Inhibition by SKF-525A of the aldehyde oxidase-mediated metabolism of the experimental antitumour agent acridine carboxamide

Involvement of aldehyde oxidase in the metabolism of aromatic and aliphatic aldehyde-odorants in the mouse olfactory epithelium

Xenobiotic-metabolizing enzymes (XMEs) expressed in the olfactory epithelium (OE) are known to metabolize odorants. Aldehyde oxidase (AOX) recognizes a wide range of substrates among which are substrates with aldehyde groups. Some of these AOX substrates are odorants, such as benzaldehyde and n-octanal. One of the mouse AOX isoforms, namely AOX2 (mAOX2), was shown to be specifically expressed in mouse OE but its role to metabolize odorants in this tissue remains unexplored. In this study, we investigated the involvement of mouse AOX isoforms in the oxidative metabolism of aldehyde-odorants in the OE. Mouse OE extracts effectively metabolized aromatic and aliphatic aldehyde-odorants. Gene expression analysis revealed that not only mAOX2 but also the mAOX3 isoform is expressed in the OE. Furthermore, evaluation of inhibitory effects using the purified recombinant enzymes led us to identify specific inhibitors of each isoform, namely chlorpromazine, 17β-estradiol, menadione, norharmane, and raloxifene. Using these specific inhibitors, we defined the contribution of mAOX2 and mAOX3 to the metabolism of aldehyde-odorants in the mouse OE. Taken together, these findings demonstrate that mAOX2 and mAOX3 are responsible for the oxidation of aromatic and aliphatic aldehyde-odorants in the mouse OE, implying their involvement in odor perception.

Selective induction of tumor cell apoptosis by a novel P450-mediated reactive oxygen species (ROS) inducer methyl 3-(4-nitrophenyl) propiolate

Induction of tumor cell apoptosis has been recognized as a valid anticancer strategy. However, therapeutic selectivity between tumor and normal cells has always been a challenge. Here, we report a novel anti-cancer compound methyl 3-(4-nitrophenyl) propiolate (NPP) preferentially induces apoptosis in tumor cells through P450-catalyzed reactive oxygen species (ROS) production. A compound sensitivity study on multiple cell lines shows that tumor cells with high basal ROS levels, low antioxidant capacities, and p53 mutations are especially sensitive to NPP. Knockdown of p53 sensitized non-transformed cells to NPP-induced cell death. Additionally, by comparing NPP with other ROS inducers, we show that the susceptibility of tumor cells to the ROS-induced cell death is influenced by the mode, amount, duration, and perhaps location of ROS production. Our studies not only discovered a unique anticancer drug candidate but also shed new light on the understanding of ROS generation and function and the potential application of a ROS-promoting strategy in cancer treatment.

Comparative metabolism of warfarin in rats and chickens

Warfarin, a coumarin rodenticide, is commonly used worldwide for rodent control, and is often reported as the cause for poisoning accidents in nontarget animals, in particular bird species. However, the metabolism of warfarin in birds is still unclear. In a previous study, we found an unknown warfarin metabolite in chicken cytosolic fractions. In the present study, we aimed to clarify the cytosolic warfarin metabolites in chickens compared with those in rats. The cytosol fractions of both chicken and rat livers showed the metabolic activity of 2 diastereomers and 2 enantiomers of warfarin alcohol. In chicken cytosol, we found that the production level of (S)-warfarin-(S)-alcohol was markedly higher (32-fold) than that in rat cytosol. From the results of the inhibition assay, we finally suggest that aldehyde oxidase may mainly contribute to the warfarin alcohol products in chicken cytosol.

Bioelectronome. Integrated Approach to Receptor Chemistry, Radicals, Electrochemistry, Cell Signaling, and Physiological Effects Based on Electron Transfer

Bioelectronome refers to the host of electron transfer (ET) reactions that occur in living systems. This review presents an integrated approach to receptor chemistry based on electron transfer, radicals, electrochemistry, cell signaling, and end result. First, receptor activity is addressed from the unifying standpoint of redox transformations in which various receptors are discussed. After a listing of receptor-binding modes, receptor chemistry is treated with focus on generation of reactive oxygen species (ROS), activation by ROS, and subsequent cell signaling involving ROS. A general electrostatic mechanism is proposed for receptor-ligand action with supporting evidence. Cell-signaling processes appear to entail electron transfer, ROS, redox chains, and relays. The widespread involvement of phosphate from phosphorylation may be rationalized electrostatically by analogy with DNA phosphate. Extensive evidence supports important participation of ET functionalities in the mechanism of drugs and toxins. The integrated approach is applied to the main ET classes, namely, quinones, metal complexes, iminium species, and aromatic nitro compounds.

