DEAMIDATED ACTIVE INTERMEDIATES IN THE IRREVERSIBLE ACID DENATURATION OF RIBONUCLEASE‐A

A study has been made on the changes in the enzymatic activity of Ribonuclease‐A**‐(RNase‐A) exposed to highly acidic (pH < 1) aqueous environment. Irreversible alterations of activity were observed when the protein was exposed to an acidic medium for a long period (20 to 60 h). Even prior to these changes in activity RNase‐A was found to form intermediates which had very nearly the same activity as the native protein. The primary process in the acid denaturation of RNase‐A was observed to be deamidation of the protein leading to the formation of active chromatographically distinct derivatives. The initial product of deamidation, a monodeamidated derivative, has been isolated by chromatography on Amberlite XE‐64. This initial deamidation reaction proceeded with very high specificity. The subsequent deamidation reaction is comparatively slower, so that nearly 50% of the native protein could be converted to this derivative before any subsequent deamidation took place. This monodeamidated derivative has been designated RNase‐Aa₁. The conversion of RNase‐A to RNase‐Aa₁ was not accompanied by any changes in the primary structure other than the observed deamidation. Apart from the differences in chromatographic and electrophoretic mobilities, RNase‐Aa₁ was found to have very nearly the same activity and physico‐chemical properties as the native enzyme. Significance of this specific and faster deamidation of RNase‐A in this denaturing medium as well as the biological significance of such deamidation reactions of proteins are discussed.