Cytochrome P-450-dependent

Clotrimazole and other azole derivatives have a different mode of action than the polyenes, eg, amphotericin B. The latter biad to the ergosterol present ia the membranes of yeasts and fungi, but azole derivatives inhibit the cytochrome P-450 dependent biosynthesis of ergosterol (8—11). This inhibition not only results in a reduction of ergosterol, but also in an accumulation of C-14 methyl sterols. They disturb membrane permeabiUty, inhibit cell rephcation, and are basically responsible, in combination with the reduction of ergosterol levels, for the antifungal action. 

Miconazole. Miconazole nitrate [22832-87-7] (Fig. 2), the 1-phenethyl-imidazole derivative first described in 1969, interferes at low doses with the cytochrome P-450 dependent ergosterol biosynthesis in yeasts and fungi. The result is accumulation of C-14 methylated sterols on the one hand and reduction of the ergosterol levels in the membranes on the other hand (12). Analogous to clotrimazole, this leads to a disturbance in the membranes it results in inhibition of ceU repHcation, mycelium development (in C. albicans) and finally, ceU death. High concentrations of miconazole, which may be achieved with topical use, disturb the orientation of phosphoHpids in the membranes, which produces leaks (13). 

Like the a2ole derivatives, it inhibits the biosynthesis of ergosterol. However, naftifine [65472-88-0] does not inhibit the cytochrome P-450 dependent C-14-demethylase, but the epoxidation of squalene. Squalene epoxidase cataly2es the first step in the conversion of squalene via lanosterol to ergosterol in yeasts and fungi or to cholesterol in mammalian cells. The squalene epoxidase in C. albicans is 150 times more sensitive to naftifine, C2 H2 N, than the en2yme in rat fiver (15). Naftifine is available as a 1% cream. 

Plasma levels of 3—5 p.g/mL are obtained two hours after adraiinistration of 200 mg ketoconazole. No accumulation in the bloodstream was noted after a 30-wk treatment with this dose. The half-life is approximately eight hours. When ketoconazole is taken with meals, higher plasma levels are obtained. Distribution studies using radioactive ketoconazole in rats show radioactivity mainly in the Hver and the connective tissue. Radioactivity is also present in the subcutaneous tissue and the sebaceous glands. After one dose of 200 mg in humans, ketoconazole is found in urine, saUva, sebum, and cenimen. Like miconazole, the mode of action is based on inhibition of the cytochrome P-450 dependent biosynthesis of ergosterol. This results in disturbed membrane permeabiUty and membrane-bound enzymes (8,10,23,25). 

FIGURE 24.27 Dicarboxylic acids can be formed by oxidation of the methyl group of fatty acids in a cytochrome P-450-dependent reaction. 

Chemical synthesis of cytochrome P-450-dependent metabolites (epoxyeico-satriene acids and other metabolites possessing heterocyclic fragments) 98MI9. 

The cytochrome P-450-dependent metabolism of trichloroethylene was studied in hepatic microsomal fractions from 23 different humans (Lipscomb et al. 1997). CYP2E1 was the predominant form of P-450 responsible for the metabolism of trichloroethylene in humans. Incubations of trichloroethylene with the microsomal preparations resulted in hyperbolic plots consistent with Michaelis-Menton kinetics. The values ranged from 12 to 55.7 pM, and were not normally distributed, and the values range from 490 to 3,455 pmol/min/mg protein and were normally distributed. The study authors concluded that the human variability in metabolism of trichloroethylene via P-450-dependent pathways was within a 10-fold range. 

Dekant W, Martens G, Vamvakas S, et al. 1987. Bioactivation of tetrachloroethylene. Role of glutathione S-transferase-catalyzed conjugation versus cytochrome P-450-dependent phospholipid alkylation. Drug Metab Dispos Biol Fate Chem 15 702-709. 

Romesser JA, DP O Keefe (1986) Induction of cytochrome P-450-dependent sulfonylurea metabolism in Streptomyces griseolus. Biochem Biophys Res Comm 140 650-659. 

Berg A, JA Gustafsson, M Ingelman-Sundberg, K Carlstrdm (1976) Characterization of a cytochrome P-450-dependent steroid hydroxylase present in Bacillus megaterium. J Biol Chem 251 2831-2838. 

Cox HHJ, BW Faber, VNM van Heiningen, H Radhoe, HI Doddema, W Harder (1996) Styrene metabolism in Exophilia jeanselmei and involvement of a cytochrome P-450-dependent styrene monooxygenase. Appl Environ Microbiol 62 1471-1474. 

Ingelman-Sundberg, M. and Ekstrom, G. (1982). Aniline is hydroxylated by the cytochrome P-450-dependent hydroxyl radical-mediated oxygenation mechanism. Biochem. Biophys. Res. Commun. 106, 625-631. 

