The oxidation of azaheterocycles with mammalian liver aldehyde oxidase

In Silico Prediction of Metabolic Reaction Catalyzed by Human Aldehyde Oxidase

Aldehyde oxidase (AOX) plays an important role in drug metabolism. Human AOX (hAOX) is widely distributed in the body, and there are some differences between species. Currently, animal models cannot accurately predict the metabolism of hAOX. Therefore, more and more in silico models have been constructed for the prediction of the hAOX metabolism. These models are based on molecular docking and quantum chemistry theory, which are time-consuming and difficult to automate. Therefore, in this study, we compared traditional machine learning methods, graph convolutional neural network methods, and sequence-based methods with limited data, and proposed a ligand-based model for the metabolism prediction catalyzed by hAOX. Compared with the published models, our model achieved better performance (ACC = 0.91, F1 = 0.77). What's more, we built a web server to predict the sites of metabolism (SOMs) for hAOX. In summary, this study provides a convenient and automatable model and builds a web server named Meta-hAOX for accelerating the drug design and optimization stage.

Evolution, expression, and substrate specificities of aldehyde oxidase enzymes in eukaryotes

Aldehyde oxidases (AOXs) are a small group of enzymes belonging to the larger family of molybdo-flavoenzymes, along with the well-characterized xanthine oxidoreductase. The two major types of reactions that are catalyzed by AOXs are the hydroxylation of heterocycles and the oxidation of aldehydes to their corresponding carboxylic acids. Different animal species have different complements of AOX genes. The two extremes are represented in humans and rodents; whereas the human genome contains a single active gene (AOX1), those of rodents, such as mice, are endowed with four genes (Aox1-4), clustering on the same chromosome, each encoding a functionally distinct AOX enzyme. It still remains enigmatic why some species have numerous AOX enzymes, whereas others harbor only one functional enzyme. At present, little is known about the physiological relevance of AOX enzymes in humans and their additional forms in other mammals. These enzymes are expressed in the liver and play an important role in the metabolisms of drugs and other xenobiotics. In this review, we discuss the expression, tissue-specific roles, and substrate specificities of the different mammalian AOX enzymes and highlight insights into their physiological roles.

Metabolism by Aldehyde Oxidase: Drug Design and Complementary Approaches to Challenges in Drug Discovery

Aldehyde oxidase (AO) catalyzes oxidations of azaheterocycles and aldehydes, amide hydrolysis, and diverse reductions. AO substrates are rare among marketed drugs, and many candidates failed due to poor pharmacokinetics, interspecies differences, and adverse effects. As most issues arise from complex and poorly understood AO biology, an effective solution is to stop or decrease AO metabolism. This perspective focuses on rational drug design approaches to modulate AO-mediated metabolism in drug discovery. AO biological aspects are also covered, as they are complementary to chemical design and important when selecting the experimental system for risk assessment. The authors' recommendation is an early consideration of AO-mediated metabolism supported by computational and in vitro experimental methods but not an automatic avoidance of AO structural flags, many of which are versatile and valuable building blocks. Preferably, consideration of AO-mediated metabolism should be part of the multiparametric drug optimization process, with the goal to improve overall drug-like properties.

Aldehyde oxidase functions as a superoxide generating NADH oxidase: an important redox regulated pathway of cellular oxygen radical formation

The enzyme aldehyde oxidase (AO) is a member of the molybdenum hydroxylase family that includes xanthine oxidoreductase (XOR); however, its physiological substrates and functions remain unclear. Moreover, little is known about its role in cellular redox stress. Utilizing electron paramagnetic resonance spin trapping, we measured the role of AO in the generation of reactive oxygen species (ROS) through the oxidation of NADH and the effects of inhibitors of AO on NADH-mediated superoxide (O(2)(•−)) generation. NADH was found to be a good substrate for AO with apparent K(m) and V(max) values of 29 μM and 12 nmol min(-1) mg(-1), respectively. From O(2)(•−) generation measurements by cytochrome c reduction the apparent K(m) and V(max) values of NADH for AO were 11 μM and 15 nmol min(-1) mg(-1), respectively. With NADH oxidation by AO, ≥65% of the total electron flux led to O(2)(•−) generation. Diphenyleneiodonium completely inhibited AO-mediated O(2)(•−) production, confirming that this occurs at the FAD site. Inhibitors of this NADH-derived O(2)(•−) generation were studied with amidone the most potent exerting complete inhibition at 100 μM concentration, while 150 μM menadione, raloxifene, or β-estradiol led to 81%, 46%, or 26% inhibition, respectively. From the kinetic data, and the levels of AO and NADH, O(2)(•−) production was estimated to be ~89 and ~4 nM/s in liver and heart, respectively, much higher than that estimated for XOR under similar conditions. Owing to the ubiquitous distribution of NADH, aldehydes, and other endogenous AO substrates, AO is predicted to have an important role in cellular redox stress and related disease pathogenesis.

