Metabolic functions mixed function oxidases

Most phase one reactions are catalyzed by the drug-metabolizing enzymes (mixed function oxidases, oxygenases) located in the endoplasmic reticulum of liver and, to a lesser extent, in intestine, kidney, and lung. These enzymes have been the subject of intensive research (G7, G8, LI). 

Rapid (< 1.5 min) Moderate (few hours) Metabolized by mixed function oxidases in liver. Some metabolites are active. Most frequently used. 

By enhancing the metabolism of trichloroethylene to its cytotoxic metabolites, compounds that induce the hepatic mixed-function oxidase system can potentiate the hepatotoxicity of trichloroethylene. 

Attempts to diminish the overall metabolism of trichloroethylene might be useful (e.g., hypothermia, mixed-function oxidase inhibitors, competitive inhibitors of trichloroethylene metabolism [i.e., P-450 substrates]), if instituted soon enough after trichloroethylene exposure. Catecholamines (especially beta agonists) act in concert with trichloroethylene, increasing the risk of cardiac arrhythmias. Hence, catecholamines should be administered to patients only in the lowest efficacious doses and for certain limited presentations of trichloroethylene poisoning. Ethanol should also be avoided because concurrent exposure to trichloroethylene and ethanol can cause vasodilation and malaise and may potentiate central nervous system depression at high dosage levels of either compound. 

Figure 5. Metabolic activation pathways of BA. MFO abbreviates for the cytochrome P-450-containing mixed-function oxidases. The absolute configurations of the metabolites are as shown.

Oxidation is intimately linked to the activation of polycyclic aromatic hydrocarbons (PAH) to carcinogens (1-3). Oxidation of PAH in animals and man is enzyme-catalyzed and is a response to the introduction of foreign compounds into the cellular environment. The most intensively studied enzyme of PAH oxidation is cytochrome P-450, which is a mixed-function oxidase that receives its electrons from NADPH via a one or two component electron transport chain (10. Some forms of this enzyme play a major role in systemic metabolism of PAH (4 ). However, there are numerous examples of carcinogens that require metabolic activation, including PAH, that induce cancer in tissues with low mixed-function oxidase activity ( 5). In order to comprehensively evaluate the metabolic activation of PAH, one must consider all cellular pathways for their oxidative activation. 

Lipid-soluble xenobiotics are commonly biotra ns formed by oxidation in the drug-metabolizing microsomal system (DMMS). For each description below, choose the component of the microsomal mixed-function oxidase system with which it is most closely associated ... 

L The answer is d. (Hardman, p 906.) Cimetidine slows the metabolism of Ca channel blockers, which are substrates for hepatic mixed-function oxidases. Inhibition of cytochrome P450 activity is peculiar to cimetidine and is not a mechanism of action of other histamine 2 (Hz) blockers. 

Rapidly metabolize PAHs by liver mixed-function oxidases, with little evidence of accumulation... 

Melius, P. and D.L. Elam. 1983. Mixed function oxidase in sea catfish. Pages 877-895 in M. Cooke and A.J. Dennis (eds.). Polynuclear Aromatic Hydrocarbons Formation, Metabolism and Measurement. Battelle Press, Columbus, OH. 

The assay can be performed using mutagenic substances that react directly with DNA or, where metabolic activation is necessary, with pre-mutagen in the presence of rat liver homogenate that is enriched in mixed function oxidases (termed S9). Metabolic oxidation (if that is what is required) results in ultimate or penultimate mutagenic forms, which act as electrophiles towards S. typhimurium. 

Hexachloroethane is metabolized by the mixed function oxidase system by way of a two-step reduction reaction involving cytochrome P-450 and either reduced nicotinamide adenine dinucleotide phosphate (NADPH) or cytochrome b5 as an electron donor. The first step of the reduction reaction results in the formation of the pentachloroethyl free radical. In the second step, tetrachloroethene is formed as the primary metabolite. Two chloride ions are released. Pentachloroethane is a minor metabolic product that is generated from the pentachloroethyl free radical. 

Liver necrosis is another concern following hexachloroethane exposure. Hexachloroethane is metabolized in the centrilobular area of the liver by way of the microsomal mixed function oxidase system. The relatively nonpolar pentachloroethyl free radical is an intermediate in this pathway. The reaction of the free radical with unsaturated lipids in the cellular or organelle membranes could contribute to hepatocyte damage and necrosis. 

Environmental agents that influence microsomal reactions will influence hexachloroethane toxicity. The production of tetrachloroethene as a metabolite is increased by agents like phenobarbital that induce certain cytochrome P-450 isozymes (Nastainczyk et al. 1982a Thompson et al. 1984). Exposure to food material or other xenobiotics that influence the availability of mixed function oxidase enzymes and/or cofactors will change the reaction rate and end products of hexachloroethane metabolism and thus influence its toxicity. 

Approximately 10-20% of -hexane absorbed by inhalation is excreted unchanged in exhaled air the remainder is metabolized. Metabolism takes place via mixed-function oxidase reactions in the liver. In a study in which metabolites were measured in workers exposed to 77-hexane (Perbellini et al. 1981), mean concentrations of 77-hexane metabolites in urine were 2,5-hexanedione, 5.4 mg/L 2,5-dimethylfuran,... 

Robertson P, White EL, Bus JS. 1989. Effects of methyl ethyl ketone pretreatment on hepatic mixed-function oxidase activity and on in vivo metabolism of -hexane. Xenobiotica 19(7) 721-729. 

Table II summarizes the results together with the detailed experimental conditions. As is evident, metabolic activities were detectable in these 3 aquatic species, but the rate was far lower as compared with mammalian hepatic enzume preparations, and the oxidative activities in snail were particularly low although the possibility was not ruled out of the presence of inhibitors of mixed-function oxidases in the fractions. The O-demethylation reaction proceeds extremely slowly in the enzyme preparation of aquatic animals, at less than one hundredth that of mammals.

---