Xenobiotic biotransforming enzymes in the central nervous system: an isoform of flavin-containing monooxygenase (FMO4) is expressed in rabbit brain

Quantitative analysis of FMO gene mRNA levels in human tissues

The developmentally and tissue-specific expression of flavin-containing monooxygenase (FMO) enzymes has been previously characterized in a number of animal species, including humans, mice, rats, and rabbits. In this study, we used sensitive real-time reverse transcription-polymerase chain reaction methodology to systematically quantify the steady-state mRNA levels of FMO1, 2, 3, 4, and 5 in human tissues. We examined the developmental regulation of these enzymes in brain tissue. FMO1 was found to be down-regulated in human adult brain. The amount of other FMO mRNAs in human brains in different age groups was not significantly different. The study also provided a systematic quantitative comparison of the steady-state mRNA levels of FMO1 to 5 in several major human organs (i.e., liver, lung, kidney, small intestine, and brain). The nature of the quantitative analysis allowed a comprehensive comparison of each FMO mRNA in different tissues as well as among FMO isoforms in the same tissue. A comparison between fetal liver and adult liver showed that FMO1 was the only FMO that was down-regulated; all other FMOs had greater amounts of mRNA in adult liver. FMO5 was the most prominent FMO form detected in fetal liver. The FMO5 mRNA level was nearly as abundant as FMO3 in adult liver. Whereas other FMOs displayed a significant, dominant tissue-specific mRNA profile (i.e., FMO1 in kidney, FMO2 in lung, FMO3 and FMO5 in adult liver), FMO4 mRNA was observed more broadly at relatively comparable levels in liver, kidney, lung, and small intestine.

Mammalian flavin-containing monooxygenases: structure/function, genetic polymorphisms and role in drug metabolism

Flavin-containing monooxygenase (FMO) oxygenates drugs and xenobiotics containing a "soft-nucleophile", usually nitrogen or sulfur. FMO, like cytochrome P450 (CYP), is a monooxygenase, utilizing the reducing equivalents of NADPH to reduce 1 atom of molecular oxygen to water, while the other atom is used to oxidize the substrate. FMO and CYP also exhibit similar tissue and cellular location, molecular weight, substrate specificity, and exist as multiple enzymes under developmental control. The human FMO functional gene family is much smaller (5 families each with a single member) than CYP. FMO does not require a reductase to transfer electrons from NADPH and the catalytic cycle of the 2 monooxygenases is strikingly different. Another distinction is the lack of induction of FMOs by xenobiotics. In general, CYP is the major contributor to oxidative xenobiotic metabolism. However, FMO activity may be of significance in a number of cases and should not be overlooked. FMO and CYP have overlapping substrate specificities, but often yield distinct metabolites with potentially significant toxicological/pharmacological consequences. The physiological function(s) of FMO are poorly understood. Three of the 5 expressed human FMO genes, FMO1, FMO2 and FMO3, exhibit genetic polymorphisms. The most studied of these is FMO3 (adult human liver) in which mutant alleles contribute to the disease known as trimethylaminuria. The consequences of these FMO genetic polymorphisms in drug metabolism and human health are areas of research requiring further exploration.

Cell-, tissue-, sex- and developmental stage-specific expression of mouse flavin-containing monooxygenases (Fmos)

The cell-, tissue-, sex- and developmental stage-specific expression profiles of five members of the flavin-containing monooxygenase (FMO) family, FMO1, 2, 3, 4 and 5, were investigated in 129/SV mice, using isoform-specific antisense RNA probes. In situ hybridization localized FMO1 and 5 mRNAs to the perivenous, and FMO 2, 3 and 4 mRNAs to the periportal, regions of the liver. In kidney, each FMO mRNA is localized to the distal and proximal tubules and collecting ducts; FMO1 mRNA is present also in the glomerulus. In lung, FMO1 and 3 mRNAs are expressed in the terminal bronchiole, and FMO1 mRNA also in the alveoli. FMO1 mRNA is present in neurons of the cerebrum and in the choroid plexus. RNase protection assays showed that the most abundant isoform in newborn liver, lung, kidney and brain, and in adult lung and kidney is FMO1, but in adult liver FMO5 is present in greatest amounts. In liver, lung and kidney, expression of Fmo1, 3 and 5 peaks at 3 or 5 weeks of age, but in the brain, Fmo1 expression is greatest in newborns. In the kidney, FMO5 mRNA abundance is fourfold greater in males than in females, at all stages of development. Our results demonstrate that Fmo1, 2, 3, 4 and 5 exhibit distinct cell-, tissue-, sex- and developmental stage-specific patterns of expression.

