Characterization of xanthine oxidase from Cellulosimicrobium funkei possessing hypoxanthine‐metabolizing activity

Aims Purine-degrading enzymes are favourable as medications and diagnostic tools for hyperuricemia. This study aimed to characterize enzymes isolated from micro-organisms, which may be useful for developing a new prophylaxis for hyperuricemia. Methods and Results Cellulosimicrobium funkei A153 was found to be a good catalyst for hypoxanthine degradation and could oxidize hypoxanthine to xanthine and further to uric acid. The enzyme catalysing this oxidation was purified, and its partial amino acid sequences were examined. Based on this information and genome sequencing results, this xanthine dehydrogenase family protein was cloned and expressed in Rhodococcus erythropolis L88. The recombinant enzyme with a His-tag was characterized. The enzyme was a xanthine oxidase as it could utilize molecular oxygen as an electron acceptor. It was stable under 50°C and exhibited maximum activity at pH 7·0. The kcat, Km and kcat/Km values for xanthine were 1·4 s−1, 0·22 mmol l−1 and 6·4 s−1 mmol−1 l, respectively. Conclusions Xanthine oxidase is favourable for hyperuricemia medication because it oxidizes hypoxanthine, an easily adsorbed purine, to xanthine and further to uric acid, which are hardly adsorbed purines. Significance and Impact of the Study The enzyme is useful for decreasing serum uric acid levels via conversion of easily absorbed purines to hardly absorbed purines in the intestine. Enzymes from micro-organisms may be used as a novel prophylaxis for hyperuricemia.