Cytochrome isozymes

The norepinephrine selective uptake inhibitor reboxetine (21) is rapidly and completely absorbed, and is metabolized mainly by the cytochrome p450 3A4 because it does not interact with CYP 2D6 there is less risk of interactions with other drugs (81 -83).Mirtazepine (14)also shows little interaction with p450 cytochrome isozymes and there is only a low risk of drug interactions (84). Mirtazepine is a racemate, and the two enantiomers are eliminated at different rates, with a twofold higher rate of elimination of the (S)-enantiomer than of the (i )-enantiomer (84). 

Zafirlukast inhibits the activity of cytochrome isozymes CYP 3A4 and CYP 2C9, resulting in the interruption of metabolism of other drugs metabolized by this pathway. Montelukast has no effect on the activity or level of these isozymes and thus has less potential for drug interaction. 

Compounds having the 16,17 ketal, eg, budesonide, amcinonide, fluocinonide, halcinonide, triamcinolone acetonide, and flurandrenohde, also undergo metabohsm by routes that parahel that of cortisol metabohsm. Unsymmetrical acetals such as budesonide are also metabolized by routes not available to the more metabohcahy stable symmetrical 16a,17a-isopropyhdiene-dioxysubstituted compounds (desonide, flunisohde, and triamcinolone acetonide). Isozymes within the cytochrome P450 3A subfamily are thought to catalyze the metabohsm of budesonide, resulting in formation of 16a-hydroxyprednisolone and 6P-hydroxybudesonide (19,20) (Fig. 3) in addition to the more common metabohc steps (oxidation via reduction of A, etc). 

The hver microsomal dmg-metabolizing system is of particular importance. This oxidative pathway is mediated by isozymes of the cytochrome family (Fig. 4). At least ten enzyme families exist to accommodate the abiUty of humans to handle many foreign molecules (21). 

Cytochrome P450 (CYP) Cytochrome P450 isozymes Cytochrome P450 mono-oxygenases Mixed function oxidases... 

Nakajima T, Wang RS, Elovaara E, et al. 1992a. A comparative study on the contribution of cytochrome P-450 isozymes to metabolism of benzene, toluene and trichloroethylene in rat liver. Biochem Pharmacol 43 251-257. 

Polymorphic metabolism Genetically determined rates of metabolism (fast vs. slow) by selected isozymes of cytochrome P-450 drug-metabolizing enzymes. 

Other drag molecules can behave as inhibitors of specific cytochrome P450s. Inhibition of cytochrome P450 isozymes can lead to a slowing down of the metab-... 

Figure 1.10 Relative contributions of different cytochrome P450 isozymes to drug metabolism in humans.

Owing to the fact that ethyl ethers are especially effective substrates for CYP1A1 [184], the probe possesses an ethyl group on the phenolic oxygen of the trimethyl lock. In vitro, fluorescence was manifested by CYP1A1 isozyme with Kcat/KM 8.8 x 103 M-1s 1 and KM 0.09 pM. In cellulo, the probe revealed the induction of cytochrome P450 activity by the carcinogen 2,3,7,8-tetrachlorodi-benzo-p-dioxin (TCDD), and its repression by the chemoprotectant resveratrol. 

Parkinson, A., Thomas, P.E., Ryan, D.E. etal. (1983) Differential time course of induction of rat liver microsomal cytochrome P 450 isozymes and epoxide hydrolase by Aroclor 1254. Archives of Biochemistry and Biophysics, 225, 203-215. 

Area under the plasma concentration-time profile Chromium-51-labeled ethylenediamine-tetraacetic acid Cytochrome P450, 3A4 isozyme... 

The virtual compounds can be screened against structural models of the metabolizing enzymes, including the known SNP variants. These procedures are becoming widely adopted for the cytochrome P450 isozymes involved in oxidative drug metabolism. 

Figure 1. The major pathways in the metabolism of BaP to BaP epoxides, dihydrodiol, and 7,8-dihydrodiol-9,10-epoxides. The absolute configurations are as shown. The position of trans-addition of water is shown by an arrow. The optical purity of the 4,5-epoxide formed in BaP metabolism is dependent on the cytochrome P-450 isozymes present in the microsomal enzyme system. EH epoxide hydrolase.

BA trans-3.4-dihvdrodiol cannot be separated from BA trans-8.9-dihydrodiol in several HPLC conditions (27-29). Quantification of BA trana-3,4-dihydrodiol by HPLC can only be accomplished after converting the 3,4-dihydrodiol to its diacetate (25.26). The BA trans-3.4-dihydrodiol formed in BA metabolism by liver microsomes from pheno-barbital-treated rats was determined to have a 3R,4R/3S,4S enantiomer ratio of 69 31 (30). Recently we have determined the optical purity of the BA trans-3.4-dihvdrodiol formed in the metabolism of BA by three liver microsomes prepared from untreated rats and rats that had been pretreated with an enzyme inducer. As shown in Table II, cytochrome P-450 isozymes contained in liver microsomes from 3-methylcholanthrene- or phenobarbital-treated rats had similar stereoselectivity toward the 3,4-double bond of BA. BA trans-3.4-dihydrodiol is formed via the 3,4-epoxide intermediate (31). 

The 8,9- and 10,11-dihydrodiols formed in the metabolism of BA and DMBA respectively are all highly enriched (>90%) in R,R enantiomers (Table III). Labeling experiments using molecular oxygen-18 in the in vitro metabolism of the respective parent compounds and subsequent mass spectral analyses of dihydrodiol metabolites and their acid-catalyzed dehydration products indicated that microsomal epoxide hydrolase-catalyzed hydration reactions occurred exclusively at the nonbenzylic carbons of the metabolically formed epoxide intermediates (unpublished results). These findings indicate that the 8,9- and 10,11-epoxide intermediates, formed in the metabolism of BA and DMBA respectively, contain predominantly the 8R,9S and 10S,11R enantiomer, respectively. These stereoselective epoxidation reactions are relatively insensitive to the cytochrome P-450 isozyme contents of different rat liver microsomal preparations (Table III). 

Saito et al. (134) found that the cytosolic nitroreductase activity was due to DT-diaphorase, aldehyde oxidase, xanthine oxidase plus other unidentified nitroreductases. As anticipated, the microsomal reduction of 1-nitropyrene was inhibited by 0 and stimulated by FMN which was attributed to this cofactor acting as an electron shuttle between NADPH-cytochrome P-450 reductase and cytochrome P-450. Carbon monoxide and type II cytochrome P-450 inhibitors decreased the rate of nitroreduction which was consistent with the involvement of cytochrome P-450. Induction of cytochromes P-450 increased rates of 1-aminopyrene formation and nitroreduction was demonstrated in a reconstituted cytochrome P-450 system, with isozyme P-448-IId catalyzing the reduction most efficiently. 

Arylthioethylsydnone 73 (TTMS) is known to act as a mechanism-based inhibitor of some cytochrome P450 isozymes and as an inducer of cytochrome P450 3A <1996MI676, 1996MI872, 1998MI739>. 

Di, L. et al. 2006. Comparison of cytochrome P450 inhibition assays for drug discovery using human liver microsomes with LC-MS, rhCYP450 isozymes with fluorescence, and double cocktail with LC/ S. Int. J. Pharmaceut. http //dx.doi.org/10.1016/j.ijpharm.2006.10.039. 

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. 

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