ExtractA simple and rapid method for determination of the hypoxanthine and xanthine concentration in plasma and urine is described. The method is based on the principle that oxygen is consumed quantitatively when hypoxanthine and xanthine are oxidized to urate by xanthine oxidase. By using Henry's law a direct measure of the hypoxanthine and xanthine concentration is obtained.The method determines these oxypurines in volumes of 200 μl in concentrations less than 5 μmol/liter in about 5 min. The average precision in the range of 0–50 μmol/liter is 2.6 μmol/liter. Of the added hypoxanthine, 99 102% is recovered in plasma. Even though xanthine oxidase is a rather nonspecific enzyme, experiments show that this method is highly specific during physiologic conditions.SpeculationOther purines which can be metabolized to hypoxanthine can be determined by this method. For instance, inosine, which is metabolized to hypoxanthine by nucleoside phosphorylase in the presence of phosphate, might also be determined according to this method.