The Structure and Function of Ribonuclease T₁

XXI. Modification of Histidine Residues in Ribonuclease T₁ with Iodoacetamide

Abstract

1. When ribonuclease T, [EC 3.1.4.8] (0.125% solution) was treated with a 760-fold molar excess of iodoacetamide at pH 8.0 and 37°, about 90% of the original activity was lost in 24hr. The half-life of the activity was about 8hr. The binding ability for 3'-GMP was lost simultaneously. Changes were detected only in histidine and the amino-terminal alanine residues upon amino acid analyses of the inactivated protein and its chymotryptic peptides. The inactivation occurred almost in parallel with the loss of two histidine residues in the enzyme.
The pH dependences of the rate of inactivation and that of loss of histidine residues were similar and indicated the implication of a histidine residue or residues with pKa 7.5 to 8 in this reaction. 3'-GMP and guanosine showed some protective effect against loss of activity and of histidine residues. The reactivity of histidine residues was also reduced by prior modifica-tion of glutamic acid-58 with iodoacetate, of lysine-41 with maleic or cis-aconitic anhydride or 2, 4, 6-trinitrobenzenesulfonate or of arginine-77 with ninhydrin.
2. Analyses of the chymotryptic peptides from oxidized samples of the iodoacetamide-inactivated enzyme showed that histidine-92 and histidine-40 reacted with iodoacetamide most rapidly and at similar rates, whereas histidine-27 was least reactive. Alkylation of histidine-92 was markedly slowed down when the GluSe-carboxymethylated enzyme was treated with iodoacetamide. On the other hand, alkylation of histidine-40 was slowed down most in the presence of 3'-GMP. These results suggest that histidine-92 and histidine-40 are involved in the catalytic action, probably forming part of the catalytic site and part of the binding site, respectively, and that histidine-27 is partially buried in the enzyme molecule or interacts strongly with some other residue, thus becoming relatively unreactive.