Biotransformation of hydroxylaminobenzene and aminophenol by Pseudomonas putida 2NP8 cells grown in the presence of 3-nitrophenol

Biotransformation products of hydroxylaminobenzene and aminophenol produced by 3-nitrophenol-grown cells of Pseudomonas putida 2NP8, a strain grown on 2- and 3-nitrophenol, were characterized. Ammonia, 2-aminophenol, 4-aminophenol, 4-benzoquinone, N-acetyl-4-aminophenol, N-acetyl-2-aminophenol, 2-aminophenoxazine-3-one, 4-hydroquinone, and catechol were produced from hydroxylaminobenzene. Ammonia, N-acetyl-2-aminophenol, and 2-aminophenoxazine-3-one were produced from 2-aminophenol. All of these metabolites were also found in the nitrobenzene transformation medium, and this demonstrated that they were metabolites of nitrobenzene transformation via hydroxylaminobenzene. Production of 2-aminophenoxazine-3-one indicated that oxidation of 2-aminophenol via imine occurred. Rapid release of ammonia from 2-aminophenol transformation indicated that hydrolysis of the imine intermediate was the dominant reaction. The low level of 2-aminophenoxazine-3-one indicated that formation of this compound was probably due to a spontaneous reaction accompanying oxidation of 2-aminophenol via imine. 4-Hydroquinone and catechol were reduction products of 2- and 4-benzoquinones. Based on these transformation products, we propose a new ammonia release pathway via oxidation of aminophenol to benzoquinone monoimine and subsequent hydrolysis for transformation of nitroaromatic compounds by 3-nitrophenol-grown cells of P. putida 2NP8. We propose a parallel mechanism for 3-nitrophenol degradation in P. putida 2NP8, in which all of the possible intermediates are postulated.     

Biotransformation of Hydroxylaminobenzene and Aminophenol by Pseudomonas putida 2NP8 Cells Grown in the Presence of 3-Nitrophenol

Biotransformation products of hydroxylaminobenzene and aminophenol produced by 3-nitrophenol-grown cells of Pseudomonas putida 2NP8, a strain grown on 2- and 3-nitrophenol, were characterized. Ammonia, 2-aminophenol, 4-aminophenol, 4-benzoquinone, N-acetyl-4-aminophenol, N-acetyl-2-aminophenol, 2-aminophenoxazine-3-one, 4-hydroquinone, and catechol were produced from hydroxylaminobenzene. Ammonia, N-acetyl-2-aminophenol, and 2-aminophenoxazine-3-one were produced from 2-aminophenol. All of these metabolites were also found in the nitrobenzene transformation medium, and this demonstrated that they were metabolites of nitrobenzene transformation via hydroxylaminobenzene. Production of 2-aminophenoxazine-3-one indicated that oxidation of 2-aminophenol via imine occurred. Rapid release of ammonia from 2-aminophenol transformation indicated that hydrolysis of the imine intermediate was the dominant reaction. The low level of 2-aminophenoxazine-3-one indicated that formation of this compound was probably due to a spontaneous reaction accompanying oxidation of 2-aminophenol via imine. 4-Hydroquinone and catechol were reduction products of 2- and 4-benzoquinones. Based on these transformation products, we propose a new ammonia release pathway via oxidation of aminophenol to benzoquinone monoimine and subsequent hydrolysis for transformation of nitroaromatic compounds by 3-nitrophenol-grown cells of P. putida 2NP8. We propose a parallel mechanism for 3-nitrophenol degradation in P. putida 2NP8, in which all of the possible intermediates are postulated.     

