Cloning, sequencing, distribution, and expression in Escherichia coli of flavin-containing monooxygenase 1C1. Evidence for a third gene subfamily in rabbits

Abstract

Two full-length cDNA clones (2.2 kilobases) encoding a newly recognized form of mammalian flavin-containing monooxygenase (FMO) have been isolated from independent libraries constructed with mRNA from different rabbits. The cDNAs encode a polypeptide of 533 amino acids which contains two putative pyrophosphate binding domains and a hydrophobic carboxyl terminus characteristic of FMOs. This sequence is 52 and 57% identical to sequences of the rabbit "hepatic" and "pulmonary" FMOs, respectively, and 55% identical to the sequence of "liver form 2" published recently by Ozols (Ozols, J. (1991) Arch. Biochem. Biophys. 290, 103-115). cDNA for the new FMO (FMO 1C1) hybridizes with two species of mRNA, one of 2.6 kilobases and one of about 5.4 kilobases, from liver or kidney, but not lung. Guinea pig, hamster, rat, and mouse all express this form of FMO in liver, kidney, and lung. FMO 1C1 has been tentatively characterized following expression in Escherichia coli. It is inactive with methimazole as substrate but highly active with n-octylamine. The temperature lability, responses to ions and detergent, and pH optimum of FMO 1C1 are similar to values reported for hepatic FMO. Sequence comparisons and analysis of rabbit and human genomic DNA indicate that FMO 1C1, as well as the pulmonary and hepatic FMOs, comprise a single gene family made up of distinct gene subfamilies (A, B,C,D, ... N), each appearing to contain a single gene. A nomenclature, based on these interrelationships and following the same designations used for classifying cytochromes P-450, is proposed.