Characterization of benzimidazole and other oxidative and conjugative metabolites of brimonidinein vitroand in ratsin vivousing on-line H/D exchange LC-MS/MS and stable-isotope tracer techniques

The characterization of brimonidine metabolites presents some challenges since brimonidine and its metabolites generate few structurally informative fragment ions in the LC-MS/MS spectra. The objective of the current study is to use on-line hydrogen/deuterium (H/D) exchange LC-MS/MS and stable-isotope tracer techniques to further characterize unknown brimonidine metabolites in vitro and in vivo. Brimonidine and D4-brimonidine were co-incubated in rat and human microsomes and rabbit aldehyde oxidase in vitro. In addition, the urine was collected from rats co-administered orally with brimonidine and D4-brimonidine. The hepatic microsomal and urinary metabolites were then characterized by H/D LC-MS/MS system. In addition to previously characterized 2-oxobrimonidine, 3-oxobrimonidine and 2,3-dioxobrimonidine, the results show that oxidation occurs at quinoxaline ring producing oxo-hydroxybrimonidine and hydroxyquinoxaline metabolites. The hydroxyquinoxaline metabolite was only observed in microsomal incubations with hydroxylation at the 7- or 8- position. The dehydro-hydroxybrimonidine metabolites were characterized as 2-oxo or 3-oxo -4', 5'-dehydrobrimonidine. A novel metabolite ((4-bromo-lH-benzoimidazol-5-yl)-imidazolidin-2-ylidene-amine) of benzimidazole derivative of brimonidine in rats in vivo was identified and confirmed with reference standard. In conclusion, on-line H/D exchange LC-MS/MS and stable-isotope tracer techniques are useful for the characterization of brimonidine metabolites.

Characterization of superoxide production from aldehyde oxidase: an important source of oxidants in biological tissues

Aldehyde oxidase, a molybdoflavoenzyme that plays an important role in aldehyde biotransformation, requires oxygen as substrate and produces reduced oxygen species. However, little information is available regarding its importance in cellular redox stress. Therefore, studies were undertaken to characterize its superoxide and hydrogen peroxide production. Aldehyde oxidase was purified to >98% purity and exhibited a single band at approximately 290 kDa on native polyacrylamide gradient gel electrophoresis. Superoxide generation was measured and quantitated by cytochrome c reduction and EPR spin trapping with p-dimethyl aminocinnamaldehyde as reducing substrate. Prominent superoxide generation was observed with an initial rate of 295 nmol min(-1) mg(-1). Electrochemical measurements of oxygen consumption and hydrogen peroxide formation yielded values of 650 and 355 nmol min(-1) mg(-1). In view of the ubiquitous distribution of aldehydes in tissues, aldehyde oxidase can be an important basal source of superoxide that would be enhanced in disease settings where cellular aldehyde levels are increased.

Degradation of 2,3-diethyl-5-methylpyrazine by a newly discovered bacterium, Mycobacterium sp. strain DM-11