Cytochrome P-450-mediated oxidation of substrates by electron-transfer; role of oxygen radicals and of 1- and 2-electron oxidation of paracetamol

The mechanism by which the hepatic cytochrome P-450 (Cyt. P-450) containing mixed-function oxidase system oxidizes the analgesic drug paracetamol (PAR) to a hepatotoxic metabolite was studied. Since previous studies excluded the possibility of oxygenation of PAR, three other mechanisms, namely direct 1-electron oxidation by a Cyt. P-450-ferrous-dioxygen complex under concomitant formation of H2O2 to N-acetyl-p-semiquinone imine (NAPSQI), direct 2-electron oxidation by a Cyt. P-450-ferric-oxene complex to N-acetyl-p-benzoquinone imine (NAPQI) and indirect oxidation by active oxygen species released from Cyt. P-450, were considered. Indirect oxidation by active oxygen species was not involved, as active oxygen scavengers such as superoxide dismutase, catalase and DMSO did not affect the oxidation of PAR in hepatic microsomes. No reaction products characteristic for a direct 1-electron oxidation of PAR by Cyt. P-450 were observed: neither NAPSQI radical formation was detectable by ESR, nor PAR-dimer formation, nor stimulation of the microsomal H2O2 production was found to occur. In fact, PAR inhibited the spontaneous microsomal H2O2 formation. Studies on the reactions of NAPSQI with glutathione (GSH) revealed that NAPSQI hardly conjugated with GSH to a 3-glutathionyl-paracetamol conjugate (PAR-GSH) conjugate. The reactions of the elusive reactive metabolite formed during microsomal oxidation of PAR in the presence of GSH closely resembled those of synthetic NAPQI: both PAR-GSH and oxidized glutathione (GSSG) formation occurred. Furthermore, in agreement with a 2-electron oxidation hypothesis, iodosobenzene-dependent oxidation of PAR by cyt. P-450 in the presence of GSH resulted in the formation of the PAR-GSH conjugate. It is concluded that bioactivation of PAR by the Cyt. P-450 containing mixed-function oxidase system consists of a direct 2-electron oxidation to NAPQI.     

Mechanisms of acetaminophen oxidation to N-acetyl-P-benzoquinone imine by horseradish peroxidase and cytochrome P-450

Horseradish peroxidase rapidly catalyzed the H2O2-dependent polymerization of acetaminophen. Acetaminophen polymerization was decreased and formation of GSSG and minor amounts of GSH-acetaminophen conjugates were detected in reaction mixtures containing GSH. These data suggest that horseradish peroxidase catalyzed the 1-electron oxidation of acetaminophen and that GSH decreased polymerization by reducing the product, N-acetyl-p-benzosemiquinone imine, back to acetaminophen. Analyses of reaction mixtures that did not contain GSH showed N-acetyl-p-benzoquinone imine formation shortly after initiation of reactions. When GSH was added to similar reaction mixtures at various times, 3-(glutathion-S-yl)-acetaminophen was formed. The formation and disappearance of this product were very similar to N-acetyl-p-benzoquinone imine formation and were consistent with the disproportionation of 2 mol of N-acetyl-p-benzosemiquinone imine to 1 mol of N-acetyl-p-benzoquinone imine and 1 mol of acetaminophen followed by the rapid reaction of N-acetyl-p-benzoquinone imine with GSH to form 3-(glutathion-S-yl)acetaminophen. When acetaminophen was incubated with NADPH, oxygen and hepatic microsomes from phenobarbital-pretreated rats, 1.2 nmol 3-(glutathion-S-yl)acetaminophen/nmol cytochrome P-450/10 min was formed. Formation of polymers was not observed indicating that N-acetyl-p-benzoquinone imine was formed via an overall 2-electron oxidation rather than a disproportionation reaction. However, when cumene hydroperoxide was replaced by NADPH in microsomal incubations, polymerization was observed suggesting that cytochrome P-450 might also catalyze the 1-electron oxidation of acetaminophen.     

Catalytic oxidation of acetaminophen by tyrosinase in the presence of L-proline: a kinetic study

A kinetic study of acetaminophen oxidation by tyrosinase in the presence of a physiological nucleophilic agent such as the amino acid L-proline is performed in the present paper. The o-quinone product of the catalytic activity, 4-acetamido-o-benzoquinone, becomes unstable through the chemical addition of L-proline, in competition with the nucleophilic addition of hydroxide ion from water. In both cases, the catechol intermediate, 3(')-hydroxyacetaminophen, is generated, as can be demonstrated by liquid chromatography. When the effect of the presence of the nucleophilic agent on the time course of the enzymatic reaction was kinetically analyzed, it was seen to decrease the duration of the lag period and increase the steady-state rate. Rate constants for the reaction of 4-acetamido-o-benzoquinone with water and L-proline were also determined. The results obtained in this paper open a new possibility to acetaminophen toxicity, that has been attributed hitherto to its corresponding p-quinone, N-acetyl-p-benzoquinone imine.     

