Fig. (2).

Hydroxylation Reaction of Purines. A; Hydroxylation order in purine catabolism. Accumulation of xanthine was observed, indicating that the reaction does not occur sequentially. The absence of formation of 6, 8-dihydroxypurine suggests that hydroxylation at the 2-position is important for hydroxylation at the 8-position. B; Concentrations of hypoxanthine, xanthine and urate were measured with HPLC during turnover from hypoxanthine to urate catalyzed by bovine milk xanthine oxidase. (T. Kusano, unpublished results).