Mercapturic acid precursors

A few alkenes react with glutathione, or cysteine, to form a mercapturic acid precursor (Section VI, F). It has been suggested that some alkyl halides may be dehalogenated to an alkcnc (James and White, 1967).The alkene could be converted to an epoxide or could add the glutathione directly by a reaction catalyzed bj an S-glutathione transferase (Boyland and Chasseaud, 1968). At present the activity of the enzyme is chiefly related to the reaction of glutathione with aS unsaturated esters which do not form mercapturic acids in vioo. 

The observation that naphthalene feeding produced cataracts in rabbits suggested a relationship between mercapturic acid formation and sulfur amino acid metabolism in mammals, In the period between World Wars I and II, most advances in the biochemistrj of mei capturic acids were made through study of tlie nutritional effects produced by mercapturic acid precursors. During this period, the concept of mercapturic acids as... 

For purposes of discussion, the mercapturic acid precursors are classified according to the mechanism of conjugation with glutathione. The conjugation is catalyzed by an enzyme which Ls specific for each type. 

The classic mercapturic acid precursors belong to Type 1. These include benzene, naphthalene, bromobenzene, chlorobenzene, etc. The halides are unreactive and arc not easily replaced. Hence such derivatives behave as hydrocarbons in the mercapturic acid forming system. 

Pentachloromethylthiobenzene (PCMTB) is metabolized in rats to bis-(methylthio)tetrachlorobenzene (bis-MTTCB) by a microflora dependent pathway that involves biliary excretion of the mercapturate precursors and the action of a microfloral C-S lyase (8). The results of metabolism studies in rats are outlined in table III. Eighty-one percent of single oral doses of labeled PCMTB were excreted with the feces as bis-MTTCB and nonextractable residues in about equal amounts. Germfree rats excreted about 88 percent of the dose with the feces as at least two mercapturic acids (I, II, fig. 3 ). The major metabolite in the urine from both conventional and germfree rats was the mercapturic acid. 

Seventy-four percent of oral doses of PCMTB-l C were excreted in the bile by conventional rats. Most of this (50 to 70%) has been characterized to be products of the MAP (8). Neither of the mercapturates shown in fig. 3 were secreted in the bile therefore, the mercapturic acids that were excreted with the germfree rat feces had to have been formed either by metabolism of the precursors of the mercapturic acid by the intestinal mucosa, or by the tissues during enterohepatic circulation of these precursors. Comparison of the rates of excretion of oral doses of PCMTB- C given to germfree and conventional rats indicate that there was enterohepatic circulation of the in the germfree rats. Conventional rats excreted more than 80 percent of the dose in the feces within two days while it took at least eight days for the germfree rats to excrete 80 percent of the dose in the feces. 

We do not know whether the S-oxidized mercapturic acid of PCMTB (II) is formed in the conventional rat or if it may be a PCMTB metabolite that is unique to the germfree state. It could result from tissue metabolism during the enterohepatic circulation of the mercapturate precursors or the mercapturate (I). If it is formed in the conventional rat, it may be a precursor for the nonextractable fecal residues because no extractable metabolite that contained an oxidized sulfur was found in conventional rat excreta. 

The more complex sulphur requirements of the marine animals are met largely by cysteine, cystine, methionine, biotin, and thiamine (Young and Maw, 1958) (Fig. 4). Cysteine is a component of the tripeptide glutathione and a precursor of taurine. Methionine is as an essential amino acid involved in biosynthesis of proteins, creatine and adrenaline. Adenosylmethionine is considered to be the active part in transmethylation, e.g. of choline. Methionine is part of the pathways to homocysteine, cystathionine and methylthioadenosine (Young and Maw, 1958). Various organisms convert cysteine and/or cystine into mercapturic acids, cysteine sulphinic acid, and thiazolidine derivatives (Zobell, 1963). 

Epoxides of aromatic rings (arene oxides) rearrange to phenols and are substrates for glutathione-S-epoxide transferase to give glutathione conjugates, the precursors of mercapturic acids (Boyland and Williams 1965). 

Although the mercapturic acids from the monohalobenzenes constitute 35-50% of the administered compounds, with most unsubstituied hydrocarbons the yield of sulfur conjugate is rather small. The proportion of the dose which is excreted as metabolism products is affected by the degree of absorption of the precursor from the gut. This varies remarkably from one compound to another. Also, precursors tend to dissolve in tissue lipid and are released slowly. 

Metabolic degradation of a mercapturic acid or its precursors decreases the urinary output (Section VII). Oxidation products, such as sulfoxides and sulfate, have been detected. Aliphatic mercapturic acids yield some carbon dioxide. Also, incorporation of the iS-substituted C3 steine into proteins undoubtedly delays excretion of the mercapturic acid. 

Enzymes which catalyze the conjugation of the precursors (glutathione-jS-transferases) and those which convert the conjugates to mercapturic acids (glutathionase and acetylases) have been found in liver and kidney. Some are possibly present in intestine, skin, or other tissues. Species deficient in one or more of the enzymes do not excrete very much mercapturic acid. 

---