A bacterium was isolated from the waste gas treatment plant at a fishmeal processing company on the basis of its capacity to use 2,3-diethyl-5-methylpyrazine (DM) as a sole carbon and energy source. The strain, designated strain DM-11, grew optimally at 25 degrees C and had a doubling time of 29.2 h. The strain did not grow on complex media like tryptic soy broth, Luria-Bertani broth, or nutrient broth or on simple carbon sources like glucose, acetate, oxoglutarate, succinate, or citrate. Only on Löwenstein-Jensen medium was growth observed. The 16S rRNA gene sequence of strain DM-11 showed the highest similarity (96.2%) to Mycobacterium poriferae strain ATCC 35087T. Therefore, strain DM-11 merits recognition as a novel species within the genus Mycobacterium. DM also served as a sole nitrogen source for the growth of strain DM-11. The degradation of DM by strain DM-11 requires molecular oxygen. The first intermediate was identified as 5,6-diethyl-2-hydroxy-3-methylpyrazine (DHM). Its disappearance was accompanied by the release of ammonium into the culture medium. No other metabolite was detected. We conclude that ring fission occurred directly after the formation of DHM and ammonium was eliminated after ring cleavage. Molecular oxygen was essential for the degradation of DHM. The expression of enzymes involved in the degradation of DM and DHM was regulated. Only cells induced by DM or DHM converted these compounds. Strain DM-11 also grew on 2-ethyl-5(6)-methylpyrazine (EMP) and 2,3,5-trimethylpyrazine (TMP) as a sole carbon, nitrogen, and energy source. In addition, the strain converted many pyrazines found in the waste gases of food industries cometabolically.

The FEMA GRAS assessment of pyrazine derivatives used as flavor ingredients. Flavor and Extract Manufacturers Association

This is the fifth in a series of safety evaluations performed by the Expert Panel of the Flavor and Extract Manufacturers Association (FEMA). In 1993, the Panel initiated a comprehensive program to re-evaluate the safety of more than 1700 GRAS flavoring substances under conditions of intended use. Elements that are fundamental to the safety evaluation of flavor ingredients include exposure, structural analogy, metabolism, pharmacokinetics and toxicology. Flavor ingredients are evaluated individually taking into account the available scientific information on the group of structurally related substances. Scientific data relevant to the safety evaluation of the use of pyrazine derivatives as flavoring ingredients is evaluated.

Characterization of brimonidine metabolism with rat, rabbit, dog, monkey and human liver fractions and rabbit liver aldehyde oxidase

1. In vitro metabolism of 14C-brimonidine by the rat, rabbit, dog, monkey and human liver fractions was studied to assess any species differences. In vitro metabolism with rabbit liver aldehyde oxidase and human liver slices, and in vivo metabolism in rats were also investigated. The hepatic and urinary metabolites were characterized by liquid chromatography and mass spectrometry. 2. Up to seven, six, 11 and 14 metabolites were detected in rat liver S9 fraction, human liver S9 fraction, human liver slices and rat urine respectively. Rabbit liver aldehyde oxidase catalysed the metabolism of brimonidine to 2-oxobrimonidine and 3-oxobrimonidine, and further oxidation to the 2,3-dioxobrimonidine. Menadione inhibited the liver aldehyde oxidase-mediated oxidation. 3. Hepatic oxidation of brimonidine to 2-oxobrimonidine, 3-oxobrimonidine and 2,3-dioxobrimonidine was a major pathway in all the species studied, except the dog whose prominent metabolites were 4',5'-dehydrobrimonidine and 5-bromo-6-guanidinoquinoxaline. 4. These results indicate extensive hepatic metabolism of brimonidine and provide evidence for aldehyde oxidase involvement in brimonidine metabolism. The species differences in hepatic brimonidine metabolism are likely related to the low activity of dog liver aldehyde oxidase. The principal metabolic pathways of brimonidine are alpha(N)-oxidation to the 2,3-dioxobrimonidine, and oxidative cleavage of the imidazoline ring to 5-bromo-6-guanidinoquinoxaline.

Molybdenum-dependent degradation of quinoline by Pseudomonas putida Chin IK and other aerobic bacteria

Eighteen different aerobic bacteria were isolated which utilized quinoline as sole source of carbon, nitrogen, and energy. Attempts were unsuccessful at isolating anaerobic quinoline-degrading bacteria. The optimal concentration of quinoline for growth was in the range of 2.5 to 5 mM. Some organisms excreted 2-hydroxyquinoline as the first intermediate. Hydroxylation of quinoline was catalyzed by a dehydrogenase which was induced in the presence of quinoline or 2-hydroxyquinoline. Quinoline dehydrogenase activity was dependent on the availability of molybdate in the growth medium. Growth on quinoline was inhibited by tungstate, an antagonist of molybdate. Partially purified quinoline dehydrogenase from Pseudomonas putida Chin IK indicated the presence of flavin, iron-sulfur centers, and molybdenum-binding pterin. Mr of quinoline dehydrogenase was about 300 kDa in all isolates investigated.

