Chemical modification of cytochrome P-450 LM4. Identification of functionally linked tyrosine residues

Cytochrome P-450 LM4 (RH, reduced flavoprotein:oxygen oxidoreductase (RH-hydroxylating), EC 1.14.14.1) from rabbit liver microsomes was chemically modified with tetranitromethane. Nitration of two tyrosine residues inhibits the p-nitrophenetole O-deethylase activity of the enzyme by about 80%. Sequencing the 3-nitrotyrosine-containing peptides after HPLC tryptic peptide mapping reveals that mainly Tyr-243 and Tyr-271 are nitrated, whereas Tyr-71, Tyr-188 and Tyr-365 are modified to a lower extent. Nitration of tyrosine residues affects the complex formation with p-nitrophenetole, alpha-naphthoflavone and metyrapone as indicated by an increased affinity towards p-nitrophenetole and by a decreased affinity for the latter compounds. Furthermore, nitration interferes with the electron transfer from NADPH-cytochrome P-450-reductase to cytochrome P-450 LM4 resulting in a slowed down reduction reaction. The results suggest that Tyr-243 and Tyr-271 of cytochrome P-450 LM4 are functionally involved in the interaction with NADPH-cytochrome P-450 reductase.