<p>Flavin-containing monooxygenases (FMOs) constitute a family of xenobiotic-metabolising enzymes [<cite idref="PUB00000158"/>]. Using an NADPH cofactor and FAD prosthetic group,these microsomal proteins catalyse the oxygenation of nucleophilic nitrogen,sulphur, phosphorous and selenium atoms in a range of structurally diversecompounds. Five mammalian forms of FMO are now known and have been designatedFMO1-FMO5 [<cite idref="PUB00002642"/>, <cite idref="PUB00002611"/>, <cite idref="PUB00004772"/>, <cite idref="PUB00000516"/>, <cite idref="PUB00002834"/>].</p><p> <taxon tax_id="10141">Cavia porcellus</taxon> (Guinea pig) and <taxon tax_id="9986">Oryctolagus cuniculus</taxon> (Rabbit) both express two variants of 'lung' FMO, observed asthree distinct phenotypes based on mobility differences in SDS-PAGE [<cite idref="PUB00004507"/>].The coding regions of the guinea pig variants differ at only two positions,both of which result in amino acid substitutions [<cite idref="PUB00004507"/>]. Similarly, thenucleotide and amino acid sequences of the rabbit variants differ at onlytwo positions. The amino sequence contains putative FAD- (GxGxxG) and NADP⁺-binding (GxGxxA) sites, a 'FATGY' motif that has also been observed in a range of siderphore biosynthetic enzymes [<cite idref="PUB00005489"/>], and a C-terminal hydrophobicsegment that is believed to anchor the monooxygenase to the microsomalmembrane [<cite idref="PUB00002549"/>].The activities of the enzymes are characteristic of the lung FMO, and themobilities of the expressed enzymes are the same as those observed for thevariants present in guinea pig pulmonary microsomal preparations [<cite idref="PUB00004507"/>]. Bothrabbit and guinea pig lung FMO are associated with a single gene. Rabbit lung FMO exists in tight association with the calcium-binding protein, calreticulin [<cite idref="PUB00000356"/>]. It is thought that complexation of calreticulin with rabbit lung FMO could account for some of the unusual physical propertiesof this FMO enzyme form.</p> Flavin monooxygenase (FMO) 2