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 zymogen, procathepsin D. Procathepsin D can convert itself, by an autoproteolytic pathway, into an active intermediate, pseudocathepsin D, while other proteases 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 procathepsin 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 chymosin. 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 recent years, increasing evidence has been documented indicating a role for this enzyme in proteolysis in cheese during ripening, most clearly in cheese where rennet activity is low, such as Swiss cheese, Quarg and Feta. Further research is required to evaluate the significance of this enzyme in more detail. (C) 2001 Elsevier Science Ltd. All rights reserved.