Oxidation products of quercetin catalyzed by mushroom tyrosinase

Quercetin was oxidized as a substrate catalyzed by mushroom tyrosinase to the corresponding o-quinone and subsequent isomerization to p-quinone methide type intermediate; followed by the addition of water on C-2 yielding a relatively stable intermediate, 2-(3,4-dihydroxybenzoyl)-2,4,6-trihydroxy-3(2H)-benzofuranone. In the presence of a catalytic amount of l-DOPA as a cofactor, the rate of this oxidation was enhanced. Fisetin, which lacks the C-5 hydroxyl group, was also oxidized but the rate of oxidation was faster than that of quercetin, indicating that the C-5 hydroxyl group is not essential but is associated with the activity.     

Mechanism of reaction of 3-hydroxyanthranilic acid with molecular oxygen

The autoxidation of the tryptophan metabolite, 3-hydroxyanthranilic acid, at pH 7 gives rise to a p-quinone dimer and cinnabarinic acid. A novel dimer formed by radical-radical coupling of 3-hydroxyanthranilic acid is also produced. Labelling studies have shown that the C-2 oxygen in the p-quinone dimer is derived from molecular oxygen. A product versus time study of this reaction has revealed that, in the absence of catalase, cinnabarinic acid is formed but undergoes decomposition by hydrogen peroxide. At pH 7, in the presence of catalase, both the p-quinone dimer and cinnabarinic acid are formed at approximately the same rate and this rate of formation increases with increasing pH. Inclusion of superoxide dismutase was found to increase the rate of formation of cinnabarinic acid, suggesting that superoxide ions may also cause decomposition of cinnabarinic acid. This was confirmed by treating cinnabarinic acid with superoxide. A mechanism involving a common anthranilyl radical intermediate is proposed to account for the formation of the different oxidation products.     

Degradation of 2,4-dichlorophenol by the lignin-degrading fungus Phanerochaete chrysosporium.

Under secondary metabolic conditions the white rot basidiomycete Phanerochaete chrysosporium mineralizes 2,4-dichlorophenol (I). The pathway for the degradation of 2,4-dichlorophenol (I) was elucidated by the characterization of fungal metabolites and of oxidation products generated by purified lignin peroxidase and manganese peroxidase. The multistep pathway involves the oxidative dechlorination of 2,4-dichlorophenol (I) to yield 1,2,4,5-tetrahydroxybenzene (VIII). The intermediate 1,2,4,5-tetrahydroxybenzene (VIII) is ring cleaved to produce, after subsequent oxidation, malonic acid. In the first step of the pathway, 2,4-dichlorophenol (I) is oxidized to 2-chloro-1,4-benzoquinone (II) by either manganese peroxidase or lignin peroxidase. 2-Chloro-1,4-benzoquinone (II) is then reduced to 2-chloro-1,4-hydroquinone (III), and the latter is methylated to form the lignin peroxidase substrate 2-chloro-1,4-dimethoxybenzene (IV). 2-Chloro-1,4-dimethoxybenzene (IV) is oxidized by lignin peroxidase to generate 2,5-dimethoxy-1,4-benzoquinone (V), which is reduced to 2,5-dimethoxy-1,4-hydroquinone (VI). 2,5-Dimethoxy-1,4-hydroquinone (VI) is oxidized by either peroxidase to generate 2,5-dihydroxy-1,4-benzoquinone (VII) which is reduced to form the tetrahydroxy intermediate 1,2,4,5-tetrahydroxybenzene (VIII). In this pathway, the substrate is oxidatively dechlorinated by lignin peroxidase or manganese peroxidase in a reaction which produces a p-quinone. The p-quinone intermediate is then recycled by reduction and methylation reactions to regenerate an intermediate which is again a substrate for peroxidase-catalyzed oxidative dechlorination. This unique pathway apparently results in the removal of both chlorine atoms before ring cleavage occurs.     