Tetraethyl and tetramethyl lead under oxidative dealkylation metabolize to the highly neurotoxic metabolites, triethyl and trimethyl lead, respectively. In the liver, the reaction is catalyzed by a cytochrome P-450 dependent monoxygenase system (Kimmel et al. 1977). Complete oxidation of alkyl lead to inorganic lead also occurs (Bolanowska 1968). 

Andrae U. Evidence for the involvement of cytochrome P-450-dependent mono-oxygenase(s) in the formation of geno-toxic metabolites from N-hydroxyurea. Biochem Biophys Res Commun 1984 118 409-415. 

Miranda, C.L., M.C. Henderson, J.L. Wang, H.S. Nakaue, and D.R. Buhler. 1987. Effects of polychlorinated biphenyls on porphyrin synthesis and cytochrome P-450-dependent monooxygenases in small intestine and liver of Japanese quail. Jour. Toxicol. Environ. Health 20 27-35. 

Schwen, R.J. and G.J. Mannering. 1982a. Hepatic cytochrome P-450-dependent monooxygenase systems of the trout, frog and snake. II. Monooxygenase activities. Comp. Biochem. Physiol. 71B 437-443. 

Bisbenzylisoquinoline alkaloids are dimeric benzyltetrahydroisoquinoline alkaloids that are known for their pharmacological activities. A well-described example is the muscle relaxant (+)-tubocurarine, which in crude form serves as an arrow poison for South American Indian tribes. In the biosynthesis of this broad class of dimeric alkaloids, it has been postulated that the mechanism of phenol coupling proceeds by generation of phenolate radicals followed by radical pairing to form either an inter- or intramolecular C - O or C - C bond. Enzyme studies on the formation of bisbenzylisoquinoline alkaloids indicated that a cytochrome P-450-dependent oxidase catalyzes C - O bound formation in the biosynthesis of berbamunine in Berberis cell suspension culture.15 This enzyme, berbamunine synthase (CYP80A1), is one of the few cytochromes P-450 that can be purified to... 

Fig. 10.3 Reaction catalyzed by the cytochrome P-450-dependent oxidase berbamunine synthase (CYP80A1). This enzyme creates a branchpoint in the (5)-reticuline biosynthetic pathway to form the bisbenzylisoquinoline alkaloids.

PAULI, H.H., KUTCHAN, T.M., Molecular cloning and functional heterologous expression of two alleles encoding (5)-Y-methylcoclaurine 3 -hydroxylase (CYP80B1), a new methyl jasmonate-inducible cytochrome P-450-dependent monooxygenase of benzylisoquinoline alkaloid biosynthesis, Plant J., 1998,13, 793-801. 

The metabolism of carbon tetrachloride proceeds via cytochrome P-450-dependent dehalogenation (Sipes et al. 1977). The first step involves cleavage of one carbon-chlorine bond to yield Cl- and a trichloromethyl free radical, which is then oxidized to the unstable intermediate trichloromethanol, the precursor of phosgene. Hydrolytic dechlorination of phosgene yields C02 and HC1 (Shah et al. 1979). Although there are similarities in the metabolism of chloroform and carbon tetrachloride, metabolic activation of chloroform produces primarily phosgene, whereas the level of phosgene production from... 

Robacker KM, Kulkarni AP, Hodgson E. 1981. Pesticide induced changes in the mouse hepatic microsomal cytochrome P-450-dependant monooxygenase system and other enzymes. J Environ Sci Health (Part B Pestic Food Contam Agric Wastes) 16(5) 529-546. 

Yarbrough JD, Grimley JM, Alley EG. 1986a. Induction of the hepatic cytochrome P-450 dependent monooxygenase system by cis- and trans-5,10-dihydrogen mirex. Toxicol Lett 32 65-71. 

In general, our studies with cytochrome P-450-dependent metabolism have emphasized the similarity of the hepatic MFO system in marine fish to that found in mammals. Thus, in the little skate (Raja erinaoea), a marine elasmobranch, enzyme activity is localized in the microsomal fraction, requires NADPH and molecular oxygen for maximum activity, and can be inhibited with CO (1, 2). Moreover, when hepatic microsomes from the little skate were solubilized and separated into cytochrome P-450, NADPH-cytochrome P-450 reductase, and lipid fractions, all three fractions were required for maximal MFO activity in the reconstituted system (3). We have also found, as have others, that the administration of polycyclic hydrocarbons (3-methylcholanthrene, 1,2,3,4-dibenzanthracene [DBA]), 2,3,7,8-tetrachlorodibenzo-p-dioxin... 

The decrease in cytochrome P-450 content correlated with a lowering of trout AE activity observed in hepatic microsomes recovered from fish fed high levels of casein, versus those from fish fed low casein diets. As shown in Table III, up to a 32% decrease in the production of the epoxide dieldrin was noted. Similar results have been observed in 10 month old rainbow trout with a nearly identical maximum decrease (unpublished data). Since AFB activation has been shown to involve a cytochrome P-450 dependent enzyme system (19, 20, 22) and trout... 

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