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 was estimated to be 0.4 microgram/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.
Larsen
L B
LB
Protein Chemistry Laboratory, University of Aarbus, Denmark.
Benfeldt
C
C
Rasmussen
L K
LK
Petersen
T E
TE
D000818:Animals; D002364:Caseins / Q000378:metabolism; D002402:Cathepsin D / Q000032:analysis / Q000378:metabolism*; D002417:Cattle; D012085:Chymosin / Q000378:metabolism; D004792:Enzyme Precursors / Q000032:analysis / Q000378:metabolism*; D004797:Enzyme-Linked Immunosorbent Assay; D007700:Kinetics; D007768:Lactalbumin / Q000378:metabolism; D007782:Lactoglobulins / Q000378:metabolism; D008892:Milk / Q000201:enzymology*; D008894:Milk Proteins / Q000032:analysis / Q000378:metabolism*; D000067816:Whey Proteins
D002364:Caseins; D004792:Enzyme Precursors; D007782:Lactoglobulins; D008894:Milk Proteins; D000067816:Whey Proteins; D007768:Lactalbumin; C058345:procathepsin D; D012085:Chymosin; D002402:Cathepsin D