The reaction of iodoacetate with ribonuclease-S.

The reaction of iodoacetic acid with ribonuclease has been studied recently by Gundlach et al. (1). Inactivation of the enzyme occurred over a wide range of pH. The authors concluded that at acid pH the reaction involved solely methionyl residues, that at pH 5.5 a histidyl residue was the principal site of attack, and that at alkaline pH values significant reaction at lysyl residues occurred. Of particular interest was the isolation of a chromatographically pure component from the reaction mixture at pH 5.5. This substance showed no enzymatic activity and its amino acid composition was identical with that of ribonuclease-Al, except for the loss of a single histidyl residue. The monocarboxymethyl histidine derivative found on analysis represented only one of the two possible ring-substituted isomers2 (also cited by Stark et al. (a)), and this isomer was the one formed in least amount with model compounds containing histidine. It was subsequently shown by Stark et al. (2) that the formation of this derivative to any detectable extent in ribonuclease-A was dependent upon the presence of the “native” configuration of the enzyme. The results of Barnard and Stein (3) on the inactivation of ribonuclease by bromoacetate indicate that the histidy1 residue involved may be the one in position 119 near the C-terminal end of the single peptide chain. A study of the reaction of iodoacetic acid with the peptide component of ribonuclease-S has been reported (4). Some further observations on the reaction of the protein component and of ribonuclease-S with the same reagent are described in this paper. The results substantiate and extend a number of the conclusions of Stark et al. (2).