Inhibitory Effect of Zn²⁺ on Rabbit Muscle Pyruvate Kinase and Reactivation by Histidine

Abstract

Histidine was found to be an activator of rabbit muscle pyruvate kinase activity with a K_1/2 value of 0.6mM. Carnosine and anserine are also effective, but only at much higher concentrations. Hyperbolic kinetics with phosphoenolpyruvate of the enzyme were found in either the presence or absence of histidine.
Of a number of divalent cations tested, only Zn²⁺ was found to be an effective inhibitor of enzyme activity at low concentrations. The kinetic data suggested that Zn²⁺ acted as inhibitor as well as activator for the enzyme activity; a high affinity binding site was associated with K_i of approximately 4.8μM Zn²⁺ and a catalytic site was associated with K_m of approximately 80μM Zn²⁺. Zn²⁺, which is associated to a high affinity binding site of the enzyme, was removed by the addition of histidine with a K_1/2 of approximately 0.6mM. From these findings, histidine including anserine and carnosine in muscle may act as a chelating agent for the enzyme activity.