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.

Kinetics of some benzothiazoles, benzoxazoles, and quinolines as substrates and inhibitors of rabbit liver aldehyde oxidase

1. Twelve oxygen and sulphur azaheterocycles were studied as potential substrates of rabbit liver aldehyde oxidase. Only benzoxazole and 1,2-benzisoxazole were found to be substrates. 2. Nine of the compounds inhibited the oxidation of quinazoline by aldehyde oxidase and in all cases mixed inhibition kinetics were observed. 3. pi-Excessive heterocycles consisting of a single 5- or 6-membered ring (thiazole, oxazole) were neither substrates or inhibitors. Addition of a carbocyclic ring (benzothiazole, benzoxazole, 1,2-benzisoxazole) allowed binding to the enzyme as a substrate and/or inhibitor. 4. The mixed inhibition exhibited by the pi-excessive azaheterocycles benzothiazole, 1,2-benzisoxazole, and 2-substituted benzoxazoles was characterised by a Ki/KI ratio greater than 1.0, where Ki is the inhibitor constant for binding to the free enzyme and KI is the inhibitor constant for binding to the ES complex. In contrast, five pi-deficient methyl-substituted quinolines, which are known substrates for aldehyde oxidase, exhibited a Ki/KI ratio of less than 1.0. 5. The pi-excessive heterocycles 2,3-benzthiophene and 2,3-benzfuran, which do not contain a nitrogen atom, exhibited weak inhibition with a very high Ki/KI ratio. 6. The results of the study indicated that whilst thiazoles and oxazoles are unlikely to be extensively metabolized by aldehyde oxidase, they may inhibit the metabolism of substrates of the enzyme.

Reaction of 1-amino- and 1-chlorophthalazine with mammalian molybdenum hydroxylases in vitro

1-Amino- and 1-chlorophthalazine were tested for possible substrate activity with partially purified rabbit-liver aldehyde oxidase and bovine-milk xanthine oxidase. 1-Chlorophthalazine was a more efficient substrate than the parent compound, phthalazine, with either aldehyde oxidase or xanthine oxidase. The oxidation product of 1-chlorophthalazine was identified as 4-chloro-1-(2H)-phthalazinone on the basis of chromatographic, infra-red and mass-spectral data. 1-Aminophthalazine was oxidized by aldehyde oxidase to 4-amino-1-(2H)-phthalazinone but was a competitive inhibitor of xanthine oxidase. Kinetic studies at different pH values indicated that, in each case, it is the unprotonated form of 1-aminophthalazine that reacts with the molybdenum hydroxylases.

Similarity in the aldehyde oxidases from guinea-pig liver and polymorphonuclear leucocytes

Partially purified enzyme from guinea-pig leucocytes has been shown to have properties similar to both guinea-pig and rabbit liver aldehyde oxidase. The presence of molybdenum in the leucocyte enzyme has been demonstrated and substrate oxidation by either guinea-pig enzyme was found to be completely inhibited by menadione, a potent inhibitor of rabbit liver aldehyde oxidase. The leucocyte enzyme resembles the guinea-pig liver enzyme in terms of substrate specificity but there is considerable variation in substrate oxidation rates between the two species.

Hydralazine: a potent inhibitor of aldehyde oxidase activity in vitro and in vivo

The interaction of the vasodilator, hydralazine, with the molybdenum hydroxylases, aldehyde oxidase and xanthine oxidase has been investigated. A potent progressive inhibition of rabbit liver aldehyde oxidase, in the presence of substrate, by low concentrations of hydralazine (0.1-1 microM) was observed in vitro but no effect was seen with bovine milk xanthine oxidase. This activity was mirrored in vivo when levels of aldehyde oxidase were significantly decreased in rabbits administered hydralazine (10 mg/kg/day for seven days) whereas hepatic xanthine oxidase activity was unaltered by hydralazine treatment. Various metabolites of hydralazine were synthesized but found to be devoid of in vitro inhibitory activity. Aldehyde oxidase prepared from either guinea pig or baboon liver was inhibited in a similar way to that of rabbit liver.

