Molybdenum-Containing Enzymes

Abstract

Purine bases such as adenine and guanine, arising from the degradation of nucleic acids are conveyed to the liver in the blood. Deamination produces hypoxanthine and xanthine, respectively, and these are ultimately oxidised to uric acid Xanthine oxidase is the enzyme that catalyses such oxidations. Uric acid levels in the blood tend to rise when cells are being destroyed and nucleoprotein liberated, for example in leukaemia and in some stages of pneumonia. Xanthine oxidase also catalyses the oxidation of aldehydes to carboxylic acids; as with purines, these oxidations are formally hydroxylations in which the hydroxyl function derives from the solvent. Although xanthine oxidase is widely distributed in mammals, milk and calf liver being particularly good sources, its biological significance is not clear. Since aldehydes do not appear in appreciable quantities in mammals and since the oxidation of purines by molecular oxygen is among the fastest reactions catalysed by xanthine oxidase, it is assumed that the catalysis of purine and aldehyde oxidations are the principal roles of the enzyme. However, the absence of xanthine-oxidase activity does not appear to have severe pathological consequences, at least in humans. A patient apparently totally lacking xanthine-oxidase activity suffered no more than a xanthine stone in the urine²⁹.