Cytochrome P450 reversible

The answer is b. (Hardmanr p 906.) Cimetidine reversibly inhibits cytochrome P450. This is important in phase I bio transformation reactions and inhibits the metabolism of such drugs as warfarin, phenytoin, propranolol, metoprolol, quinidine, and theophylline. None of the other enzymes are significantly affected. 

Ketoconazole inhibits a variety of cytochrome P450 enzymes, including 11-hydroxylase and 17-hydroxylase. It is highly effective in lowering cortisol in Cushing s disease, and patients can be maintained successfully on therapy for months to years. The most common adverse effects are reversible elevation of hepatic transaminases and GI upset. It can cause gynecomastia and lower plasma testosterone values. 

Wadleigh RW, Yu SJ (1988) Detoxification of iso thiocyanate allelochemicals by glutathione transferase in three lepidopterous species. J Chem Ecol 14 1279-1288 Werck-Reichhart D, Feyereisen R (2000) Cytochromes P450 a success story. Genome Biol 1 1-9 Williams AB, Jacobs RS (1993) A marine natural product, patellamide D, reverses multidrug resistance in a human leukemic cell line. Cancer Lett 71 97-102 Yazaki K (2006) ABC transporters involved in the transport of plant secondary metabolites. FEBS Lett 580 1183-1191... 

Nishimura M, Yaguti H, Yoshitsugu H, Naito S, Satoh T (2003) Tissue distribution of mRNA expression of human cytochrome P450 isoforms assessed by high-sensitivity real-time reverse transcription PCR. Yakugaku Zasshi 123(5) 369-375. 

Ezetimibe does not appear to be a substrate for cytochrome P450 enzymes. Experience to date reveals a low incidence of reversible impaired hepatic function with a small increase in incidence when given with a reductase inhibitor. Myositis has been reported rarely. 

Rifabutin is derived from rifamycin and is related to rifampin. It has significant activity against M tuberculosis, M avium-intracellulare, and M fortuitum (see below). Its activity is similar to that of rifampin, and cross-resistance with rifampin is virtually complete. Some rifampin-resistant strains may appear susceptible to rifabutin in vitro, but a clinical response is unlikely because the molecular basis of resistance, rpoB mutation, is the same. Rifabutin is both substrate and inducer of cytochrome P450 enzymes. Because it is a less potent inducer, rifabutin is indicated in place of rifampin for treatment of tuberculosis in HIV-infected patients who are receiving concurrent antiretroviral therapy with a protease inhibitor or nonnucleoside reverse transcriptase inhibitor (eg, efavirenz)—drugs that also are cytochrome P450 substrates. 

Several additional studies were carried out to obtain information about the precise behavior of the various components in the model system. The interplay between the manganese porphyrin and the rhodium cofactor was found to be crucial for an efficient catalytic performance of the whole assembly and, hence, their properties were studied in detail at different pH values in vesicle bilayers composed of various types of amphiphiles, viz. cationic (DODAC), anionic (DHP), and zwitterionic (DPPC) [30]. At pH values where the reduced rhodium species is expected to be present as Rh only, the rate of the reduction of 13 by formate increased in the series DPPC < DHP < DODAC, which is in line with an expected higher concentration of formate ions at the surface of the cationic vesicles. The reduction rates of 12 incorporated in the vesicle bilayers catalyzed by 13-formate increased in the same order, because formation of the Rh-formate complex is the rate-determining step in this reduction. When the rates of epoxidation of styrene were studied at pH 7, however, the relative rates were found to be reversed DODAC DPPC < DHP. Apparently, for epoxidation to occur, an efficient supply of protons to the vesicle surface is essential, probably for the step in which the Mn -02 complex breaks down into the active epoxidizing Mn =0 species and water. Using a-pinene as the substrate in the DHP-based system, a turnover number of 360 was observed, which is comparable to the turnover numbers observed for cytochrome P450 itself. 

Farrell H, Correia MA. Structural and functional reconstitution of hepatic cytochrome P450 in vivo. Reversal of allylisopropylacetamide-mediated destruction of the hemoprotein by exogenous heme. J Biol Chem 1980 255 10128-10133. 

Microsomal cytochromes P450 often form hydrogen peroxide as a side product. This may arise directly from the Fe-O-O intermediate shown in Eq. 18-57. Some cytochromes P450 use this reaction in reverse to carry out hydroxylation utilizing peroxides instead of 02 (Eq. 18-58). 

Reverse transcriptase Hypothetical protein Ubiquitin Soma ferritin Cytochrome p450 NADH dehydrogenase Ribosomal proteins Transcription initation protein T ransposase Primosomal protein N ... 

Ames Test The Ames test, developed by Bruce Ames and his coworkers at the University of California, Berkeley, depends on the ability of mutagenic chemicals to bring about reverse mutations in Salmonella typhimurium strains that have defects in the histidine biosynthesis pathway. These strains will not grow in the absence of histidine but can be caused to mutate back to the wild type, which can synthesize histidine and hence can grow in its absence. The postmitochondrial supernatant (S-9 fraction), obtained from homogenates of livers of rats previously treated with PCBs in order to induce certain cytochrome P450 isoforms, is also included in order to provide the activating enzymes involved in the production of the potent electrophiles often involved in the toxicity of chemicals to animals. 

Patients with hepatic insufficiency may not tolerate the drug at usual doses, however, because of increased area under the concentration curve of both parent drugs and metabolites. This may necessitate a dose reduction to 7.5 mg/kg every 12 hours or 5 mg/kg every 8 hours in some patients. Quinupristin and dalfopristin are not metabolized by cytochrome P450 enzymes but significantly inhibit CYP 3 A4, which metabolizes warfarin, diazepam, astemizole, terfenadine, cisapride, nonnucleo- side reverse transcriptase inhibitors, and cyclosporine, among others. Dosage reduction of cyclosporine may be necessary. 

Interactions between NO and ferric hemes also result in reversible inhibition of cytochrome P450, catalase, and cytochrome c oxidase activity (66-68), through occupation of the active site and consequent competitive hindrance of mitochondrial respiration. 

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