The kinetic theory of substrate reaction during the modification of enzyme activity [Duggleby (1986) J. Theor. Biol. 123, 67–80; Wang and Tsou (1990) J. Theor. Biol. 142, 531–549] has been applied to a study of the inactivation kinetics of ribonuclease A by bromopyruvic acid. The results show that irreversible inhibition belongs to a non-competitive complexing type inhibition. On the basis of the kinetic equation of substrate reaction in the presence of the inhibitor, all microscopic kinetic constants for the free enzyme, the enzyme–substrate complex and the enzyme–product complex have been determined. The non-competitive inhibition type indicates that neither the substrate nor the product affects the binding of bromopyruvic acid to the enzyme and that the ionization state of His-119 may be the same in both the enzyme–substrate and the enzyme–product complexes.

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