Multiple Carboxymethylation of Histidines in Bovine Ribonuclease A.

Reaction of RNAase A with bromoacetate at pH 5.5 for 1–42 days results in multiple reactions. Alkylation of residues proceeds in the sequence: (1) N-1 of histidine-119; (2) methionine (probably methionine-30); (3) N-3 of histidine-12; (4) N-3 of histidine-105 and N-3 of 1-carboxymethyl histidine-119; and (5) lysine-1. Both histidine-12 and histidine-119 of the same active site are carboxymethylated. A derivative carboxymethylated at both active site histidines is obtained in 1 day and probably some of this derivative is obtained in short reaction times. This is contrary to the conclusions of earlier investigations. Histidine-48 undergoes little or no reaction. The results are in accord with the X-ray structure of RNAase. [ABSTRACT FROM AUTHOR]