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A method for determining chymosin and bovine pepsin A in commercial extracts of bovine veils, based on the use of the synthetic hexapeptide (LeuSer-Phe(NO 2 )-Nle-Ala-Leu-OMe) as reference substrate, is reported. Chymosin and bovine pepsin A were separated chromatographically from extracts and assayed for clotting activity on a reconstituted skim-milk standardized with reference chymosin and bovine pepsin A, themselves standardized in relation to the hexapeptide. The effect of pH on the absorbance difference between the hexapeptide and the LeuSer-Phe(NO 2 ) tripeptide resulting from its hydrolysis was studied. It was found that the ' optimal' pH for determining the activities of the reference enzyme solutions was 4-7.Six chymosin and 3 bovine pepsin A preparations were assayed on the hexapeptide to define the relation between the proteolytic activity and the amount of active enzyme. At pH 4-7 and 30 °C 1 mg chymosin and 1 mg bovine pepsin A hydrolysed 100 and 2700 /ai-peptide/s respectively. The clotting activity of these preparations was assayed on a reconstituted skim-milk to standardize it and thus define the relation between the clotting time and the amount of active enzyme. Chymosin had a specific clotting activity twice that of bovine pepsin A. At equal clotting activities, bovine pepsin A was 55 times more active than chymosin on the hexapeptide at pH 4-7.

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