Chemical S-thiolation of reduced ribonuclease A using S-(2-amino-2-carboxy-ethylsulfonyl)-L-cysteine (Sacsyl-cysteine) : preparation and characterization of ribonuclease A-cysteine mixed disulfide

Ribonuclease A-cysteine mixed disulfide (RNaseA-Cys) was prepared by S-thiolation of reduced bovine pancreatic ribonuclease A (RNaseA) using S-(2-amino-2-carboxyethylsulfonyl)-L-cysteine (Sacsyl-cysteine) as an S-thiolating reagent. Total cysteine content of RNaseA-Cys was 16 moles per mole of RNaseA. Cysteine was not separated from RNaseA-Cys preparation by gel filtration, but the treatment with dithiothreitol liberated 8 moles of cysteine per mole of RNaseA. These findings indicate that 8 cysteine residues in reduced RNaseA were S-thiolated with cysteine