The catalytic mechanism of cytochrome P-450. Spin-trapping evidence for one-electron substrate oxidation

Olefin oxygenation and N-dealkylation by dopamine beta-monooxygenase: catalysis and mechanism-based inhibition

In an initial communication [May, S. W., Mueller, P. W., Padgette, S. R., Herman, H. H., & Phillips, R. S. (1983) Biochem. Biophys. Res. Commun. 110, 161-168], we reported that 1-phenyl-1-(aminomethyl)ethene hydrochloride (PAME) is an olefinic substrate for dopamine beta-monooxygenase (DBM; EC 1.14.17.1) which inactivates the enzyme in an apparent mechanism-based manner. The present study further characterizes this reaction. The inactivation reaction yields kinact = 0.23 min-1 at pH 5.0 and 37 degrees C and is strictly dependent on reductant (ascorbate) and oxygen. The DBM/PAME substrate reaction (apparent kcat = 14 s-1), shown to be stimulated by fumarate, gives the corresponding epoxide as product, identified by derivatization with 4-(p-nitrobenzyl)pyridine. However, the lack of DBM inhibition by alpha-methylstyrene oxide, and the observation of identical PAME/DBM inactivation rates in the absence and presence of preformed enzymatic PAME epoxide, indicates that free epoxide is not the inactivating species. A structure-activity study revealed that 4-hydroxylation of PAME (to give 4-HOPAME) increases both kinact (0.81 min-1) and apparent kcat (56 s-1) values, while 3-hydroxylation (to give 3-HOPAME) greatly diminishes inactivation activity while retaining substrate activity (apparent kcat = 47 s-1). 4-Hydroxy-alpha-methylstyrene was found to be a DBM inhibitor (kinact = 0.53 min-1) with weak substrate activity (apparent kcat = 0.71 s-1), while 3-hydroxy-alpha-methylstyrene and alpha-(cyanomethyl) styrene were found not to exhibit detectable DBM substrate activity and only weak inhibitory activity. 3-Phenylpropargylamine hydrochloride showed no detectable DBM substrate activity but rapidly inactivated the enzyme. A new substrate activity for DBM was discovered, N-dealkylation of N-phenylethylenediamine and N-methyl-N-phenylethylenediamine, and the lack of O-dealkylation activity with phenyl 2-aminoethyl ether and 4-hydroxyphenyl 2-aminoethyl ether indicates that DBM N-dealkylation proceeds via initial one-electron abstraction from the benzylic nitrogen heteroatom. With this new substrate and inhibitor reactivity information in hand, along with the other known substrate reactions, a DBM oxygenation mechanism analogous to that for cytochrome P-450 is proposed.

2-Acetamido-p-benzoquinone: a reactive arylating metabolite of 3'-hydroxyacetanilide

The covalent binding to protein of 3'-hydroxyacetanilide (3HAA), its primary metabolite 2',5'-dihydroxyacetanilide (2,5DHAA), and a putative secondary metabolite thereof, 2-acetamido-p-benzoquinone (APBQ), was studied in hepatic microsomal preparations from phenobarbital-pretreated mice. All compounds were found to bind irreversibly to microsomal protein, APBQ being by far the most effective member of the group. In the case of 3HAA, binding was dependent upon the presence in incubation media of the co-factor NADPH, indicating that metabolism of 3HAA was necessary for the generation of a reactive intermediate. In contrast, NADPH decreased by more than 2-fold the binding of both 2,5DHAA and APBQ. The free radical spin-trapping agent alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone (POBN) did not reduce the binding of 3HAA to protein. These results support the contention that metabolic activation of 3HAA is a two-step process which involves initial aromatic hydroxylation to give the substituted hydroquinone, 2,5DHAA, followed by a second oxidation reaction (which may not be enzyme-mediated) to produce the benzoquinone derivative, APBQ. This quinone is a reactive, electrophilic intermediate which may either undergo reduction back to 2,5DHAA or bind covalently to cellular macromolecules.

Affinity-dependent cross-linking to neurotoxin sites of the acetylcholine receptor mediated by catechol oxidation

The choline homologue 3-[(trimethylammonio)methyl]catechol (TMC) has been synthesized, and the controllable features of its complex oxidation have been examined spectroscopically and correlated with its toxin binding inactivating reactions with the acetylcholine receptor (AcChR) from Torpedo californica electroplax. Affinity-dependent reactions of early intermediates in the oxidation of TMC are suggested to intercede covalently in this inactivation. At pH 7.4, where the oxidative polymerization of catechols proceeds spontaneously, pyrocatechol produced no effect on the toxin binding function of AcChR, whereas comparable concentrations of TMC led to inactivation of half of all available sites. Lower concentrations of TMC converted via oxidation with ceric salts to an in situ mixture of monohydroxylated catechols were shown to be effective in short-term incubations in inactivating approximately half of the toxin binding sites by covalent labeling of the receptor. Mixtures of dihydroxycatechol intermediates, hydroxy-p-quinones, and polymeric products led to nonspecific toxin binding site inactivation of AcChR in excess of half of all available sites. Collectively, the results suggest that both covalent labeling and oxygen reduction product inactivating mechanisms are operative in these model macromolecular site reactions and that catechol-containing affinity reagents may be useful in elucidating the molecular features of sites to which they are directed.

