Covalent structure of liver microsomal flavin-containing monooxygenase form 1

Liver microsomal, flavin-containing monooxygenases catalyze NADPH- and oxygen-dependent oxidation of a wide variety of antipsychotic and narcotic drugs. Two forms of these enzymes have been isolated and partially characterized (Ozols, J. (1989) Biochem. Biophys. Res. Commun. 163, 49-55). The amino acid sequence of form 1 is presented here. Sequence determination has been achieved by automated Edman degradation of peptides generated by chemical and enzymatic cleavages. The NH2 terminus of form 1 oxygenase is blocked. Partial acid hydrolysis of the blocked peptides removed acetyl groups and permitted their analysis by Edman degradation. Form 1 monooxygenase contains 536 residues. A peptide of 32 residues at the COOH terminus of the protein could not be sequenced in a gas-phase or pulsed liquid-phase sequenator, due to its extreme hydrophobicity. Covalent coupling of this peptide to an aryl amine membrane by means of carbodiimide, followed by automated solid-phase sequencing, established the order of 30 amino acid residues. The hydrophobic segment at the COOH terminus presumably functions to anchor the monooxygenase to the microsomal membrane. The amino acid sequence of form 1 monooxygenase, despite overlapping substrate specificity, is not related to the cytochrome P-450 superfamily. Comparison of the sequence of form 1 oxygenase with other known sequences, except for some short segments similar to those in the bacterial flavin-containing monooxygenases, did not reveal significant sequence similarities that would suggest a structural or evolutionary relationship.