THE INFLUENCE OF ESTERIFICATION OF CARBOXYL GROUPS OF RIBONUCLEASE‐A ON ITS STRUCTURE AND IMMUNOLOGICAL ACTIVITY

The effect of modification of carboxyl groups of Ribonuclease‐A^a on the enzymatic activity and the antigenic structure of the protein has been studied. Modification of four of the eleven free carboxyl groups of the protein by esterification in anhydrous methanol/0.1 M hydrochloric acid resulted in nearly 80% loss in enzymatic activity but had very little influence on the antigenic structure of the protein. Further increases in the modification of the carboxyl groups caused a progressive loss in immunological activity, and the fully methylated RNase‐A exhibited nearly 30% immunological activity. Concomitant with this change in the antigenic structure of the protein, the ability of the molecule to complement with RNase‐S‐protein increased, clearly indicating the unfolding of the peptide “tail” from the remainder of the molecule. The susceptibility to proteolysis, accessibility of methionine residues for orthobenzoquinone reaction and the loss in immunological activity of the more extensively esterified derivatives of RNase‐A are suggestive of the more flexible conformation of these derivatives as compared with the compact native conformation. The fact that even the fully methylated RNase‐A retains nearly 30% of its immunological activity suggested that the modified protein contained antibody recognizable residual native structure, which presumably accommodates some antigenic determinants.