Flavin-containing monooxygenase

Abstract

Abstracts 29. Aldehyde oxidases and other molybdenum hydroxylases Christine Beedham Clinical Sciences, University of Bradford, Bradford W Yorks, United Kingdom, BD7 1DP Drug oxidation (and reduction) mediated by the molybdenum containing enzyme, aldehyde oxidase (AO), is often overlooked in comparison to reactions catalyzed by cytochrome P450. However, aldehyde oxidase and the closely related xanthine oxidoreductase (XOR) catalyse the oxidation of a wide range of N-heterocyclic drugs in addition to aldehyde oxidation and reductive reactions. Typically, in vivo conversion of drugs to AO-generated metabolites is rapid and metabolites are excreted directly without conjugation. Consequently, drugs may be quickly inactivated or bioavailability may be reduced due to AO or XOR-catalysed oxidation in liver (AO and XOR) or gut (XOR). It is dif- ficult to express AO and XOR activity in recombinant systems and until recently, information on these enzymes has predominantly come from in vitro animal studies. This has provided valuable data on substrate/inhibitor specificity and possible clinical implications. Many in vitro AO inhibitors have been identified but, to date, there is little clinical indication that inhibitory drug interactions are significant in vivo. In contrast, co-administration of XOR inhibitors can be used to modulate drug therapeutics. Recently, full elucidation of the crystal structure of bovine milk XOR and Desulfovibria gigas AO and determination of their gene structure has expanded our knowledge of these enzymes considerably. AO and XOR each have a single functional gene in human liver (hAOX1 and XOR) but, unlike XOR, the AO gene is not conserved in all species and thus results from animal studies have to be interpreted with caution. Human AO and XOR genes appear to be controlled by complex but differential mechanisms at both transcriptional and post-translational levels. For example, studies in mice have indicated that both enzymes may be induced via the aryl hydrocarbon receptor (AhR) pathway, however, there is no clinical evidence that metabolism of drugs catalyzed by these enzymes is altered in smokers or during exposure to other AhR inducers. There are conflicting reports on the implications of AO polymorphism but some preliminary studies have indicated that human AO single nucleotide poly- morphisms (SNPs) can influence drug efficacy whereas XOR regulation and polymorphism may be more important in various physiological and pathophysiological mechanisms. 30. Flavin-Containing monooxygenases Ronald N. Hines 1 , David E. Klick 1 , Sevasti B. Koukouritaki 2 , Eugene W. Gerner 3 , Patricia Thompson 3 , and Frank L. Meyskens Jr. 4 Depts. of Pediatrics and Pharmacology/Toxicology & Children’s Research Institute, Medical College of Wisconsin & Children’s Hospital & Health System, Milwaukee, WI, USA, 53226, Depts. of Pediatrics & Children’s Research Institute, Medical College of Wisconsin & Children’s Hospital & Health System, Milwaukee, WI, USA, 53226, Arizona Cancer Center, University of Arizona, Tucson, AZ, USA, 85724, Chao Family Cancer Center, University of California-Irvine, Irvine, CA, USA, 92668 The flavin-containing monoxygenases (FMOs) oxidatively metabolize numerous toxicants and approximately 2% of clinically relevant drugs. In the human, 5 FMO genes encode functional enzymes (FMO1-5) with FMO1, 2 and 3 being most important for drug and toxicant metabolism. FMO1 is expressed at high levels in the fetal liver, small intestine and kidney while FMO3 is expressed at high levels in the adult liver. FMO2 is primarily a lung-specific enzyme, but its impact is minimized by a premature stop codon common in populations outside of Africa. In the human, FMO1 is expressed at its highest level in the 1st trimester fetal liver, then declines and is silenced a few days after birth. In contrast, FMO3 is essentially absent in the fetal and neonatal liver, but is detectable in most individuals by 1 to 2 years of age. Intermediate expression is observed in individuals between ages 2 and 11. Adult expression is usually seen by age 18. FMO3 promoter analyses revealed the presence of an NFY (position -75 to -59), Pbx 2 /HOX (position -115 to -103), HNF4A (position -167 to -152), YY1 (position -258 to -248) and C/EBPB (position -456 to -444) responsive elements. The NFY, HNF4A and C/EBPB sites appear most important for constitutive expression in the adult while developmental changes in the C/EBPB LAP:LIP ratio likely are involved in regulating FMO3 developmental expression. Several functional FMO3 variants have been identified. Hypomorphic variants have been associated with increased sulindac chemoprevention efficacy in familial adenomatous polyposis (FAP) patients. However, preliminary analysis of data from a recently com- pleted phase III randomized placebo-controlled trial in which patients with sporadic colorectal polyps received a combination of difluoromethylornithine (DFMO) and sulindac (N=191) or placebo (N=184) do not appear to corroborate these earlier findings. A significant association was observed with decreased colonic mucosa PGE 2