Evidence for a single active-site cysteinyl residue in the streptococcal NADH peroxidase.

Substrate reduction of the streptococcal flavoprotein NADH peroxidase, followed by anaerobic denaturation and titration with 5,5'-dithiobis(2-nitrobenzoate), yields a stoichiometry of one protein thiol per mole of FAD. Analysis of the NADH peroxidase, purified from cultures of Streptococcus faecalis 10Cl grown on a chemically-defined medium containing [35S]cysteine, confirms the stoichiometry of one cysteinyl residue per subunit and allows the isolation and sequencing of the corresponding cysteinyl peptide. The amino acid sequence of the single cysteinyl peptide thus identified shows a striking difference from the active-site cysteinyl peptides of the flavoprotein disulfide and dimercaptide reductases.