Liver metabolic activation capability

Fish liver microsomes are capable of both hydrolytic and oxidative metabolism of phthalate esters. In addition, trout liver cytosol and blood serum exhibited esterase activity against DEHP. 

The converse is true of drugs requiring metabolic activation for toxicity. For example, paracetamol is less hepatotoxic to newborn than to adult mice, as less is metabolically activated in the neonate. This is due to the lower levels of cytochromes P-450 in neonatal liver (Fig. 5.30). Also involved in this is the hepatic level of glutathione, which is required for detoxication. Although levels of this tripeptide are reduced at birth, development is sufficiently in advance of cytochrome P-450 levels to ensure adequate detoxication (Fig. 5.30). The same effect has been observed with the hepatotoxin bromobenzene. (For further details of paracetamol and bromobenzene see chap. 7.) Similarly, carbon tetrachloride is not hepatotoxic in newborn rats as metabolic activation is required for this toxic effect, and the metabolic capability is low in the neonatal rat. 

Identification of the endogenous substrate(s) of an enzyme has an immediate effect on the biochemical characterization of the protein, but it can also have a broader impact on downstream metabolic and signaling studies. For example, the discovery of NATs naturally implies the presence of a previously unappreciated biosynthetic pathway responsible for the production of these compounds. Indeed, follow-up experiments confirmed the existence of a membrane-bound enzyme activity in the liver and kidney capable of producing NATs from acyl-CoAs and taurines [29] (Fig. 7a). 

A number of chemicals with demonstrable suppression of immune function produce this action via indirect effects. By and large, the approach that has been most frequently used to support an indirect mechanism of action is to show immune suppression after in vivo exposure but no immune suppression after in vitro exposure to relevant concentrations. One of the most often cited mechanisms for an indirect action is centered around the limited metabolic capabilities of immunocompetent cells and tissues. A number of chemicals have caused immune suppression when administered to animals but were essentially devoid of any potency when added directly to suspensions of lymphocytes and macrophages. Many of these chemicals are capable of being metabolized to reactive metabolites, including dime-thylnitrosamine, aflatoxin Bi, and carbon tetrachloride. Interestingly, a similar profile of activity (i.e., suppression after in vivo exposure but no activity after in vitro exposure) has been demonstrated with the potent immunosuppressive drug cyclophosphamide. With the exception of the PAHs, few chemicals have been demonstrated to be metabolized when added directly to immunocompetent cells in culture. A primary role for a reactive intermediate in the immune suppression by dimethylnitrosamine, aflatoxin Bi, carbon tetrachloride, and cyclophosphamide has been confirmed in studies in which these xenobiotics were incubated with suspensions of immunocompetent cells in the presence of metabolic activation systems (MASs). Examples of MASs include primary hepatocytes, liver microsomes, and liver homogenates. In most cases, confirmation of a primary role for a reactive metabolite has been provided by in vivo studies in which the metabolic capability was either enhanced or suppressed by the administration of an enzyme inducer or a metabolic inhibitor, respectively. 

In vitro studies showed that rat liver microsomes activated with NADPH and molecular oxygen metabolized MMT (Hanzlik et al. 1980b). Preliminary studies with pooled liver microsomes from 5-6 normal or pheno-barbital-induced rats showed that reaction rates of metabolism were linear for the first 20 minutes. MMT and aminopyrine, a positive control compound that is metabolized exclusively by cytochrome P450, showed parallel responses to changes in incubation conditions (i.e. NADPH dependence, inhibition by carbon monoxide, induction by phenobarbital). Liver microsomes metabolized MMT with an estimated of 78 pM and a Vni of 3.12 nmol/mg protein/min. When the studies were done with liver microsomes from phenobarbital-treated rats, the remained the same, but the doubled (Hanzlik et al. 1980b). Lung microsomes were equally capable of metabolizing MMT, but phenobarbital induction did not enhance the response. 

Extracts from cooked beef are clearly mutagenic in the frameshift sensitive Ames/Salmonella strains. Aroclor-induced metabolic activation is required for mutagenicity, but the human liver appears to be environmentally induced so that it is capable of metabolic activation. The synthetic mutagens IQ, a minor component of cooked beef basic extract (Felton et al. 

The sulfhydryls of cysteine, cysteine peptides including glutathione, and proteins are 1 found capable of reacting with a suitably activated hydrocarbon moiety in vitro. As described in Section VI, E, such reactions app rntly occur, though to a minor extent, in biologic sterns. When the reaction product is an S-substituted ej steine, acetylation can occur in liver or kidney mid a mercapturic acid results (Section VI, I). Similarly, hydrocarbons bound to proteins or peptides are degraded to S-substituted cysteines by metabolic activity (Section VI, H, 2). 

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