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.

Biotransformation of xenobiotics by amine oxidases

Although the cytochrome P450 (CYP) system ranks first in terms of catalytic versatility and the wide range of xenobiotics it detoxifies or activates to reactive intermediates, the contribution of amine oxidases and in particular of monoamine oxidases (MAOs) to the metabolism of xenobiotics is far from negligible but has been largely neglected. In this review on the involvement of amine oxidases in the metabolism of xenobiotics, the major characteristics reported for the CYP system (protein, reaction, tissue distribution, subcellular localisation, substrates, inhibitors, inducers, genetic polymorphism, impact of different physiopathological conditions on the activity, turnover) will be compared, whenever possible, with the corresponding characteristics of amine oxidases (MAOs in particular). The knowledge of the involvement of MAO-A, -B or both in the metabolism of a drug allows us to predict interactions with selective or non-selective MAO inhibitors (e.g. the metabolism of a drug deaminated by both forms of MAO is not necessarily inhibited in vivo by a selective MAO-A or -B inhibitor). If a drug is metabolized by MAOs, competitive interactions can occur with other drugs that are MAO substrates, e.g. with beta-adrenoceptor agonists and antagonists, prodrugs of dopamine, serotonin 5-HT1-receptor agonists as well as with primaquine, flurazepam and citalopram. Moreover, the knowledge of the involvement of MAOs in the metabolism of a drug may suggest possible, although not obligatory, interactions with tyramine-containing food or drink, with over the counter medicines sold to relieve the symptoms of coughs and colds (generally containing the indirectly-acting sympathomimetic amine phenylpropanolamine) or with phenylephrine-containing preparations. Finally, biotransformation by amine oxidases, as by CYP, does not always lead to detoxication but can produce toxic compounds.

Inter-species variation in the metabolism and inhibition of N-[(2'-dimethylamino)ethyl]acridine-4-carboxamide (DACA) by aldehyde oxidase

N-[(2'-Dimethylamino)ethyl]acridine-4-carboxamide (DACA) is a new anticancer agent currently undergoing clinical trials. The metabolism of DACA to acridone metabolites by aldehyde oxidase (AO) (EC 1.2.3.1) appears to play a major role in its elimination in human patients and rodents. The aim of this study was to compare the ability of human, guinea pig, and rat AO preparations to metabolise DACA, and to determine if either animal model was appropriate for predicting AO-mediated DACA-drug interactions in humans. Both human and rodent liver samples were homogenised in buffer before sequential centrifugation to produce the cytosol fraction. Human supernatant underwent an additional ammonium sulphate precipitation procedure, which produced a 2-fold increase in enzyme activity per milligram of protein. After incubations with DACA (range, 0-200 microM), DACA-9(10H)-acridone formation was determined by HPLC analysis. Michaelis-Menten parameters, Km and Vmax, were determined from the best fit curves by nonlinear regression. Three of the four human liver preparations had similar DACA intrinsic clearance values (Vmax/Km) ranging from 0.27 to 0.35 mL/min/mg protein, whereas both the rat and guinea pig had approximately 7- and 160-fold greater intrinsic clearances, due to lower Km values in rats (4.5 +/- 0.7 microM) and guinea pigs (0.15 +/- 0.1 microM) compared with humans (28.3 +/- 8.3 microM, N = 4). Amsacrine, menadione, and 7-hydroxy-DACA were potent inhibitors of DACA metabolism in all three species, but 10-fold differences in IC50 values were apparent between species. In addition, SKF-525A was a potent inhibitor of the metabolism of DACA in rat cytosol but caused minimal inhibition in the guinea pig or human preparations. These results suggest that neither rat nor guinea pig AO preparations are suitable for predicting AO-mediated DACA-drug interactions in humans.