Primary deuterium isotope effect in the oxidation of an iminium ion by aldehyde oxidase

Studies of the rates of aldehyde oxidase-mediated oxidation of iminium ion (I) and its specifically labeled counterpart containing deuterium at the methine and methylene carbons of its seven-membered ring (I-d3), are reported. In separate incubations, the isotope effect (kH/kD) on Vmax was 3.3 and that on Km was 1.5. Co-incubation of equimolar amounts of I and I-d3 with aldehyde oxidase resulted in an isotope effect of 2.2 on product ratio. These results support hydrogen transfer from the methine carbon of the seven-membered ring to the molybdenum center of the enzyme as the rate-determining step in the conversion of I to its lactam metabolite (II).

Oxidative metabolism of carbazeran in vitro by liver cytosol of baboon and man

The metabolism of carbazeran has been investigated in vitro using liver cytosol from dog, baboon and man. Carbazeran was not metabolized in cytosol prepared from dog liver but was rapidly metabolized to a single product in baboon- and human-liver cytosol. The product was identified as 4-hydroxy carbazeran. The enzyme responsible for the 4-hydroxylation of carbazeran in vitro was shown by the use of inhibitors to be liver aldehyde oxidase. Species differences in the metabolism of carbazeran in vitro correlate well with studies in vivo; these showed that following an oral dose to man and baboon, the compound was almost completely cleared via pre-systemic 4-hydroxylation, whereas in the dog, this metabolic route appeared unimportant.

A species difference in the presystemic metabolism of carbazeran in dog and man

The bioavailability of carbazeran and the metabolism of carbon-14 labelled drug have been studied in the dog and man following oral administration. The drug was moderately well absorbed in both species, but there was a marked difference in bioavailability and in routes of metabolism. In the dog, systemic bioavailability was approx. 68% and biotransformation involved mainly O-demethylation. In man, bioavailability was not measurable and carbazeran was almost completely cleared via 4-hydroxylation of the phthalazine moiety. Thus the lack of detectable pharmacological effect in man following oral administration of the drug appears to be due to presystemic metabolism by a particularly active pathway not found in the dog.

Elevation of molybdenum hydroxylase levels in rabbit liver after ingestion of phthalazine or its hydroxylated metabolite

Oral administration of phthalazine (50 mg/kg/day) or 1-hydroxyphthalazine (10 mg/kg/day) to female rabbits caused an increase in the specific activity of the hepatic molybdenum hydroxylases aldehyde oxidase and xanthine oxidase, whereas no effect on microsomal cytochrome P-450 activity was observed. The rise in the specific activity of purified aldehyde oxidase fractions was accompanied by a similar increase in molybdenum content. A significant lowering of the Km value for phthalazine was demonstrated with enzyme from treated rabbits whereas Km values for structurally similar substrates such as isoquinoline were unchanged from control values. Iso-electric focusing of DEAE-cellulose fractions showed the presence of an additional band of activity indicating that genuine induction of aldehyde oxidase had occurred in rabbits treated with phthalazine or 1-hydroxyphthalazine.

Simultaneous formation of 2- and 4-quinolones from quinolinium cations catalysed by aldehyde oxidase

Quinolinium salts were incubated with partially purified aldehyde oxidase, and the products were separated by high-pressure liquid chromatography and fully characterized by u.v. spectroscopy, i.r. spectroscopy and mass spectrometry. Oxidation of N-methylquinolinium salts with either rabbit or guinea-pig liver aldehyde oxidase in vitro gave two isomeric products, N-methyl-4-quinolone and N-methyl-2-quinolone. Incubation of N-phenylquinolinium perchlorate similarly yielded two oxidation products, N-phenyl-4-quinolone and N-phenyl-2-quinolone. The ratio of 2- to 4-quinolone production was species-dependent, the proportion of 4-quinolone with the guinea-pig enzyme being greater than that obtained with the rabbit liver enzyme. Kinetic constants were determined spectrophotometrically for both the quinolinium salts and a number of related quaternary compounds. In general, quaternization facilitated oxidation of a substrate, but a number of exceptions were noted, e.g. N-methylisoquinolinium and N-methylphen-anthridinium. Km values varied with the nature of electron acceptor employed, and this difference was more marked for quaternary substrates than the unquaternized counterparts. The product ratio obtained from N-methylquinolinium salts was found to be constant under various conditions, including purification of the enzyme and the use of either induced or inhibited aldehyde oxidase, but a change in the ratio was found at high pH values and in the presence of a competing substrate, N-methylphenanthridinium. This may indicate that a quaternary substrate binds to aldehyde oxidase in two alternative positions.

