抄録
The kinetic parameters, Km and log Vmax, of the cleavage of dinucleoside phosphates, GpU, GpC and ApU, by RNase Rh from a Rhizopus sp. were measured at various pH's. Analysis of the pH profiles of Km and Vmax of these dinucleoside phosphates according to Dixon's theory suggested that two functional groups having pKa values of 7.0-7.3 and 3.25-3.5 are present in the active site of RNase Rh. The former value may correspond to a histidine residue. The pH dependence of the rates of inactivation of RNase Rh by photoxidation and carbethoxylation also indicated that a functional group having pKa about 7.0 was involved in the active site. Amino acid analysis of photooxidized RNase Rh showed that only a histidine residue had been destroyed. In the carbethoxylation of RNase Rh, the formation of carbethoxyhistidine caused a corresponding loss of activity of RNase Rh without modification of tyrosyl residues. It was concluded that a histidine residue having pKa about 7.0 is involved in the active site of RNase Rh.
