Christine Beedham

Molybdenum hydroxylases: biological distribution and substrate-inhibitor specificity

Publisher Summary This chapter discusses the biological distribution and substrate-inhibitor specificity of molybdenum hydroxylases. Aldehyde oxidase and xanthine oxidase are molybdenum-containing enzymes that catalyze the oxidation of many aldehydes and nitrogen heterocycles. They have also been shown to catalyze, under in vitro conditions, a number of reductions. Although aldehyde oxidase is known to oxidize physiological compounds, such as N 1 -methylnicotinamide and pyridoxal, there does not yet appear to be a specific endogenous role for this enzyme. Xanthine oxidase is involved in endogenous purine catabolism, catalyzing the oxidation of hypoxanthine and xanthine to uric acid. This enzyme also has wide substrate specificity, and different functions have been assigned to it. One of the most plausible proposals is that both molybdenum hydroxylases may provide a biochemical barrier against ingested purines and pyrimidines, rendering them harmless. This is similar to the view put forward in a recent review on the role of the molybdenum hydroxylases as drug-metabolizing enzymes, except that the substrate specificity is not restricted to purines and pyrimidines.

Human Liver Aldehyde Oxidase: Inhibition by 239 Drugs

The authors tested 239 frequently used drugs and other compounds for their potential to inhibit the drug‐metabolizing enzyme, aldehyde oxidase, in human liver cytosol. A sensitive, moderate throughput HPLC‐MS assay was developed for 1‐phthalazinone, the aldehyde oxidase–catalyzed product of phthalazine oxidation. Inhibition of this activity was examined for the 239 drugs and other compounds of interest at a test concentration of 50 μM. Thirty‐six compounds exhibited greater than 80% inhibition and were further examined for measurement of IC50. The most potent inhibitor observed was the selective estrogen receptor modulator, raloxifene (IC50 = 2.9 nM), and tamoxifen, estradiol, and ethinyl estradiol were also potent inhibitors. Other classes of drugs that demonstrated inhibition of aldehyde oxidase included phenothiazines, tricyclic antidepressants, tricyclic atypical antipsychotic agents, and dihydropyridine calcium channel blockers, along with some other drugs, including loratadine, cyclobenzaprine, amodiaquine, maprotiline, ondansetron, propafenone, domperidone, quinacrine, ketoconazole, verapamil, tacrine, and salmeterol. These findings are discussed in context to potential drug interactions that could be observed between these agents and drugs for which aldehyde oxidase is involved in metabolism and warrant investigation of the possibility of clinical drug interactions mediated by inhibition of this enzyme.

Ziprasidone metabolism, aldehyde oxidase, and clinical implications

Ziprasidone (Geodon, Zeldox), a recently approved atypical antipsychotic agent for the treatment of schizophrenia, undergoes extensive metabolism in humans with very little (<5%) of the dose excreted as unchanged drug. Two enzyme systems have been implicated in ziprasidone metabolism: the cytosolic enzyme, aldehyde oxidase, catalyzes the predominant reductive pathway, and cytochrome P4503A4 (CYP3A4) is responsible for two alternative oxidation pathways. The involvement of two competing pathways in ziprasidone metabolism greatly reduces the potential for pharmacokinetic interactions between ziprasidone and other drugs. Because CYP3A4 only mediates one third of ziprasidone metabolism, the likelihood of interactions between ziprasidone and CYP3A4 inhibitors/ substrates is low. Furthermore, aldehyde oxidase activity does not appear to be altered when drugs or xenobiotics are coadministered. Aldehyde oxidase, a molybdenum-containing enzyme, catalyzes the oxidation of N-heterocyclic drugs such as famciclovir and zaleplon, in addition to reducing some agents such as zonisamide. Both reactions can occur simultaneously. Although in vitro inhibitors of aldehyde oxidase have been identified, there are no reported clinical interactions with aldehyde oxidase inhibitors or inducers. There is no evidence of genetic polymorphism in aldehyde oxidase, and thus it not surprising that ziprasidone exposure demonstrates unimodality in humans. Aldehyde oxidase is unrelated to the similarly named enzyme aldehyde dehydrogenase, which is predominantly responsible for the oxidation of acetaldehyde during ethanol metabolism. Consequently, it is unlikely that there would be any pharmacokinetic interaction between ethanol and ziprasidone.

