Chemistry and biochemistry of 4-hydroxynonenal, malonaldehyde and related aldehydes.

International programme on chemical safety

Publisher Summary This chapter discusses the role of glutathione (GSH) and glutathione s-transferases in metabolism of chemical carcinogens and other electrophilic agents. GSH is a tripeptide (I) that is present in nearly all living cells and is the most abundant sulfhydryl compound present in animal tissues, mainly in the cytosol. The chapter illustrates the wide range of electrophilic agents, including several known mutagens and carcinogens, which conjugate with GSH, a process usually catalyzed by the GSH S-transferases. This conjugation is probably a protective mechanism and is the initial stage in mercapturic acid biosynthesis for the elimination of foreign compounds from the body. GSH S-transferases provide protection not only by catalyzing the conjugation of a potential toxicant with GSH, but also by preferentially binding, even covalently, that toxicant. The reactive electrophiles that conjugate with GSH also bind to DNA, RNA, and protein and identification of GSH conjugates provide information on the nature of these biologically active intermediates or even their immediate precursors. Thus, the knowledge of the way in which mutagens and carcinogens are metabolized is essential to a better understanding of their mode of action and of the processes for their detoxication.

The role of glutathione and glutathione S-transferases in the metabolism of chemical carcinogens and other electrophilic agents.

1. Several aromatic and olefinic compounds are converted to intermediate arene and alkene oxides by mammalian mono-oxygenases. Intermediate arene oxides rearrange non-enzymically to phenols. Arene and alkene oxides are converted by epoxide hydrases to vicinal diols and by glutathione S-epoxide conjugases to glutathione conjugates. Due to their high electrophilic reactivity, such oxiranes also bind to proteins, RNA and DNA. Mutagenic, carcinogenic and cytotoxic effects of several aromatic and olefinic compounds appear to be due to the formation of intermediate epoxides and their reaction with tissue constituents. Whether a given aromatic or olefinic compound produces such an effect would thus depend on a variety of factors, such as the relative rate of formation and degradation of the intermediate oxirane, on its stability with respect to spontaneous isomerization to the corresponding phenol and on its chemical electrophilic reactivity.2. Epoxide hydrases, which convert such intermediate oxiranes t...

Mammalian epoxide hydrases: inducible enzymes catalysing the inactivation of carcinogenic and cytotoxic metabolites derived from aromatic and olefinic compounds.

Publisher Summary This chapter provides information on the metabolism and the biological activities of polycyclic aromatic hydrocarbons. Three types of products—dihydrodiols, glutathione conjugates, and phenols—have been found as the metabolites of aromatic hydrocarbons in tissue preparations. Oxidative metabolism at the double bonds of aromatic substrates leads to the formation of epoxides. Epoxide metabolites are responsible for many of the biological effects that have been attributed to the parent hydrocarbons. The chapter discusses the role of intermediates such as free radicals, radical cations, and carbonium ions in polycyclic hydrocarbon carcinogenesis. It also discusses the factors that can influence the formation, further metabolism, and biological effectiveness of polycyclic hydrocarbon epoxides. Epoxides may prove to be biologically important derivatives that are formed from polycyclic hydrocarbons by microsomal metabolism; alternatively, they may not. The exact molecular mechanism that leads to the initiation of malignancy by hydrocarbons remains obscure, although strong suspicions persist that somatic mutations induced by metabolites are involved.

Epoxides in polycyclic aromatic hydrocarbon metabolism and carcinogenesis.

The oxidation in rabbit liver microsomes of styrene, cyclohexene and indene to epoxides has been demonstrated by gas chromatography, by thin-layer chromatography of the epoxide picrate derivatives and by accumulation of radioactivity in a styrene oxide pool from labeled styrene. Hydration of these epoxides to glycols had previously been demonstrated; in the cases of the cycloalkenes, the glycol products were the same isomers as those formed by overall oxidation of the alkenes. Epoxides are therefore intermediates in the oxidation of these alkenes to glycols in liver microsomes.

Epoxide intermediates in microsomal oxidation of olefins to glycols

Metyrapone [2-methyl-1,2-bis(3-pyridyl)-1-propanone], an inhibitor of adrenal steroid hydroxylation, has been shown to affect a number of liver microsomal drug-oxidizing enzyme systems. The metabolism of hexobarbital and that of aminopyrine are inhibited in vitro by this drug in microsomal preparations from control and phenobarbital-treated rats. Hexobarbital sleeping time is prolonged by metyrapone in both groups of rats. Demethylation of morphine and hydroxylation of tyramine are inhibited to a lesser extent, and NADPH oxidase is little affected. In contrast, the effect of metyrapone on the formation of phenolic metabolites from acetanilide follows a biphasic relationship with respect to metyrapone concentration: inhibition occurs only at very high concentrations, and the reaction is greatly enhanced at lower concentrations with both control and phenobarbital-induced preparations. Trichloroethylene oxidation is also enhanced by metyrapone in both preparations, but the enhancement in phenobarbital-induced preparations is superimposed on an inhibitory effect. Studies with the latter pathway show that both effects are independent of the NADPH-generating system, and are exerted in a reversible manner.

Effects of metyrapone on liver microsomal drug oxidations.

The product of metabolism of trichloroethylene in rat liver microsomes has been characterized as chloral hydrate. In common with chloral hydrate, the metabolite is converted to dichloroacetic acid upon reaction with basic cyanide; to chloroglyoxal bis-2,4-dinitrophenylhydrazone upon treatment with 2,4-dinitrophenylhydrazine in acid; to trichloroethanol by the action of liver alcohol dehydrogenase; and to trichloroacetic acid upon oxidation mediated by chloral hydrate dehydrogenase. Possible mechanisms of the conversion of trichloroethylene to chloral hydrate are discussed.

ACKNOWLEDGMENTS The authors are greatly indebted to Mr. Howard J. Lambert for aid in gas chromatography, and to him and Dr. E. R. Garrett for making available to them their chromatographic method in advance of publication. They also acknowledge the technical assistance of Mr. James D. Hindman in developing colorimetric methods. This work was supported in Part by U. S. Public Health Service Research Grant No. OH-00124.

Metabolism of Trichloroethylene in Liver Microsomes: II. Identification of the Reaction Product as Chloral Hydrate

Oxidation of styrene in liver microsomes.

The conversion of trichloroethylene to chloral hydrate was shown to take place in liver microsomes of rats, rabbits, and dogs, in a reaction requiring NADPH and oxygen. A stable, lyophilized preparation of microsomal and soluble fractions of liver homogenates in Tris buffer was used, from which a fraction could be obtained which resembled fresh microsomes in behavior. The effects of a number of activators and inhibitors of the enzyme system are described.

Kenneth C. Leibman

Papers

Epoxide intermediates in microsomal oxidation of olefins to glycols

Effects of metyrapone on liver microsomal drug oxidations.

Metabolism of Trichloroethylene in Liver Microsomes: II. Identification of the Reaction Product as Chloral Hydrate

Oxidation of styrene in liver microsomes.

Metabolism of trichloroethylene in liver microsomes. I. Characteristics of the reaction.