Products of Performic Acid Oxidation and Acid Hydrolysis of S-Carboxymethylcysteine and Related Compounds

Abstract

1. S-Carboxymethylcysteine was submitted to performic acid oxidation and subsequent acid hydrolysis and the products obtained at each step were analyzed with an amino acid analyzer. Upon oxidation at −10 to −20°C, the major product was S-CM-cysteine sulfoxide (80%), and a small amount of S-CM-cysteine sulfone was also formed. When the oxidation was performed at 37°C, the product was exclusively S-CM-cysteine sulfone. 2. Upon acid hydrolysis, S-CM-cysteine sulfoxide was completely decomposed to give cystine plus cysteine as the major products (47–80%) and S-CM-cysteine as a minor product (maximally 8%). S-CM-cysteine sulfone was also decomposed extensively (maximally 81%) on acid hydrolysis, but gave no ninhydrin-positive amino acid detectable on amino acid analysis. 3. Similar results were obtained with a reduced carboxymethylated derivative of glutathione and of ribonuclease T₁ [EC 2.7.7.26] and with reduced carboxamido-methylated ribonuclease A [EC 2.7.7.16]. The yields of cystine plus cysteine from the S-carboxymethyl- or S-carboxamidomethylcysteine residues in these samples upon oxidation and acid hydrolysis were in the range of 40 to 50%. 4. The results obtained with S-CM-cysteine were compared with those obtained with S-methylcysteine and methionine under the same conditions. The sulfoxides of these latter amino acids did not yield cystine and cysteine on acid hydrolysis. The presence of carboxymethyl group on the sulfur atom of S-CM-cysteine sulfoxide appears to be responsible for the preferential cleavage of the C-S bond in the $HOOC-CH2-S↑O-$ group of this amino acid over the cleavage of the S-O bond.