Cysteine Oxygenase

Abstract

Abstract The mechanism of the conversion of l-cysteine to l-cysteinesulfinic acid in the presence of oxygen, reduced pyridine nucleotide, Fe2+, and an enzyme from rat liver cytoplasm was studied with the aid of 18O2 and H218O. A procedure is described for the isolation of small quantities of cysteinesulfinic acid from enzyme reaction mixtures. Mass spectrometric analysis of enzymatically synthesized cysteinesulfinic acid showed that both oxygen atoms originated from molecular O2. When corrected for nonenzymatic exchange reactions, no oxygen originating from water could be detected in the sulfinyl group. Thus the conversion of cysteine to cysteinesulfinic acid does not involve dehydrogenation steps. Since reduced pyridine nucleotide is not stoichiometrically utilized in the reaction, the reaction appears to be catalyzed by a dioxygenase rather than by two hydroxylases (mixed function oxidases) acting sequentially.