Thyroid peroxidase selects the mechanism of either 1- or 2-electron oxidation of phenols, depending on their substituents

Unlike lactoperoxidase and horseradish peroxidase, thyroid peroxidase catalyzed the oxidation of hydroquinone mostly by way of 2-electron transfer. This conclusion could be derived from three independent experiments: ESR measurements of p-benzosemiquinone, trapping the unpaired electron by cytochrome c, and spectrophotometric analysis of catalytic intermediates of the enzymes. The 1-electron flux for hydroquinone oxidation was found to be 15-19% in the reaction of thyroid peroxidase, while it was nearly 100% in the reactions of lactoperoxidase and horseradish peroxidase. From the spectrophotometric analysis of the catalytic intermediates of enzyme, it was suggested that the mechanism of oxidation catalyzed by thyroid peroxidase changes from a 2-electron to a 1-electron type as the substituents at 2- and 6-positions of phenol become bulky or heavy. On the other hand, the mechanism was invariably a 1-electron type when the oxidation of phenols was catalyzed by lactoperoxidase or horseradish peroxidase. These three peroxidases all catalyzed 1-electron oxidation of ascorbate.     

Production of 2-amino-5-phenoxyphenol from 4-nitrobiphenyl ether using nitrobenzene nitroreductase and hydroxylaminobenzene mutase from Pseudomonas pseudoalcaligenes JS45

Microbial metabolism of nitroarenes via o-aminophenols requires the participation of two key enzymes, a nitroreductase and an hydroxylaminobenzene mutase. The broad substrate ranges of the enzymes suggested that they could be used as biocatalysts for the production of substituted o-aminophenols. We have used enzymes from Pseudomonas pseudoalcaligenes JS45 for the conversion of 4-nitrobiphenyl ether to the corresponding o-aminophenol. Partially purified nitrobenzene nitroreductase reduced 4-nitrobiphenyl ether to the corresponding 4-hydroxylaminobiphenyl ether. Partially purified hydroxylaminobenzene mutase stoichiometrically converted the intermediate to 2-amino-5-phenoxyphenol. The results indicate that the enzyme system can be applied for the production of o-aminophenols useful as intermediates for synthesis of commercially important materials. Journal of Industrial Microbiology & Biotechnology (2000) 24, 301–305. Received 13 October 1999/ Accepted in revised form 31 January 2000     

Bacterial conversion of hydroxylamino aromatic compounds by both lyase and mutase enzymes involves intramolecular transfer of hydroxyl groups

Hydroxylamino aromatic compounds are converted to either the corresponding aminophenols or protocatechuate during the bacterial degradation of nitroaromatic compounds. The origin of the hydroxyl group of the products could be the substrate itself (intramolecular transfer mechanism) or the solvent water (intermolecular transfer mechanism). The conversion of hydroxylaminobenzene to 2-aminophenol catalyzed by a mutase from Pseudomonas pseudoalcaligenes JS45 proceeds by an intramolecular hydroxyl transfer. The conversions of hydroxylaminobenzene to 2- and 4-aminophenol by a mutase from Ralstonia eutropha JMP134 and to 4-hydroxylaminobenzoate to protocatechuate by a lyase from Comamonas acidovorans NBA-10 and Pseudomonas sp. strain 4NT were proposed, but not experimentally proved, to proceed by the intermolecular transfer mechanism. GC-MS analysis of the reaction products formed in H(2)(18)O did not indicate any (18)O-label incorporation during the conversion of hydroxylaminobenzene to 2- and 4-aminophenols catalyzed by the mutase from R. eutropha JMP134. During the conversion of 4-hydroxylaminobenzoate catalyzed by the hydroxylaminolyase from Pseudomonas sp. strain 4NT, only one of the two hydroxyl groups in the product, protocatechuate, was (18)O labeled. The other hydroxyl group in the product must have come from the substrate. The mutase in strain JS45 converted 4-hydroxylaminobenzoate to 4-amino-3-hydroxybenzoate, and the lyase in Pseudomonas strain 4NT converted hydroxylaminobenzene to aniline and 2-aminophenol but not to catechol. The results indicate that all three types of enzyme-catalyzed rearrangements of hydroxylamino aromatic compounds proceed via intramolecular transfer of hydroxyl groups.     

