The milk acid proteinase cathepsin D: a review

Cathepsin D is a lysosomal aspartic proteinase with a low pH optimum, found in many tissues and in bovine milk. It is synthesised as its inactive zymo gen, procathepsin D. Procathepsin D can convert itself, by an autoproteolyt ic pathway, into an active intermediate, pseudocathepsin D, while other pro teases are involved in the processing to mature cathepsin D. Cathepsin D is known to participate in a range of physiological processes. Pro-, pseudo- and mature cathepsin D have been purified from bovine milk, but procathepsi n D is the major form present in milk. Cathepsin D activity in milk appears correlated with somatic cell count. The active forms of the enzyme readily hydrolyse alpha (s1)-, alpha (s2)- and beta -caseins, in the case of alpha (s1)-casein and beta -casein with a specificity similar to that of chymosi n. Cathepsin D can also produce para-kappa -casein from kappa -casein and, at high concentrations, can coagulate milk. Cathepsin D appears to be able to at least partially survive commercial pasteurisation processes. In recen t years, increasing evidence has been documented indicating a role for this enzyme in proteolysis in cheese during ripening, most clearly in cheese wh ere rennet activity is low, such as Swiss cheese, Quarg and Feta. Further r esearch is required to evaluate the significance of this enzyme in more det ail. (C) 2001 Elsevier Science Ltd. All rights reserved.