Conversion of N-hydroxyamphetamine to phenylacetone oxime by rat liver microsomes

A Chemical Perspective of Pharmacology and Toxicology

My chemical training provided a somewhat different perspective of biolo-gical problems, in the problem itself and approaches to its solution. I was fortunate to have in my laboratory postdocs and students who shared this perspective and used appropriate tools to address problems in amphetamine pharmacology and air pollution toxicology. These apparently disparate areas of research shared two chemical reactions: prooxidant-based generation of reactive oxygen and formation of covalent bonds between electrophiles and biological nucleophiles. This article is an attempt to summarize that research and to identify those individuals who made the contributions.

Primary aliphatic amine metabolism: the formation of a stable imine metabolite

The possible role of hydroxylamine and imine intermediates in the metabolism of amphetamine and related primary amines is discussed. The inherent instability of imines, has hitherto precluded the recognition of these compounds as metabolites of primary amines. During studies on the metabolism of o-methylbenzhydrylamine using washed hepatic microsomes from a number of mammalian species, the formation of o-methylbenzophenoneimine as a metabolite was observed. This metabolite was identified by preparative thin layer chromatography followed by gas-chromatography combined with mass spectrometry.

A new metabolite of methamphetamine; evidence for formation of N-[(1-methyl-2-phenyl)ethyl]ethanimine N-oxide

1. N-Hydroxyamphetamine and N-hydroxymethamphetamine, metabolic intermediates of amphetamine and methamphetamine, showed no reactivity towards endogeneous protein, amino acids, nucleic acids and fatty acids. In contrast, formaldehyde, acetaldehyde and propionaldehyde reacted well with N-hydroxyamphetamine and slightly with N-hydroxymethamphetamine under mild conditions (pH 7.4, 37 degrees C). 2. Two products were isolated from the reaction mixture of acetaldehyde and N-hydroxyamphetamine. These were characterized as N-[(1-methyl-2-phenyl)ethyl]ethanimine N-oxide and N-[(1-methyl-2-phenyl)ethyl]butenimine N-oxide by mass and n.m.r. spectrometries. 3. N-[(1-Methyl-2-phenyl)ethyl]ethanimine N-oxide was formed by incubating methamphetamine with liver 9000 g supernatants of rats and guinea-pigs. The butenimine N-oxide derivative, however, could not be detected as a metabolite of methamphetamine in vitro. 4. N-[(1-Methyl-2-phenyl)ethyl]ethanimine N-oxide was also detected as a urinary metabolite of methamphetamine in rats and guinea-pigs. Thus, the ethanimine N-oxide was established as a novel metabolite of methamphetamine.

Metabolic activation of the serotonergic neurotoxin para-chloroamphetamine to chemically reactive intermediates by hepatic and brain microsomal preparations

Para-chloroamphetamine (PCA) is selectively toxic to serotonergic neurons in laboratory animals. Acute, reversible neurotoxicity is followed by long-term effects which include inactivation of tryptophan hydroxylase and destruction of neurons. We have studied the metabolic formation of reactive intermediates that might be involved in long-term PCA neurotoxicity. Incubation of [3H]PCA with rat hepatic microsomes resulted in NADPH-dependent and oxygen-dependent covalent binding of radioactivity to microsomal protein. Addition of SKF-525A and glutathione to incubation mixtures inhibited [3H]PCA covalent binding 30% and 92% respectively. No inhibition of radiolabeled covalent binding was observed in an atmosphere of carbon monoxide/oxygen (80/20). 7,8-Benzoflavone was more effective than metyrapone in inhibiting [3H]PCA covalent binding. The extent of [3H]PCA covalent binding to microsomal protein was unchanged after phenobarbital pretreatment of rats, whereas 3-methylcholanthrene pretreatment increased [3H]PCA covalent binding (175%). NADPH-dependent and oxygen-dependent covalent binding of radioactivity was also observed when [3H]PCA was incubated with rat brain microsomal preparations. Addition of SKF-525A and glutathione to incubation mixtures inhibited covalent binding 10 and 40% respectively. There were no significant differences in total, NADPH-independent or NADPH-dependent covalent binding of radiolabeled R,S(+/-)-, R(-)-, or S(+)-PCA to rat hepatic microsomal protein. Less covalent binding was observed when [3H]amphetamine was incubated with rat liver microsomal preparations as compared to results with [3H]PCA. Minimal covalent binding was observed when [3H]PCA was incubated with liver microsomal preparations from rabbits, a species resistant to PCA neurotoxicity. Results of these metabolism studies are consistent with the hypothesis that oxidative metabolic activation of PCA to reactive and toxic metabolites is related to the long-term neurotoxicity of this agent.