Sensitive fluorometric assay for the activity of chymosin

Fluorogenic substrates for chymosin [Dns-Leu-Ser-Phe-Trp-Ala-Leu-OCH2Py (I), Dns-Leu-Ser-Phe-Met-Trp-Leu-OCH2Py (II), Dns-Leu-Ser-Leu-Trp-Ala-Leu-OCH2Py (III), Dns-Leu-Ala-Phe-Trp-Ala-Leu-OCH2Py (IV), Dns-Leu-Ser-Phe-Leu-Ala-Leu-OCH2Py (V) and Dns-Leu-Ser-Phe-Phe-Ala-Leu-OCH2Py (VI)] were synthesized by a solution method. The obtained substrates I-VI were cleaved specifically (between the Phe and Trp residues for substrates I and IV, the Phe and Met residues for substrate II, the Leu and Trp residues for substrate III, the Phe and Leu residues for substrate V, and the Phe and Phe residues for substrate VI) by chymosin. The fluorescence of substrates I-IV (345 nm) increased with their hydrolysis, and hydrolysis rates were obtained by measuring the increase in fluorescence. The minimum detectable chymosin concentrations for substrates I and IV were about 1 nM; those for substrates II and III were about 4 and 2 nM. This assay method is very sensitive, and it is possible to determine the chymosin activity rapidly and easily. Substrates I and IV-VI were hydrolyzed by chymosin two times faster than substrates II and III. The effect of the amino-acid residues of the substrates on the hydrolysis rate is discussed.

Quantification of chymosin action on nonlabeled kappa-casein-related peptide substrates by ultraviolet spectrophotometry: description of kinetics by the analysis of progress curves

A method is described for quantifying the proteolytic action of the milk-clotting enzyme chymosin on small and medium-sized peptide substrates by monitoring the decrease of absorbance at 230 nm during cleavage. The method is illustrated by the determination of the kinetic parameters of the specific splitting of a kappa-casein-related hexa- and pentadecapeptide by chymosin. The results are in good agreement with those found earlier with the same enzyme/substrate system by using an automated ninhydrin method. Erroneous results were obtained when the kinetic data were derived from one single progress curve. The significance of initial rate measurements for calculating correct kinetic parameters is briefly discussed. The usefulness of single progress curves measured at different initial substrate concentrations for obtaining information about the mechanism of the enzymic reaction is demonstrated.

Kinetic study of the action of bovine chymosin and pepsin A on bovine kappa-casein

A study was carried out to determine the Michaelian parameters relative to the action of chymosin and pepsin A on bond Phe105-Met106 of bovine kappa0-casein (carbohydrate-free fraction in micellar state). The reaction was performed in citrate buffer, pH 6.2, at 30 degrees C. The reaction mixture was analysed by reverse phase HPLC. Dosages of peptide 106-169 (caseino macropeptide) at different reaction times from recordings of its absorbance at 220 nm gave the initial rates of reaction at each substrate concentration. From these values the following parameters were determined: kcat = 68.5 s-1, Km = 0.048 mM, kcat/Km = 1,413 mM-1 s-1 for chymosin, and kcat = 45 s-1, Km = 0.018 mM, kcat/Km = 2,439 mM-1 s-1 for pepsin A. For chymosin they are similar to those obtained previously in dimethyl glutarate buffer, pH 6.6, at 30 degrees C, using fragment 98-111 of kappa-casein as substrate. It can thus be concluded that neither the micellar state nor the presence of the whole peptide chain of kappa-casein (our conditions) significantly affect the action of chymosin on fragment 98-111, which seems to contain all information that makes bond 105-106 highly sensitive to chymosin. For pepsin A, only the information contained in fragment 103-108 appears to be required.

Hydrolysis of the synthetic chromophoric hexapeptide Leu-Ser-Phe(NO2)-Nle-Ala-Leu-OMe catalyzed by bovine pepsin A. Dependence on pH and effect of enzyme phosphorylation level

Steady-state kinetic parameters for the hydrolysis of the chromophoric hexapeptide Leu-Ser-Phe(NO2)-Nle-Ala-Leu-OMe catalyzed by bovine gastricsin and pepsin A were determined. It was shown that the phosphate content of bovine pepsin A is without any significant effect on that parameters. At pH 4.7, the specificity constant (kcat/Km) was 2455 and 2150 mM-1 X s-1 for the most phosphorylated bovine pepsin A (2.58 phosphate groups per molecule), before and after treatment by potato acid phosphatase, respectively. The kcat/Km ratio found for bovine gastricsin (1314 mM-1 X s-1) was closer to that of bovine pepsin A than that previously reported for chymosin (25 mM-1 X s-1). The spectral properties of the chromophoric tripeptide Leu-Ser-Phe(NO2) in the pH range 1-3.6 were investigated. We have shown that the hexapeptide hydrolysis could be followed by difference spectrophotometry at 295 nm (delta epsilon = -235 M-1 X cm-1 at pH 1.0) thus allowing to study the effect of pH on bovine pepsin A activity in a pH range which could not be explored earlier. The pH-dependence of kcat/Km ratio of unphosphorylated bovine pepsin A indicated that enzyme activity was dependent upon the ionization of two groups of the enzyme whose pK are 1.2 and 5.0. These pK values strongly suggest the involvement of two carboxyl groups probably corresponding to the two reactive aspartyl residues (Asp32 and Asp215) identified through active site-directed reagents for all the aspartic proteinases so far tested.