Carboxamidomethylation of a ribonuclease from Aspergillus saitoi.

1. The inactivation of a RNase from Aspergillus saitoi (RNase Ms) was studied to obtain information on its active site. 2) Inactivation of RNase Ms by iodoacetamide was greater at an alkaline pH, and was protected more by 2',(3')-AMP than by 2',(3')-GMP. 3) Analysis of the hydrolysis products with 6 N HCl and alkaline treatment of carboxamidomethylated RNase Ms showed that the sites of reaction were one carboxyl group and one histidine residue. 4) Since the incorporation of a carboxamidomethyl group into carboxylic acid was not protected by 2',(3')-AMP, it was concluded that the formation of N1-carboxamidomethylhistidine was responsible for the loss of enzymatic activity of RNase Ms.