Cloning, sequencing and tissue distribution of rat flavin-containing monooxygenase 4: two different forms are produced by tissue-specific alternative splicing

The nucleotide sequence of rat flavin-containing monooxygenase 4 (FMO4) mRNA was obtained by reverse transcription-polymerase chain reaction (RT-PCR) and 5'/3' terminal extension. Complete cDNA was amplified, cloned, and sequenced from the mRNA obtained from rat kidney and brain. Two different transcripts (short and long) stemming from the splicing of an internal region of 189 bases pair, corresponding to exon 4 were identified. This alternative splicing seems to be specific of the brain. The long cDNA encodes a protein of 560 amino acids with a predicted molecular mass of 63,395 Da. The short cDNA encodes a protein of 497 amino acids with a predicted molecular mass of 55,871 Da. Both of these encoded sequences contain the NADPH- and FAD-binding sites and a hydrophilic carboxyl terminus. These sequences are 80 and 79% identical to the sequences of human and rabbit FMO4. By Northern blotting and/or RT-PCR, the long-form FMO4 mRNA was detected in the rat kidney, intestine, and liver and the short form particularly in the brain. For the first time, the expression of FMO4 protein was demonstrated. By Western blotting using the two different forms of FMO4 antibodies, a long FMO4 protein was detected in the rat kidney, whereas in the rat brain, only the short form of FMO4 was observed.

Ethnic differences in human flavin-containing monooxygenase 2 (FMO2) polymorphisms: detection of expressed protein in African-Americans

The flavin-containing monooxygenases (FMOs) are a family of xenobiotic-metabolizing enzymes that are expressed in a species- and tissue-specific manner. FMO2 expression has been observed in pulmonary tissue from several species, but not human. Two human FMO2 point mutations have been reported: a cytosine to thymidine transition at position 1414 resulting in a premature stop codon and a thymidine insertion at position 1589 resulting in a frameshift. To define the frequency of these sequence variations and explore their significance, unrelated African-American, Caucasian, and Korean individuals were genotyped. In the African-American population tested (n = 180), the 1414C allele occurred at a 13% frequency; however, all of the tested Caucasians (n = 52) and Koreans (n = 100) were homozygous for the 1414T allele. The T1589 allele occurred at frequencies of 6.9 and 13.0% in African-Americans (n = 175) and Caucasians (n = 23), respectively, and appears to segregate with the 1414T allele. Thus, it would have no further impact on FMO2 activity. Western blot analysis of pulmonary microsomes failed to detect immunoreactive protein in 1414T homozygotes. A heterozygotic individual did exhibit a single band of the expected size, but no detectable FMO activity in the corresponding lung microsomes. Sequence analysis, however, was consistent with the 1414C allele encoding an active FMO2 enzyme. FMO2 mRNA expression was observed in most individuals, but failed to correlate with genotype or protein expression. In summary, functional FMO2 is expressed in only a small percentage of the overall population. However, in certain ethnic groups, active pulmonary FMO2 enzyme will be present in a significant number of individuals.

Flavin-containing monooxygenase mediated metabolism of benzydamine in perfused brain and liver

Benzydamine (BZY) N-oxidation mediated by flavin-containing monooxygenase (FMO) was evaluated in perfused brain and liver. Following 20 min of perfusion with modified Ringer solution, the infusion of BZY into brain or liver led to production of BZY N-oxide. BZY N-oxide, a metabolite of BZY oxidized exclusively by FMO, was mostly recovered in the effluent without undergoing further metabolism or reduction back to the parent substrate. The BZY N-oxide formation rate increased as the infusion concentration of BZY increased both in perfused brain and perfused liver. BZY N-oxidation activities in perfused rat brain and liver were 4.2 nmol/g brain/min and 50 nmol/g liver/min, respectively, although the BZY N-oxidation activity in brain homogenates was one 4000th that in liver homogenates. This is the first study of FMO activity in brain in situ.

Expression and characterization of a modified flavin-containing monooxygenase 4 from humans

The inability to obtain flavin-containing monooxygenase 4 (FMO4) in heterologous systems has hampered efforts to characterize this isoform of the FMO gene family. Neither the human nor the rabbit ortholog of FMO4, each of which has been cloned and sequenced, has been expressed. Attempts to achieve expression of FMO4 have been made with Escherichia coli, baculovirus, yeast, and COS systems. The cDNAs encoding FMO4 have extended coding regions compared with those encoding other FMO isoforms. The derived amino acid sequences of FMO1, -2, -3, and -5 from all species examined contain about the same number of residues (531-535 residues), whereas the derived sequences of human and rabbit FMO4 contain 558 and 555 residues, respectively. We have investigated whether the elongation of the FMO4 coding region is related to the inability to achieve expression. The cDNA encoding human FMO4 has been modified by a single base change that introduces a stop codon at the consensus position. This modification allows for expression in E. coli. Lack of expression of intact FMO4 is caused by a problem that occurs following transcription, a problem that is overcome completely by relocation of the stop codon 81 bases to 5' of its normal position. Truncated FMO4 is expressed as an active enzyme with characteristics typical of an FMO isoform. Possible functional changes resulting from altering the 3'-end of an FMO were investigated with human FMO3. Elongation of the coding region of the FMO3 cDNA to the next available stop codon (FMO3*) resulted in the expression of an enzyme with properties very similar to those of unmodified FMO3. Elongation of FMO3 lowered the level of expression in E. coli but did not eliminate it. As with FMO4, the difference in expression levels between FMO3 and elongated FMO3 (FMO3*) appears to be related to translation rather than transcription. The functional characteristics of FMO3 and FMO3* are not significantly different.