L-lactate-2-monooxygenase. Sequence of peptides containing residues modified by 1-fluoro-2,4-dinitrobenzene

L-Lactate-2-monooxygenase (EC 1.13.12.4) inactivated by 1-fluoro-2,4-dinitrobenzene essentially as described previously (Choong, Y. S., Shepherd, M. G., and Sullivan, P. A. (1978) Biochem. J. 173, 255-262) incorporated 2.8 mol of the dinitrophenyl (DNP) moiety per mole of flavin. The inhibitors 2-methyl lactate or sulfite decreased the incorporation to 0.9 mol of DNP per mole of flavin. Peptide mapping by high performance liquid chromatography of radioactively labeled protein digested with trypsin showed three peaks of radioactivity. DNP-amino acid analysis and peptide sequencing showed that 2 distinct cysteine residues and a histidine residue had been modified. Both cysteine peptides were protected from modification by either of the inhibitors, whereas the histidine was only partially protected. The sum of the 2 cysteine peptides accounted for nearly 1 mole of label per mole of monomer. Since both of the cysteines are protected by inhibitors, they both must be in or near the substrate-binding site of the enzyme and appear to be modified in a mutually exclusive fashion. The histidine, on the other hand, does not lie directly in the substrate-binding site. It is possible that this histidine is the positively charged residue that is postulated to be near the N-1 position of the flavin.