Activity loss and histidine modification in guanine deaminase.

Photooxidation of guanine deaminase (E.C.3.5.4.3.) in the presence of rose bengal as sensitizer resulted in decay in enzyme activity. The pH profile of inhibition suggests modification of histidyl residue(s). Amino acid analysis during photooxidation showed a decrease in unmodified hystidine. Diethylpyrocarbonate inhibits the enzyme activity whilst 5-aminoimidazole-4-carboxamide, a competitive inhibitor of guanine deaminase, partially protects against Et2PC action. Hydroxylamine partially reverses inhibition. These results are consistent with the presence of the imidazole moiety of the putative histidyl residue(s) at or near the active site of guanine deaminase.