Spin-trapping of methyl radical in the oxidative metabolism of 1,2-dimethylhydrazine

A carbon-centered free radical formed during oxidative metabolism of 1,2-dimethylhydrazine has been spin-trapped with alpha-(4-pyridyl-1-oxide)N-tert-butyl nitrone and 2-methyl-2-nitrosopropane. In the horseradish peroxidase/H2O2 catalyzed oxidation, the trapped species was identified as the methyl radical by the characteristic 1:3:3:1 quartet pattern of the 2-methyl-2-nitroso propane adduct. A carbon-centered radical is also formed during microsomal oxidation of 1,2-dimethylhydrazine in the presence of NADPH. However, the alpha-(4-pyridyl-1-oxide)N-tert-butyl nitrone trapped radical has not been unambiguously identified in this latter instance. These results may be of importance in regard to both carcinogenic and antitumor properties of 1,2-disubstituted hydrazine derivatives.

Enzymatic activation of chemicals to toxic metabolites

A variety of enzymes function in the oxygenation, oxidation-reduction, conjugation, and hydrolysis of drugs and other foreign chemicals. Often these enzymes detoxicate chemicals to prevent detrimental effects. In this review we will, however, concentrate on cases in which metabolism activates chemicals to reactive species which cause cellular damage. Particular attention will be given to mixed-function oxidases, which carry out a variety of oxygenations, as well as other reactions. (We will focus on cellular toxicity as opposed to initiation of tumorigenesis in this review.) In many cases, considerable circumstantial evidence exists linking these enzymes to enhanced toxicity of chemicals, although causal relationships have seldom been demonstrated. Further, in very few cases is the explicit cause of toxicity known. Modification of critical protein residues is suspected, although oxidative stress may also be involved in some cases. We discuss general aspects of mechanisms of toxic action, briefly list all cases in which metabolism is suspected to play a role in enhancing toxicity, and review a few examples in detail where substantial chemical and enzymatic information is available. The latter instances would involve knowledge of the enzymes involved, chemical evidence on the structures of the reactive metabolites, identification of adducts, and some inference into the biological processes which are effected to elicit toxicity. We consider, in this regard, vinyl halides (which have been a focus in our own laboratory), acetaminophen, pyrrolizidine alkaloids, and fluoroxene.

Production of superoxide during the metabolism of nitrazepam

Nitrazepam is metabolized in both humans and rats to 7-amino-nitrazepam OFFicating that this drug is reduced to a number of metabolic intermediates including several free radical species. When rat-hepatic microsomes are incubated with NADPH in the presence of nitrazepam, its nitro anion free radical was observed under anaerobic conditions. In the presence of oxygen, this free radical reduced oxygen giving nitrazepam and superoxide. 7-Nitroxyl-nitrazepam was produced by the chemical oxidation of 7-amino-nitrazepam using m-chloroperbenzoic acid. Reaction of this reactive free radical with hepatic microsomes led to the covalent spin labelling of microsomal protein. This phenomenon was also observed by the enzymic oxidation of 7-amino-nitrazepam with hepatic microsomes, obtained from a phenobarbital-induced rat, in the presence of a NADPH-generating system. With the generation of superoxide and hydrogen peroxide (arising from the dismutation of superoxide), it is not surprising that nitrazepam-enhanced lipid peroxidation was demonstrated by monitoring the production of lipid peroxyl radicals using spin-trapping techniques.

Study of H2O2-supported N-demethylations catalyzed by cytochrome P-450 and horseradish peroxidase

H2O2-supported oxidative demethylation reactions catalyzed by cytochrome P-450 and horseradish peroxidase have been compared. In contrast to peroxidase catalyzed reactions no free substrate radicals could be detected by EPR stopped flow measurements in demethylation reactions catalyzed by highly purified cytochrome P-450 although the rate of product formation for both enzyme systems was identical. These findings cause doubts in a general peroxide dependent demethylation mechanism valid for all hemoproteins and in the hypothesis that free substrate radicals are principally formed during cytochrome P-450 catalysis.