Unusual hypoxanthine hydroxylation system in hepatopancreas of Helix pomatia (Gastropoda).

The enzymatic system in hepatopancreas of H. pomatia (terrestrial purinotelic gastropod) hydroxylates hypoxanthine to xanthine and uric acid but fails to hydroxylate adenine, nicotinic acid and 3-methyl-6- hydroxypurine ; allopurinol is hydroxylated to oxypurinol 7 times faster than hypoxanthine to xanthine; at concentration of 10(-6) M it inhibits hydroxylation of hypoxanthine by 55%. Two protein fractions [precipitated at 0-0.30 (I) and 0.30-0.45 (II) saturation with (NH4)2 SO4] hydroxylate hypoxanthine with NAD+ as a cosubstrate but only fraction I, predominating during the active life, hydroxylates also xanthine and is inhibited by NADH. Protein fraction II, dominant during winter sleep, does not hydroxylate xanthine and its hypoxanthine-hydroxylating activity is not inhibited by NADH. The latter property may enable continuous operation of the protein catabolic pathway under anaerobiosis.