Xanthine

Xanthine is the downstream oxidative catabolite of hypoxanthine, produced by xanthine oxidase during purine nucleotide degradation, and itself a substrate for uric acid production. The conversion of hypoxanthine to xanthine via xanthine oxidase generates reactive oxygen species, directly linking purine catabolism to oxidative stress during exercise recovery. Salivary xanthine, used in the xanthine/hypoxanthine ratio, provides a measure of xanthine oxidase flux and free radical generation proportional to exercise intensity.