Active site residues and mechanism of UDP-glucose dehydrogenase.

UDP-glucose dehydrogenase catalyzes the NAD+-dependent twofold oxidation of UDP-glucose to give UDP-glucuronic acid. A sequestered aldehyde intermediate is produced in the first oxidation step and a covalently bound thioester is produced in the second oxidation step. This work demonstrates that the Streptococcus pyogenes enzyme incorporates a single solvent-derived oxygen atom during catalysis and probably does not generate an imine intermediate. The reaction of UDP-[6",6"-di-2H]-d-glucose is not accompanied by a primary kinetic isotope effect, indicating that hydride transfer is not rate determining in this reaction. Studies with a mutant of the key active site nucleophile, Cys260Ala, show that it is capable of both reducing the aldehyde intermediate, and oxidizing the hydrated form of the aldehyde intermediate but is incapable of oxidizing UDP-glucose to UDP-glucuronic acid. In the latter case, a ternary Cys260Ala/aldehyde intermediate/NADH complex is presumably formed, but it does not proceed to product as both release and hydration of the bound aldehyde occur slowly. A washout experiment demonstrates that the NADH in this ternary complex is not exchangeable with external NADH, indicating that dissociation only occurs after the addition of a nucleophile to the aldehyde carbonyl. Studies on Thr118Ala show that the value of kcat is reduced 160-fold by this mutation, and that the reaction of UDP-D-[6",6"-di-2H]-glucose is now accompanied by a primary kinetic isotope effect. This indicates that the barriers for the hydride transfer steps have been selectively increased and supports a mechanism in which an ordered water molecule (H-bonded to Thr118) serves as the catalytic base in these steps.     

Bacterial Conversion of Hydroxylamino Aromatic Compounds by both Lyase and Mutase Enzymes Involves Intramolecular Transfer of Hydroxyl Groups

Hydroxylamino aromatic compounds are converted to either the corresponding aminophenols or protocatechuate during the bacterial degradation of nitroaromatic compounds. The origin of the hydroxyl group of the products could be the substrate itself (intramolecular transfer mechanism) or the solvent water (intermolecular transfer mechanism). The conversion of hydroxylaminobenzene to 2-aminophenol catalyzed by a mutase from Pseudomonas pseudoalcaligenes JS45 proceeds by an intramolecular hydroxyl transfer. The conversions of hydroxylaminobenzene to 2- and 4-aminophenol by a mutase from Ralstonia eutropha JMP134 and to 4-hydroxylaminobenzoate to protocatechuate by a lyase from Comamonas acidovorans NBA-10 and Pseudomonas sp. strain 4NT were proposed, but not experimentally proved, to proceed by the intermolecular transfer mechanism. GC-MS analysis of the reaction products formed in H₂¹⁸O did not indicate any ¹⁸O-label incorporation during the conversion of hydroxylaminobenzene to 2- and 4-aminophenols catalyzed by the mutase from R. eutropha JMP134. During the conversion of 4-hydroxylaminobenzoate catalyzed by the hydroxylaminolyase from Pseudomonas sp. strain 4NT, only one of the two hydroxyl groups in the product, protocatechuate, was ¹⁸O labeled. The other hydroxyl group in the product must have come from the substrate. The mutase in strain JS45 converted 4-hydroxylaminobenzoate to 4-amino-3-hydroxybenzoate, and the lyase in Pseudomonas strain 4NT converted hydroxylaminobenzene to aniline and 2-aminophenol but not to catechol. The results indicate that all three types of enzyme-catalyzed rearrangements of hydroxylamino aromatic compounds proceed via intramolecular transfer of hydroxyl groups.     

