Cytochrome P-450-dependent mixed function

Benzene metabolism in humans and animals follows many similar pathways. Benzene is metabolized by cytochrome P-450-dependent mixed-function oxidase enzymes. Cytochrome P-450s are ubiquitous in all tissues. However, since the predominant repository of cytochrome P-450 is the liver, benzene is primarily metabolized in the liver. The metabolism of benzene by hepatic cytochrome P-450 may play an important role in the bioactivation of benzene. Sammett et al. (1979) provided corroborative evidence for this role by showing that partial hepatectomy of rats diminished both the rate of metabolism of benzene and its toxicity, suggesting that a metabolite and/or metabolites formed in the liver are necessary for toxicity. 

James, M.O. and J.R. Bend. Polycyclic aromatic hydrocarbon induction of cytochrome P-450-dependent mixed-function oxidases in marine fish. Toxicol. Appl. Pharmacol. 54 117—133, 1980. 

The National Institute of Environmental Health Sciences is funding research at the State University of New York at Albany to conduct an epidemiologic study of Mohawk women and infants to test the hypotheses that exposure to PAHs, polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) from a nearby Superfund hazardous waste site elevates body burdens and effects the cytochrome P-450-dependent mixed function... 

James MO, Little PJ (1980) Characterization of cytochrome P-450 dependent mixed-function oxidation in the spiny lobster, Panulirus argus. In Gustafsson JA, Carlstedt-Duke J, Mode A, Rafter J (eds) Biochemistry, biophysics and regulation of cytochrome P-450. Elsevier/North-Holland, Amsterdam, pp 113-120... 

White, I. N. H. A continuous fluorometric assay for cytochrome P-450-dependent mixed function oxidases using 3-cyano-7-ethoxycoumarin. Anal. Biochem. 1988,172, 304—310. 

Several pesticides and other xenobiotics are metabolized in plants by mixed-function oxidases which are cytochrome P-450-dependent [28]. Inhibitors of such enzymes might therefore serve as synergists of herbicides or other crop protection compounds which are oxidatively degraded. Therefore, it has been logical to test whether the growth retardants believed to be oxygenase inhibitors would interfere with xenobiotic metabolism. Under in vitro conditions it has indeed been observed that tetcyclacis at relatively low concentrations inhibits the metabolism of the herbicide chlortoluron [10]. Similarly, the herbicidal activity of bentazon could be enhanced by tetcyclacis which, most likely, reduced its oxidative inactivation [18]. Further studies will, however, be required to find out whether such effects can be used under practical conditions. 

A potentially powerful probe for sorting out the contribution of hydroperoxide-dependent and mixed-function oxidase-dependent polycyclic hydrocarbon oxidation is stereochemistry. Figure 9 summarizes the stereochemical differences in epoxidation of ( )-BP-7,8-dihydrodiol by hydroperoxide-dependent and mixed-function oxidase-dependent pathways (31,55,56). The (-)-enantiomer of BP-7,8-dihydrodiol is converted primarily to the (+)-anti-diol epoxide by both pathways whereas the (+)-enantiomer of BP-7,8-dihydrodiol is converted primarily to the (-)-anti-diol epoxide by hydroperoxide-dependent oxidation and to the (+)-syn-diol epoxide by mixed-function oxidases. The stereochemical course of oxidation by cytochrome P-450 isoenzymes was first elucidated for the methycholanthrene-inducible form but we have detected the same stereochemical profile using rat liver microsomes from control, phenobarbital-, or methyl-cholanthrene-induced animals (32). The only difference between the microsomal preparations is the rate of oxidation. 

An effect of ozone on lung microsomes has been suggested by morpholine studies that indicated alterations in the endoplasmic reticulum, Biochemical evidence of an effect on microsomal enzymes was originally obtained in the studies of Palmer et who demonstrated that ozone exposure (0.75-10 ppm for 3 h) resulted in a decrease in activity of Syrian hamster lung benzopyrene hydroxylase, a mixed-function oxidase that depends on cytochrome P-450. No changes in hepatic activities of this enzyme were observed, and the results were similar in animals in which high activities of benzopyrene hydroxylase had been induced. The maximal effect was not observed until a few days after the single ozone exposure. Palmer et also reported a decrease in rabbit tracheobronchial mucosal benzopyrene hydroxylase activity after exposure to similar ozone concentrations. 

Ricinoleic acid (Figure 3.8) is the major fatty acid found in castor oil from seeds of the castor oil plant (Ricinus communis Euphorbiaceae), and is the 12-hydroxy derivative of oleic acid. It is formed by direct hydroxylation of oleic acid (usually esterified as part of a phospholipid) by the action of an 02- and NADPH-dependent mixed function oxidase, but this is not of the cytochrome P-450 type. Castor oil has a long history of use as a domestic purgative, but it is now mainly employed as a cream base. Undecenoic acid (A9-undecenoic acid) can be obtained from ricinoleic acid by thermal degradation, and as the zinc salt or in ester form is used in fungistatic preparations. 

Clofibrate at a concentration of 0.5 mmol in culture medium maintained the cytochrome P-450 content of rat hepatocytes for up to 96 h. This effect was associated with a marked induction of lauric acid hydroxylation whereas little effect was observed on the metabolism of three other cytochrome p450 dependent mixed function oxidase substrates. 

Correia, M.A. and G.X Mannering (1973). Reduced diphosphopyridine nucleotide synergism of the reduced triphosphopyridine nucleotide dependent mixed function oxidase of hepatic microsomes. Role of the Type I drug binding site of cytochrome P-450. Mol. Pharmacol. 9, 470-485. 

---