Identification of the ligand trans to thiolate in cytochrome P-450 LM2 by chemical modification.

About 3 tyrosine residues of cytochrome P-450 LM2 are accessible to chemical modification with tetranitromethane. Nitration of two tyrosines inactivates the enzyme to about 20%. The partial formation of a hyper-porphyrin spectrum originating from the pK shift by nitration and formation of a tyrosinate is prevented by modification in the presence of the inhibitor metyrapone. These findings support the assumption of a tyrosine residue as sixth ligand of the heme iron in cytochrome P-450 LM2.