The streptococcal flavoprotein NADH peroxidase: Purification, analysis of structural and redox properties, and identification of the active-site cysteinyl derivate

The NADH peroxidase of Streptococcus faecalis 10C1, purified to homogeneity, was studied using a variety of structural and spectroscopic techniques. The cofactor content of the enzyme was established using standard techniques, including atomic absorption analyses for the metal content. The native and subunit molecular weights of the protein were obtained through a combination of analytical ultracentrifugation, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, amino acid composition, and flavin content. Redox properties were studied by spectral titrations with substrates and/or chemical reductants. An essential oxidized cysteinyl derivative within the enzyme was identified through thio titrations of oxidized and reduced enzyme with 5,5'-dithiobis(2-nitrobenzoic acid), reductive alkylation of the enzyme with iodo[2-14C]acetamide, and performic acid oxidation of enzyme labelled metabolically with [35S]cysteine. Proteolytic digestion of radiolabelled enzyme followed by peptide purification by high performance liquid chromatography and automated Edman degradation yielded amino acid sequences of the cysteine-containing tryptic and chymotryptic peptides. Preliminary mass spectral analysis of the chymotryptic peptide was performed to probe the structure of the oxidized cysteinyl derivative