Metabolism further, conjugates

The bio transformation of a chemical is determined by its structure, physicochemical properties, and enzymes in the tissues exposed. Biotransformation can be divided into phase 1 (oxidation, reduction, and hydrolysis) and phase 2 (conjugation). Further metabolism of conjugates has been termed phase 3. 

Thiram is absorbed via the skin, mucous membranes, respiratory, and gastrointestinal tracts. Thiram is rapidly absorbed from the gastrointestinal tract. Thiram and other dimethyldithiocarbamates are metabolized to diethyldithiocarbamic (DDC) acid, diethylamine, and carbon disulfide. DDC is rapidly absorbed by the gastrointestinal tract and further metabolized by hepatic enzymes. A portion of the acid is excreted unchanged or as glucuronide conjugate. Further metabolism can result in the formation of dimethylamine and carbon disulfide residues. 

Figure 9. Identification of metabolites A through D in urine of rats treated with CDA led to the proposed hidden metabolites shown here in brackets. The reactive sulfate conjugate apparently undergoes glutathione conjugation, further metabolism via the mercapturic acid pathway and, finally, C-S cleavage, methylation and sulfur oxidation of C to yield D.

Enterohepatic circulation can lead to toxic effects. For example, the drug chloramphenicol is metabolized to a conjugate that is excreted in bile by the rat. Once in the gut, the conjugate is broken down to release a phase 1 metabolite that undergoes further metabolism to yield toxic products. When these are reabsorbed, they can cause toxicity. The rabbit, by contrast, excretes chloramphenicol conjugates in urine, and there are no toxic effects at the dose rates in question. 

Bilirubin formed in peripheral tissues is transported to the hver by plasma albumin. The further metabolism of bihtubin occuts primarily in the hver. It can be divided into thtee processes (1) uptake of bilirubin by hver parenchymal cells, (2) conjugation of bilirubin with glucuronate in the endoplasmic reticulum, and (3) secretion of conjugated bilirubin into the bile. Each of these processes will be considered separately. 

Alternatively, acrylonitrile is metabolized to 2-cyanoethylene oxide by the microsomal enzyme system. 2-Cyanoethylene oxide can react directly with tissue macromolecules or it can be further metabolized to oxidation products that release cyanide. Cyanide is converted to thiocyanate and excreted in the urine. 2-Cyanoethylene oxide is also conjugated with glutathione and metabolized to 2- hydroxyethylmercapturic acid which is excreted in the urine. 

Biosynthesis is performed in one step by the enzyme L-histidine decarboxylase (HDC, E.C. 4.1.1.22). Histamine metabolism occurs mainly by two pathways. Oxidation is carried out by diamine oxidase (DAO, E.C. 1.4.3.6), leading to imidazole acetic acid (IAA), whereas methyla-tion is effected by histamine N-methyltransferase (HMT, E.C. 2.1.1.8), producing fe/e-methylhistamine (t-MH). IAA can exist as a riboside or ribotide conjugate. t-MH is further metabolized by monoamine oxidase (MAO)-B, producing fe/e-methylimidazole acetic acid (t-MIAA). Note that histamine is a substrate for DAO but not for MAO. Aldehyde intermediates, formed by the oxidation of both histamine and t-MH, are thought to be quickly oxidized to acids under normal circumstances. In the vertebrate CNS, histamine is almost exclusively methylated... 

Phenol is a hydrolyzed metabolite of benzene and is itself further hydrolyzed or conjugated to produce other compounds. Therefore, the toxic effects of phenol exposure may be due to a combination of the parent compound and its metabolites. The major tissues in which metabolism appears to occur are the liver, gut, lung, and kidney (Cassidy and Houston 1984 Powell et al 1974 Quebbemann and Anders 1973 Tremaine et al. 1984). Since phenol, benzene, and their major metabolites all seem to compete for the same P450 and conjugating enzymes, metabolic reactions are presumed to be saturable. 

All of these metabolites possess retinoid activity that is in some in vitro models more than that of the parent isotretinoin. However, the clinical significance of these models is unknown. After multiple oral dose administration of isotretinoin to adult cystic acne patients (18 years of age and older), the exposure of patients to 4-oxo-isotretinoin at steady state under fasted and fed conditions was approximately 3.4 times higher than that of isotretinoin. In vitro studies indicated that the primary P450 isoforms involved in isotretinoin metabolism are 2C8, 2C9, 3A4, and 2B6. Isotretinoin and its metabolites are further metabolized into conjugates, which are then excreted in urine and feces. 

