Urea and guanidine actions on chicken LDH

Abstract

Urea and guanidine tested at low concentrations inhibit the hepatic lactic dchydro- genase in chickens (homogenated) where it exists in the form of a single iso-enzyme (Ht). The inhibitive action increases in proportion to the time that the effector is in prior contact with the enzyme, with the former’s concentraron and with the temperature.

Inhibition of the LDH activity in the interval of temperatures between 5° C and 30° C is reversible up to concentrations of urea of 1.2 M and of guanidine of 0.72 M But if the temperature is 55° C, it proves to be irreversible or partially reversible for all concentration tested.

The inhibition of both cffcctors is competitive with respect to the pyruvate and non- competitive with respect to the other substrates. NADH proteets the inhibition induced by 2 M and 3 M urea, whilst the pyruvate and the L-lactate, do not protect. The contact between LDH and NAD, in the absence of lactate, gives rise to the formation of a stable inactive complex, where, after 24 hous of diálisis, no elimination of the co-factor isobserved.

The anti-rhcumatic pharmacons, salicilic acid and phenilbutazone are reversible inhibitors of LDH and acts by competing with the co-factors NAD or NADH, the inhibition not being of a competitive nature with respect to the pyruvate and the L-lactate.