Flavin redox chemistry precedes substrate chlorination during the reaction of the flavin-dependent halogenase RebH

The flavin-dependent halogenase RebH catalyzes chlorination at the C7 position of tryptophan as the initial step in the biosynthesis of the chemotherapeutic agent rebeccamycin. The reaction requires reduced FADH(2) (provided by a partner flavin reductase), chloride ion, and oxygen as cosubstrates. Given the similarity of its sequence to those of flavoprotein monooxygenases and their common cosubstrate requirements, the reaction of FADH(2) and O(2) in the halogenase active site was presumed to form the typical FAD(C4a)-OOH intermediate observed in monooxygenase reactions. By using stopped-flow spectroscopy, formation of a FAD(C4a)-OOH intermediate was detected during the RebH reaction. This intermediate decayed to yield a FAD(C4a)-OH intermediate. The order of addition of FADH(2) and O(2) was critical for accumulation of the FAD(C4a)-OOH intermediate and for subsequent product formation, indicating that conformational dynamics may be important for protection of labile intermediates formed during the reaction. Formation of flavin intermediates did not require tryptophan, nor were their rates of formation affected by the presence of tryptophan, suggesting that tryptophan likely does not react directly with any flavin intermediates. Furthermore, although final oxidation to FAD occurred with a rate constant of 0.12 s(-)(1), quenched-flow kinetic data showed that the rate constant for 7-chlorotryptophan formation was 0.05 s(-)(1) at 25 degrees C. The kinetic analysis establishes that substrate chlorination occurs after completion of flavin redox reactions. These findings are consistent with a mechanism whereby hypochlorite is generated in the RebH active site from the reaction of FADH(2), chloride ion, and O(2).     

Reaction pathway of bovine aortic lysyl oxidase

The catalysis of amine oxidation by lysyl oxidase has been probed to assess for the likely order of substrate binding and product release and to discriminate between mechanistic alternatives previously proposed for other copper-dependent amine oxidases using molecular oxygen as a substrate. Lineweaver-Burk plots revealed a pattern of parallel lines when the oxidation of n-butylamine was followed at different fixed concentrations of oxygen consistent with a "ping-pong" kinetic mechanism in which the aldehyde is produced and released before the binding of oxygen, the second substrate. Initial burst experiments revealed the ability of lysyl oxidase to form and release n-butyraldehyde in amounts stoichiometric with functional active site content in the absence of oxygen, consistent with the ping-pong kinetics obtained. Reciprocal plots of n-butylamine oxidation in the presence of fixed concentrations of the reaction products were consistent with a Uni Uni Uni Bi ping-pong kinetic mechanism with the aldehyde being the first, H2O2 the second, and ammonia the last departing product. Moreover, spectral studies of the oxidation of p-hydroxybenzylamine by lysyl oxidase indicated that the enzyme does not process the amine substrate to a noncovalently bound p-hydroxybenzaldimine intermediate subsequently to be hydrolyzed to p-hydroxybenzaldehyde. The kinetic mechanism of lysyl oxidase thus appears to be similar to those described for diamine oxidase and pig plasma monoamine oxidase.     

Mechanisms of action of histidinol dehydrogenase and UDP-Glc dehydrogenase. Evidence that the half-reactions proceed on separate subunits.

Histidinol dehydrogenase and UDP-Glc dehydrogenase catalyze 4-electron dehydrogenations that convert primary alcohol groups to the corresponding acids. Both reactions proceed in two distinct steps involving the oxidation of the primary alcohol to a bound form of the intermediate aldehyde, followed by oxidation of this to the corresponding acid. The enzymes have subunit structure, the former is made up of two subunits and the latter of six (beef liver enzyme). Evidence is presented that the two half-reactions proceed independently of the overall reaction. Histidinol dehydrogenase preparations that approach total dissociation into subunits also approach total inhibition of the overall reaction, while the second half reaction is completely unaffected and 50% of the first half-reaction survives. Further, the fraction of overall activity surviving in partially dissociated preparations follows the weight fraction of residual dimer. UDP-Glc dehydrogenase behaves in an analogous fashion. These data are interpreted on the basis that both enzymes function by carrying out first oxidation step at a site on one subunit and then pass the intermediate to a vicinal site on the adjacent subunit, where the reaction is completed.     