Species variation in hepatic aldehyde oxidase activity

SummaryThe activity of hepatic aldehyde oxidase from rabbit, guinea pig, rat, marmoset, dog, baboon and man was investigated in vitro with charged and uncharged N-heterocyclic substrates: Km and Vmax values were determined for phthalazine, 6,7-dimethoxy-l-[-4-(ethylcarbamoyloxy)piperidino]phthalazine (carbazeran), quinine and quinidine. The oxidation of N-phenylquinolinium chloride to N-phenyl-2-quinolone and N-phenyl-4-quinolone was followed spectrophotometrically. Rat or dog liver showed low and negligible enzyme activity respectively, whereas baboon liver contained a highly active aldehyde oxidase. Enzyme from marmoset and guinea pig liver had the closest spectrum of activity to human liver aldehyde oxidase. Unlike that from man, rabbit hepatic aldehyde oxidase was refractory towards carbazeran and converted N-phenylquinolinium chloride predominantly to the 2-quinolone. N-Phenyl-4-quinolone was the major oxidation product with enzyme from guinea pig, marmoset, baboon and man.

Aldehyde oxidase-catalysed oxidation of methotrexate in the liver of guinea-pig, rabbit and man

Although 7‐hydroxymethotrexate is a major metabolite of methotrexate during high‐dose therapy, negligible methotrexate‐oxidizing activity has been found in‐vitro in the liver in man. The goals of this study were to determine the role of aldehyde oxidase in the metabolism of methotrexate to 7‐hydroxymethotrexate in the liver and to study the effects of inhibitors and other substrates on the metabolism of methotrexate. Methotrexate, (±)‐methotrexate and (‐)‐methotrexate were incubated with partially purified aldehyde oxidase from the liver of rabbit, guinea‐pig and man and the products analysed by HPLC. Rabbit liver aldehyde oxidase was used for purposes of comparison.

The role of non—P450 enzymes in drug oxidation.

In addition to cytochrome P450, oxidation of drugs and other xenobiotics can also be mediated by non–P450 enzymes, the most significant of which are flavin monooxygenase, monoamine oxidase, alcohol dehydrogenase, aldehyde dehydrogenase, aldehyde oxidase and xanthine oxidase. This article highlights the importance of these non–P450 enzymes in drug metabolism. A brief introduction to each of the non–P450 oxidizing enzymes is given in this review and the oxidative reactions have been illustrated with clinical examples. Drug oxidation catalyzed by enzymes such as flavin monooxygenase and monoamine oxidase may often produce the same metabolites as those generated by P450 and thus drug interactions may be difficult to predict without a clear knowledge of the underlying enzymology. In contrast, oxidation via aldehyde oxidase and xanthine oxidase gives different metabolites to those resulting from P450 hydroxylation. Although oxidation catalyzed by non-P450 enzymes can lead to drug inactivation, oxidation may be essential for the generation of active metabolite(s). The activation of a number of prodrugs by non–P450 enzymes is thus described. It is concluded that there is still much to learn about factors affecting the non–P450 enzymes in the clinical situation.

1-Substituted phthalazines as probes of the substrate-binding site of mammalian molybdenum hydroxylases

The interaction of a series of 1-substituted phthalazine derivatives with partially purified aldehyde oxidase from rabbit, guinea-pig and baboon liver, and with bovine milk xanthine oxidase, has been investigated. Of the 18 compounds examined, rabbit liver aldehyde oxidase metabolized 10, whereas guinea-pig and baboon liver enzyme oxidized 13 and 14, respectively. Where metabolites were characterized, oxidation was shown to occur at position four of the phthalazine ring. Km values ranged from 0.003 to 1.8 mM. In contrast, most compounds were competitive inhibitors of bovine milk xanthine oxidase with Ki values ranging from 0.015 to 1.3 mM; the cationic derivative 2-methylphthalazinium iodide was oxidized to 2-methyl-1-phthalazinone by both aldehyde oxidase and, with a much reduced affinity, by xanthine oxidase. In terms of structure-metabolism relationships, Vmax values were relatively insensitive to the electronic effects of substituents, but a trend for the more lipophilic derivatives to show increased affinities (Km and Vmax/Km) towards aldehyde oxidase could be seen. However, calculations of molecular size revealed a species-dependent cut-off threshold above which compounds were not metabolized. Results suggest that the relative size of the active site for hepatic aldehyde oxidase is in the order baboon greater than guinea-pig greater than rabbit, and that in spatial terms the active site of bovine milk xanthine oxidase is similar to that of baboon liver aldehyde oxidase. Thus, the binding site of rabbit liver aldehyde oxidase, a widely used source of the oxidase, is apparently more restricted than in some other species.

