Enzymatic deamination of isoxanthopterin by the silkworm,Bombyx mori

Isoxanthopterin deaminase was discovered in the silkworm, Bombyx mori, and distinguished from a previously reported sepiapterin deaminase. This enzyme catalysed the irreversible hydroxylative deamination of isoxanthopterin. The pH and temperature optima for the deaminase were 6.6 and 37°C respectively. Only isoxanthopterin was deaminated of all pteridines, purines, and pyrimidines tested, while xanthopterin, guanine, KF and KCN inhibited the enzyme. The K_m of the deaminase towards isoxanthopterin was found to be 1.2 × 10⁻⁵ M.