Abstract
Ribonuclease T1 [EC 2. 7. 7. 26] was photooxidized with and without protection by the substrate analog, a mixture of guanosine 2'-phosphate and guanosine 3'-phosphate. In the absence of the substrate analog, the enzymatic activity gradually decreased with loss of two to three histidine residues, while in the presence of the substrate analog, full activity was maintained and two histidine residues were protected against photo-oxidation. The tryptophan residue was oxidized with simultaneous O2-uptake and the rate was scarcely affected by the presence of the substrate analog. These results demonstrate that two histidine residues are involved in the active centre of ribonuclease T1 but that tryptophan is not involved.