Enzymatic Oxidation of Vanillin, Isovanillin and Protocatechuic Aldehyde with Freshly Prepared Guinea Pig Liver Slices

Background/Aims: The oxidation of xenobiotic-derived aromatic aldehydes with freshly prepared liver slices has not been previously reported. The present investigation compares the relative contribution of aldehyde oxidase, xanthine oxidase and aldehyde dehydrogenase activities in the oxidation of vanillin, isovanillin and protocatechuic aldehyde with freshly prepared liver slices. Methods: Vanillin, isovanillin or protocatechuic aldehyde was incubated with liver slices in the presence/absence of specific inhibitors of each enzyme, followed by HPLC. Results: Vanillin was rapidly converted to vanillic acid. Vanillic acid formation was completely inhibited by isovanillin (aldehyde oxidase inhibitor), whereas disulfiram (aldehyde dehydrogenase inhibitor) inhibited acid formation by 16% and allopurinol (xanthine oxidase inhibitor) had no effect. Isovanillin was rapidly converted to isovanillic acid. The formation of isovanillic acid was not altered by allopurinol, but considerably inhibited by disulfiram. Protocatechuic aldehyde was converted to protocatechuic acid at a lower rate than that of vanillin or isovanillin. Allopurinol only slightly inhibited protocatechuic aldehyde oxidation, isovanillin had little effect, whereas disulfiram inhibited protocatechuic acid formation by 50%. Conclusions: In freshly prepared liver slices, vanillin is rapidly oxidized by aldehyde oxidase with little contribution from xanthine oxidase or aldehyde dehydrogenase. Isovanillin is not a substrate for aldehyde oxidase and therefore it is metabolized to isovanillic acid predominantly by aldehyde dehydrogenase. All three enzymes contribute to the oxidation of protocatechuic aldehyde to its acid.

Chapter 34 Role of aldehyde oxidase in biogenic amine metabolism

Publisher Summary This chapter discusses the role of aldehyde oxidase in biogenic amine metabolism. It shows that both homovanillyl aldehyde and 5-hydroxy-3-indoleacetaldehyde are substrates for guinea pig liver aldehyde oxidase. The results obtained with both in vivo and in vitro inhibitors indicate that aldehyde oxidase plays a major role in homovanillic (HV) and 5-hydroxytryptamine (5-HT) metabolism in guinea pig liver. In view of the similarity between the guinea pig and human liver aldehyde oxidase, it is likely that human hepatic aldehyde oxidase may also be important in biogenic amine metabolism. The urinary metabolites of 5-HT in have been examined in oriental subjects, but no correlation was found between 5-hydroxyindoleacetic acid (5-HIAA) production and mitochondrial aldehyde dehydrogenase genotype. It was suggested that cytosolic aldehyde dehydrogenase could oxidize physiological concentrations of 5-hydroxyindoleacetaldehyde when the mitochondrial isozyme is absent. However, this oxidation could equally well be carried out by aldehyde oxidase. Although peripheral plasma levels of HV acid—the deaminated and O-methylated metabolite of dopamine—are often used as an indicator of central dopaminergic activity. Lambert have shown that HVA is produced locally, perhaps from circulating DOPA.

Tissue distribution of the molybdenum hydroxylases, aldehyde oxidase and xanthine oxidase, in male and female guinea pigs

SummaryThe activity of the molybdenum hydroxylase, aldehyde oxidase, was determined in crude homogenates and (NH4)2S04 fractions prepared from guinea pig liver, lung, kidney, intestine, spleen and heart. Xanthine oxidase was also measured in (NH4)2S04 fractions. In each case, xanthine oxidase levels were lower than those of aldehyde oxidase; activity of the latter enzyme was highest in the liver, whereas xanthine oxidase was predominant in the small intestine. There was no significant difference in the activity of either molybdenum hydroxylase between tissues taken from male and female guinea pigs.