Photo-oxidation of Jack bean urease in the presence of methylene blue.

Photo-oxidation of Jack bean urease was performed in the presence of a low concentration of methylene blue, which led to the complete loss of the enzymatic activity. The inactivation was more remarkable in an alkaline region than in an acidic region and prevented by the addition of histidine or methionine. Amino acid analysis of the oxidized enzyme revealed that the number of histidine residues had decreased to 73% that of the native enzyme, but the numbers of other amino acid residues were not significantly affected. Benzohydroxamic acid, a specific urease inhibitor, protected the active site of the enzyme against photo-oxidation. On the other hand, oxidation of the enzyme decreased its binding ability with caprylo- and benzohydroxamic acid to one-third. These results suggest that histidine residues are modified by photo-oxidation and are essential to both the enzymatic activity and the binding ability with hydroxamic acid.