Stereoselective biotransformation of the selective serotonin reuptake inhibitor citalopram and its demethylated metabolites by monoamine oxidases in human liver

Citalopram (CIT) is an antidepressive drug of the group of selective serotonin reuptake inhibitors (SSRIs). The tertiary amine CIT is given as a racemic drug, but its pharmacological activity resides mainly in S-CIT. CIT is metabolised by cytochrome P450 (CYP) to N-demethylcitalopram (DCIT) and N-didemethylcitalopram (DDCIT). The citalopram propionic acid derivative (CIT-PROP) is another, but pharmacologically inactive, metabolite, the formation of which has been poorly characterised but is postulated to occur by deamination of CIT, DCIT and/or DDCIT. The aim of the present investigation was to study the formation of the enantiomers of CIT-PROP from CIT and its two N-demethylated metabolites, DCIT and DDCIT, in an in vitro incubation system (microsomal and cytosolic fractions) obtained from human livers. The production of CIT-PROP was measured by a stereospecific HPLC method. Incubation of rac-CIT, rac-DCIT and rac-DDCIT (500 microM each, separately) in the presence (or absence) of NADP showed that CIT-PROP formation was substrate-dependent and essentially NADP-independent. Monoamine oxidases (MAO) type A and B and aldehyde oxidase were identified as the probable enzymes involved in the formation of CIT-PROP from CIT, DCIT and DDCIT. Indeed, the irreversible monoamine oxidase type A inhibitor clorgyline and the irreversible monoamine oxidase type B inhibitor selegiline (both at 0.5 microM in the incubation mixture) inhibited CIT-PROP formation, depending on the substrate, up to 70% and 88%, respectively. The participation of aldehyde oxidase in the subsequent step is suggested by the inhibition caused by menadione (50 microM) in CIT-PROP formation. Preliminary experiments suggest the presence of four unknown metabolites, probably products of deamination, which were detected in plasma and urine samples of patients treated with CIT as well as in in vitro biotransformations. Their presence confirms the importance of deamination in the biotransformation of CIT and its demethylated metabolites, especially in the brain where, in contrast to the liver, the role of cytochrome P450 appears to be low.

Involvement of growth hormone as a regulating factor in sex differences of mouse hepatic aldehyde oxidase

The participation of circulating growth hormone (GH) as a regulator of sex differences in hepatic aldehyde oxidase (AO) activity in ddy mice was examined. The 2- to 3-fold higher activities in adult male mice compared with adult female mice were decreased to the female levels by neonatal pretreatment with monosodium glutamate (MSG) or monosodium aspartate (MSA), either of which is known to reduce circulating GH levels. A decline of the activities in the MSG-treated male mice was restored nearly to the male control levels by subsequent injections of human GH every 12 hr for 7 days. These changes in AO activities in male mice caused by the excitotoxic amino acids were not observed in females. Hypophysectomy markedly decreased hepatic AO activities in male mice and partially in female mice. The activities in hypophysectomized male mice were restored again to levels similar to the control males by intermittent injections of human GH. Administration of testosterone propionate (TP) significantly increased the activities of hepatic AO in intact female mice, but not in MSA-treated or hypophysectomized females. On the other hand, the AO activities in adult male mice were decreased partially by the administration of estradiol benzoate. These results indicate that the pituitary GH is involved as one of the major regulatory factors of sex differences in the activities of hepatic AO in mice and TP also contributes to maintaining the higher activity in male mice mainly through the hypothalamus-pituitary system.

Methadone: a potent inhibitor of rat liver aldehyde oxidase

Several drugs with structural similarities to SKF-525A were tested for their ability to inhibit rat liver aldehyde oxidase using the experimental antitumour agent N-[(2'-dimethylamino)ethyl]acridine-4-carboxamide (AC; NSC 601316; acridine carboxamide) as substrate. The antihistamine D-chlorpheniramine, and the antiarrhythmics disopyramide, procainamide and lignocaine were ineffective in inhibiting this reaction. The antihistamines diphenhydramine, pheniramine, doxylamine, orphenadrine, methapyrilene and pyrilamine, gave IC50 values of 100-500 microM. The narcotic analgesics D-propoxyphene and, in particular, methadone were potent inhibitors of acridine formation with IC50 values of 15.5 and 0.31 microM, respectively. Further analysis indicates mixed non-competitive type inhibition by methadone with inhibition constants (Kis and Kii, respectively) of 0.03 +/- 0.01 (SE) and 0.57 +/- 0.12 microM.