Carbamylation Reactions. -Alkyl, -benzyl and -chloroallyl thiocarbamates do not readily react with GSH. In contrast, their sulfoxide derivatives ( and 6,) are very effective carbamylating agents for many thiols including GSH (19, ). The GSH conjugates formed vivo via 3 and 6 are quickly cleaved, acetylated and further metabolized as follows (19-21. 23. 24). 

Carnitine, L-3-hydroxy-4-(trimethylammonium)butyrate, is a water-soluble, tri-methylammonium derivative of y-amino-jS-hydroxybutyric acid, which is formed from trimethyllysine via y-butyrobetaine [40]. About 75% of carnitine is obtained from dietary intake of meat, fish, and dairy products containing proteins with trimethyllysine residues. Under normal conditions, endogenous synthesis from lysine and methionine plays a minor role, but can be stimulated by a diet low in carnitine. Carnitine is not further metabolized and is excreted in urine and bile as free carnitine or as conjugated carnitine esters [1, 41, 42]. Adequate intracellular levels of carnitine are therefore maintained by mechanisms that modulate dietary intake, endogenous synthesis, reabsorption, and cellular uptake. 

Under physiologic conditions, carnitine is primarily required to shuttle long-chain fatty acids across the inner mitochondrial membrane for FAO and products of peroxisomal /1-oxidation to the mitochondria for further metabolism in the citric acid cycle [40, 43]. Acylcarnitines are formed by conjugating acyl-CoA moieties to carnitine, which in the case of activated long-chain fatty acids is accomplished by CPT type I (CPT-I) [8, 44]. The acyl-group of the activated fatty acid (fatty acyl-CoA) is transferred by CPT-I from the sulfur atom of CoA to the hydroxyl group of carnitine (Fig. 3.2.1). Carnitine acylcarnitine translocase (CACT) then transfers the long-chain acylcarnitines across the inner mitochondrial membrane, where CPT-II reverses the action of CPT-I by the formation of acyl-CoA and release of free un-esterified carnitine. 

Other authors suggested that catechins are converted to glucuronyl derivatives in the intestinal mucosa and are further metabolized by methylation, sulfation and conjugation with glucuronic acid, sulfate and glycine [101]. 

It is supposed that, once fermentation products have crossed the intestinal barrier, they reach the liver through the portal vein, where they are further metabolized by dehydroxylation, methylation or conjugation to sulfate esters or glucuronides as it has been shown for other flavonoids [59]. 

This transporter is particularly important in the small intestine, in the gut wall enterocytes, where its activity in humans is sevenfold higher than liver tissue. In the gut, pGp, acting in concert with cytochrome P-450 (CYP3A4) (see chap. 4), functions to keep chemicals, which may be potential toxicants, out of the body by pumping them back into the lumen of the gut. The CYP3A4 converts them into more polar compounds, which are less readily absorbed or further metabolized into water-soluble conjugates. 

Oxidation Sulfation Further metabolism of glutathione conjugates... 

The further metabolism of suitably stable epoxides may occur, with the formation of dihydrodiols as discussed later. Dihydrodiols may also be further metabolized to catechols. Other products of aromatic hydroxylation via epoxidation are glutathione conjugates. These may be formed by enzymic or nonenzymic means or both, depending on the reactivity of the epoxide in question. 

As some of the newer drugs such as hormones, growth factors, and cytokines now being produced are peptides and certain toxins are also peptides or proteins, the role of peptidases may be important. Peptidases are especially active in the lumen of the gut, and consequently many such drugs are administered intravenously. Also some natural protein toxins may bypass the gut by via bites or stings into tissue. However, peptidase activity is also found in blood and other tissues. Peptidases are also important in the further metabolism of glutathione conjugates (see below). 

Another example of a glutathione conjugate responsible for toxicity is the industrial chemical hexachlorobutadiene discussed in chapter 7. The diglutathione conjugate of bromobenzene is believed to be involved in the nephrotoxicity after further metabolic activation (chap. 7, Fig. 7.31). 

Figure 4.65 Oxidation of hydroquinone to quinone and multiple conjugation with glutathione. Biliary excretion of the conjugate and reabsorption allow further metabolism (phase 3) in the gut and kidney to the cysteine conjugate, which is nephrotoxic.

The first two of these are discussed in chapter 4, and there are specific examples in chapter 7. The products are either excreted directly into the bile or further metabolized and excreted into the urine as cysteine or N-acetylcysteine conjugates. There are, however, examples of GSH conjugates being involved in toxicity as indicated in chapters 4 and 7. 

---