Kinetic studies on hepatic zanthine dehydrogenase. II. Inhibition by various purines

Abstract

Adenine and purine behave as reversible inhibitors of chicken liver xanthine dehydro- genase. Purine inhibits competitively the Xanthine-Xanthine dehydrogenase-NAD system; ho- wever, when the NAD concentraron is low, the inhibition induced by adenine shows to be non-competitive. Operating with NAD as the variable substrate and with xanthine as the fixed one, adenine behaves as a non-competitive inhibitor of the dehydrogenase. When the enzyme is also inhibitid by one of its own substratos —xanthine or hypoxanthine—, purine and adenine appear to satisfy the general kinetics typical of competitive inhibitors upon a Sys­ tem already inhibited by substrate. Urea inhibits xanthine dehydrogenase in a less effective way, although the mechanism is apparently similar to the one exhibited by adenine. Adenine also inhibits competitively the Xanthine-Xanthine dehydrogenase-DCI system, and non-compe- titively when reduced NAD — instead of xanthine — is used as the electrón donor.

The preceding observations suggest that all purines must compete for the same active cen- ter of the dehydrogenase oxidized form. Adenine also seems to partially combine with the free enzyme reduced form that is produced during the catalytical redox reaction.

Salicylate inhibits competitively the Xanthine-Xanthine dehydrogenase-NAD or DCI Sys­ tems and uncompetitively the reduced NAD-Xanthine dehydrogenase-DCI system. This beha- viour of xanthine dehydrogenase induced by salicylate shows to be different from that of most of the hitherto studied dehydrogenases.