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Complementary action of dipeptidyl peptidase IV and aminopeptidase M in the digestion of β-casein

SUMMARY Purified bovine β-casein was digested in vitro with varying mixtures of purified proteinases and peptidases including trypsin, chymotrypsin, dipeptidyl peptidase IV (DP IV), aminopeptidase M and prolidase. In digestion mixtures without DP IV the yield of free amino acids was considerably lower than in the corresponding assays with this peptidase. Especially, the release of proline increases drastically from almost zero to the theoretical amount in the presence of DP IV. Quantitative results indicated that the specificities of the two microvillar peptidases (aminopeptidase M and DP IV) optimally complemented each other. This effect elucidates the hitherto obscure physiological role of intestinal DP IV. A similar effect may also apply to other caseins and nutritional proteins.