Xanthine oxidase activity was demonstrated in rat brain preparations by a spectrofluorometric method using pterine as a substrate. The activity of the enzyme in brain cytosol was 10 ± 4.3 nmoles/g protein per h (mean ± S.D.) as compared with an activity of 46.0 ± 24.9 μmoles/g protein per h in liver cytosol. Column chromatography of brain cytosol on Sephadex G-150 dextran resulted in elution of two peaks of 280 nm absorption. The first peak contained xanthine oxidase activity with a 174-fold increase in total activity over that measurable in the cytosol applied to the column, and an increase in specific activity to 2620.4 nmoles/g protein per h. The second peak contained an inhibitor to the enzyme assay. Similar chromatography of liver cytosol resulted in only a 1.54-fold increase in the yield of xanthine oxidase activity, a slight increase in specific activity to 54.9 μmoles/g protein per h and no detection of an inhibitor. The inhibitor separated from brain xanthine oxidase was identified as hypoxanthine by paper chromatography and its characteristic ultraviolet spectra. The brain enzyme had temperature and pH optima, apparent molecular weight, and kinetic properties similar to those of the liver enzyme. The presence of xanthine oxidase activity in brain tissue could contribute to the regulation of purine metabolism in the brain.
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