The Modification of Tyrosyl Residues of a Minor Ribonuclease from Aspergillus saitoi

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The role of tyrosyl residues in the enzymatic activity and the state of tyrosyl residues of guanine preferential ribonuclease (RNase Ms) from Aspergillus saitoi were studied. 1) The solvent perturbation difference spectra of RNase Ms measured with ethylene glycol and polyethylene glycol as perturbants indicated that ca. 5 tyrosyl residues and one tryptophan residue were exposed to solvents. 2) The spectrophotometric titration of RNase Ms indicated that ca. 5 tyrosyl residues were titrated first, giving an apparent pK_a value of 10.5,and the rest of the tyrosine residues gave higher pK_a values. 3) About 8 tyrosine residues in RNase Ms were acetylated by excess N-acetylimidazole and about 6 of them were reactive towards a relatively low concentration of the reagent. Extrapolation of the curve showing the relation between the tyrosyl residues modified and the remaining activity indicated that the enzyme was inactivated when 4-5 tyrosyl residues were modified. 4) The chemical modification of RNase Ms with diazonium 1 (H)-tetrazole was also studied. The plot of the relationship between the residual activity and the tyrosyl residues modified showed that 1-1.5 tyrosyl residue (s) was responsible for the loss in enzymatic activity. 5) The enzymatic activities of RNase Ms modified by the two kinds of tyrosine-modifying reagents were measured with ribonucleic acid (RNA) and guanyl (3'→5') cytidine (GpC) as substrates at pH 5.0. The loss in enzymatic activity was more marked when RNA was used as a substrate than with GpC as a substrate. 6) Based on the reported (Heinemann and Saenger, Nature, 299,27 (1982)) three-dimensional structure of RNase T_1,which is about 60% sequence-homologous with RNase Ms, the possible site of the diazotization is discussed.
公益社団法人日本薬学会の論文
1985-10-25

The Modification of Tyrosyl Residues of a Minor Ribonuclease from Aspergillus saitoi

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