Identification of a new reaction intermediate in the oxidation of methylamine dehydrogenase by amicyanin

The two-electron oxidation of tryptophan tryptophylquinone (TTQ) in substrate-reduced methylamine dehydrogenase (MADH) by amicyanin is known to proceed via an N-semiquinone intermediate in which the substrate-derived amino group remains covalently attached to TTQ [Bishop, G. R., and Davidson, V. L. (1996) Biochemistry 35, 8948-8954]. A new method for the stoichiometric formation of the N-semiquinone in vitro has allowed the study of the oxidation of the N-semiquinone by amicyanin in greater detail than was previously possible. Conversion of N-semiquinone TTQ to the quinone requires two biochemical events, electron transfer to amicyanin and release of ammonia from TTQ. Using rapid-scanning stopped-flow spectroscopy, it is shown that this occurs by a sequential mechanism in which oxidation to an imine (N-quinone) precedes hydrolysis by water and ammonia release. Under certain reaction conditions, the N-quinone intermediate accumulates prior to the relatively slow hydrolysis step. Correlation of these transient kinetic data with steady-state kinetic data indicates that the slow hydrolysis of the N-quinone by water does not occur in the steady state. In the presence of excess substrate, the next methylamine molecule initiates a nucleophilic attack of the N-quinone TTQ, causing release of ammonia that is concomitant with the formation of the next enzyme-substrate cofactor adduct. In light of these results, the usually accepted steady-state reaction mechanism of MADH is revised and clarified to indicate that reactions of the quinone form of TTQ are side reactions of the normal catalytic pathway. The relevance of these conclusions to the reaction mechanisms of other enzymes with carbonyl cofactors, the reactions of which proceed via Schiff base intermediates, is also discussed.     

On the stoichiometry of the oxidase and monooxygenase reactions catalyzed by liver microsomal cytochrome P-450. Products of oxygen reduction

This laboratory has recently reported that, in a reconstituted enzyme system containing alcohol-induced isozyme 3a of liver microsomal cytochrome P-450, the sum of acetaldehyde generated by the monooxygenation of ethanol and of hydrogen peroxide produced by the NADPH oxidase activity is inadequate to account for the O2 and NADPH consumed. Studies on the stoichiometry have revealed the occurrence of an additional reaction involving an overall 4-electron transfer to molecular oxygen which is presumed to yield water: O2 + 2 NADPH + 2H+----2 H2O + 2 NADP+. The occurrence of a peroxidase reaction in which free H2O2 is reduced to water by NADPH was ruled out. When the 4-electron oxidase activity is taken into account, measurements of NADPH oxidation and O2 consumption are in accord with the amounts of products formed in the presence of various P-450 isozymes, either in the absence or presence of typical substrates, including those which undergo hydroxylation, N- or O-demethylation, or oxidation of hydroxymethyl to aldehyde groups. Of the substrates examined, some had no effect on the oxidase reaction yielding hydrogen peroxide or the 4-electron oxidase reaction, some were inhibitory, and some were stimulatory, but the same substrate did not necessarily have the same effect on the two reactions.     

The mechanism of the oxidation of d-amphetamine by rabbit liver oxygenase. Oxygen-18 studies

The oxidation of d-amphetamine by rabbit liver microsomes has been studied using oxygen-18 as the source of oxygen. Incorporation of heavy oxygen into the two major metabolites phenylacetone oxime and phenylacetone, was 93–95% and 25–31% respectively. These data are consistant with a mechanism in which the initial step is the hydroxylation of the substrate at the carbon atom α to the amino group. The carbinol amine which is formed by this reaction then serves as the key intermediate from which ketone and oxime are formed. Thus, oxime can form from carbinol amine in two step, (1) dehydration of carbinol amine and (2) oxygenation of the resulting imine. Phenylacetone can form by two pathways (1) loss of a molecule of ammonia from carbinol amine (incorporation of oxygen from molecular oxygen) and (2) hydrolysis of oxime (incorporation of oxygen from water). In the case of d-amphetamine the hydrolytic route appears to be the more important as suggested by Hucker, et al. (4, 5).