CHARACTERIZATION OF HEPATIC FLAVIN MONOOXYGENASE FROM THE MARINE TELEOST TURBOT (SCOPHTHALMUS-MAXIMUS L)

1. The presence and properties of flavin monooxygenase (FMO) in liver of the marine teleost, turbot (Scophthalmus maximus) were examined in relation to organic xenobiotic metabolism and osmoregulation. 2. Hepat ic microsomes of sexually mature fish contained NADPH-dependent FMO as evidenced by the conversion of N,N-dimethylaniline (DMA) to DMA-N-oxi de, and immunorecognition of single bands (approximate apparent molecu lar weight of 55 kDa) by antibodies to mammalian FMO 1 and FMO 2. Addi tionally, Northern analysis using a full-length cDNA probe to mammalia n FMO 1 revealed a single hybridizing band of approximately 2.5 kb. 3. No significant differences were seen between male and female turbot F MO with respect to DMA N-oxidase activity, levels of immunoreactive pr otein (with anti-FMO 1 or anti-FMO 2) and gene expression (hybridizing mRNA). 4. Hepatic microsomal DMA N-oxidase activity was inhibited by methimazole (an FMO substrate) and trimethylamine (TMA), but not by pi peronpl butoxide (a P450 inhibitor). Inhibition by TMA is indicative o f a role for FMO in osmoregulation, catalysing the conversion of TMA t o TMA N-oxide. DMA N-oxidase activity was optimal at pH 8.8 and 25 deg rees C, and displayed Michaelis-Menten kinetics with respect to DMA (a pparent Km = 88 mu M).