The Regulation of Rat Liver Xanthine Oxidase

Abstract

The aerobic oxidation of xanthine by rat liver supernatant was greatly stimulated by the addition of methylene blue or of NAD+: the latter was reduced during the reaction.Storage of the supernatant at -20 ° brought about an enhancement of the xanthine oxidation rate measured without addition of cofactors.A similar "activation" was caused by prior incubation at 37 ° of the unfractionated liver homogenate, or of the supernatant separated after sonic disruption of the homogenate.The same effect was obtained by treatment with solvents, or by prior incubation at 37 ° of the supernatant in the presence of proteolytic enzymes or under anaerobic conditions.The presence of xanthine accelerated the effect of proteolytic enzymes and of anaerobiosis.Only the changes caused by anaerobiosis could be reversed by incubating the supernatant in air before the assay.The reaction rate was apparently unaffected by these treatments if activity of the enzyme was measured in the presence of methylene blue or of NAD+.The latter, however, was not reduced during the oxidation of xanthine by "activated" supernatants stored at -20 ° if the reaction was run in the presence of oxygen.If the reaction was in anaerobiosis, uric acid and a corresponding amount of NADH were formed by fresh supernatant, and by supernatants activated at -20 ° or by prior incubation in anaerobiosis, but not by supernatant activated by trypsin.The hypothesis is formulated that most of the xanthine oxidase of rat liver supernatant is a dehydrogenase (Type D), and may be converted (activated) into an oxidase (Type O).