the caseinomacropeptide from purified kappa-casein; and to coagulate bovine skim milk. This de monstrated that milk contains an indigenous coagulation enzyme present mainly in the whey fraction.