The fate of dibenz[b,f]-1,4-oxazepine (CR) in the rat. Part II. Metabolism in vitro

CR (dibenz[b,f]-1,4-oxazepine) is metabolized by rat liver 105 000 g supernatant fractions by (a) ring opening and reduction to 2-amino-2'-hydroxymethyldiphenyl ether and (b) oxidation at C11 to give a cyclic lactam. Reaction (a) is NADPH-dependent, decreased by dialysis and methylene blue, whereas reaction (b) is heat-resistant, inactivated by dialysis, inhibited by CN-, p-chloromercuribenzoate, amytal and menadione, and stimulated by methylene blue, phenazine methosulphate and 2,6-dichlorophenol indophenol. Reaction (a) is similar to that of aldehyde reductases (E.C.1.1.1.2) and reaction (b) to that of molybdenum hydroxylases (E.C.1.2.3.1). Reaction (a) is also catalysed by an NADH-dependent enzyme in liver microsomes and subsequent hydroxylation of the lactam also occurs in this cell fraction. Some extrahepatic metabolism of CR occurs via the same routes in kidney, small intestine and lung, though the yield is limited. Digestive gland extract of Helix pomatia converts CR to its lactam in significant amounts. The metabolism of CR in vitro is similar to that predicted from observations in vivo.

Species differences in the metabolic C- and N-oxidation , and N-methylation of [14C]pyridine in vivo

1. The metabolism of [2,6-14C]pyridine in vivo has been investigated in the rat, hamster, mouse, gerbil, rabbit, guinea-pig, cat and man, and the quantitative determination of the various urinary metabolites carried out by radiochromatographic analysis. 2. Unchanged pyridine and its N-methylated metabolite, N-methylpyridinium ion, were determined using a Partisil-10 SCX cation-exchange h.p.l.c. column, whereas the C- and N-oxidation products were assayed by reverse-phase chromatography, using a Partisil-10 ODS column. 3. Most of the species studied produce pyridine N-oxide, N-methylpyridinium ion, 2-pyridone, 3-hydroxypyridine and 4-pyridone as metabolites, but the proportion of the dose excreted as each of these metabolites is species-dependent.

Studies by electron-paramagnetic-resonance spectroscopy of the molybdenum centre of aldehyde oxidase

Molybdenum(V) e.p.r. spectra from reduced forms of aldehyde oxidase were obtained and compared with those from xanthine oxidase. Inhibited and Desulpho Inhibited signals from aldehyde oxidase were fully characterized, and parameters were obtained with the help of computer simulations. These differ slightly but significantly from the corresponding parameters for the xanthine oxidase signals. Rapid type 1 and type 2 and Slow signals were obtained from aldehyde oxidase, but were not fully characterized. From the general similarities of the signals from the two enzymes, it is concluded that the ligands of molybdenum must be identical and that the overall co-ordination geometries must be closely similar in the enzymes. The striking differences in substrate specificity must relate primarily to structural differences in a part of the active centre concerned with substrate binding and not involving the catalytically important molybdenum site.

Further evidence for an acetylator phenotype difference in the metabolism of hydralazine in man

1 The 0-24 h urine from hypertensive patients treated with hydralazine (100 mg twice daily) has been analysed by gas chromatography and high pressure liquid chromatography. 2 4-N-Acetylhydrazinophthalazine-1-one (NAcHPZ), s-triazolo [3, 4-a] phthalazine (TP), phthalazinone (PZ) and hydralazine (free, H; acid-labile hydrazones, HH) were detected and assayed. 3 The results indicate that slow acetylators excrete less NAcHPZ and TP than rapid acetylators but more PZ and HH. 4 Free hydralazine was present in low levels and was only detected in some urine samples. 5 The ratios of the metabolites NAcHPZ/HH; TP/HH; NAcHPZ/PZ and PZ/TP are different in the two acetylator phenotypes. 6 It is possible the ratio PZ/TP may be used for determination of acetylator phenotype. 7 It is concluded that hydralazine metabolism is dependent on the acetylator phenotype.