A study of the alkaline mesentericopeptidase active site by means of peptide chloromethyl ketones

Abstract The kinetics of inactivation of alkaline mesentericopeptidase was studied using chloromethyl ketone derivatives of amino acids and peptides. It was shown that Tos-LysCH 2 Cl and Tos-PheCH 2 Cl did not influence the enzyme activity, while the inhibitory effect of Cbz-Ala-Gly-PheCH 2 Cl was 35 times that of Cbz-Ala-PheCH 2 Cl. The dependence of the pseudo-first order rate constant of the enzyme inactivation by Cbz-Ala-Gly-PheCH 2 Cl on pH and temperature indicated that a group with a p K of 6.59 and ΔH i of 7.7 kcal/mol was the site of the inhibigor's attack. Amino acid analysis of the modified totally inactive enzyme revealed a definite loss of histidine and after performic acid oxidation a recovery of 3-carboxymethyl histidine. The whole set of experimental data is convincing evidence on behalf of a selective alkylation of the N -3 of the active site histidine after treatment with Cbz-Ala-PheCH 2 Cl and Cbz-Ala-Gly-PheCH 2 Cl. Alkaline mesentericopeptidase possesses an extended active site and only a peptide chloromethyl ketone, covering a determined sequence of the enzyme molecule (S 3 , S 2 , S′ 1 , S 1 , S′ 2 , S′ 3 …) is able to provide effective inhibition. The values of the inactivation constant ( k inact ) for Cbz-Ala-PheCH 2 Cl and Cbz-Ala-Gly-PheCH 2 Cl are close to the corresponding values reported for subtilisin Amylosacchariticus .