Cytochrome P-450 isoforms

Werck-Reichhart, D., Gabriac, B., Teutsch, H., and Durst, F., Two cytochrome P-450 isoforms catalyzing O-de-ethylation of ethoxycoumarin and ethoxyresorufin in higher plants, Biochem. J., 270, 729-735, 1990. 

Suzuki A, lida I, Tanaka F, Akimoto M, Fukushima K, Tani M, Ishizaki T, Chiba K. Identification of human cytochrome P-450 isoforms involved in metabolism of R(+)- and S(-)-gallopamil utility of in vitro disappearance rate. Drug Metab Dispos 1999 27(ll) 1254-9. 

Murray, M. (1989) Inhibition of hepatic drug metabolism by phenothiazine tranquilizers quantitative structure-activity relationships and selective inhibition of cytochrome P-450 isoform-specific activities. Chcm. Res. Toxicol., 2, 240-246. 

Turin A, Amato G, Longo V, Gervasi PG (1998) Oxidation of methyl- and eth)d-tertiary-butyl ethers in rat liver microsomes role of the cytochrome P-450 isoforms. Arch Toxicol 72 207-214... 

Chiba, M. Nishime, J.A. Lin, J.H. Potent and selective inactivation of human liver microsomal cytochrome P-450 isoforms by L-754,394, an investigationed human immune deficiency virus protease inhibitor. J.Pharmacol.Exp.Ther., 1995, 275, 1527—1534... 

Kudo, S., M.G. Okumura, and T. Ishizaki (1999), Cytochrome P-450 isoforms involved in carboxylic acid ester cleavage of Hantzsch pyridine ester of pranidipine. Drug Metab. Dispos. 27, 303-308. 

Rifkind, A.B., A. Kanetoshi, J. Orlinick, J.H. Capdevila, and C. Lee (1994). Purification and biochemical characterization of two major cytochrome P-450 isoforms induced by 2,3,7,8-tetrachlorodibenzo-/7-dioxin in chick embryo liver. J. Biol. Chem. 269, 3387-3396. 

Suzuki A lida I, Tanaka F, et al. Identification of Human Cytochrome P-450 Isoforms Involved in Metabolism of R(-F)- and S(-)-Gallopamil Utility of In Vitro Disappearance Rate. Drug Metab Dispos. 1999 27 1254-1259. 

CYP2D6 The principal isoenzyme (isoform) of cytochrome P-450 involved in the metabolism of psychotropic drugs. 

SADP or sulfo-SADP also have been used to study the phenylalanine-methionine-arginine-phenylalanine-amide-activated sodium channel (Coscoy et al., 1998), various apolipoprotein E isoforms (Mann et al., 1995), the high-affinity phenylalkylamine Ca2+ antagonist binding protein from guinea pig (Moebius et al., 1994), the interaction of non-histone proteins with nucleosome core particles (Reeves and Nissen, 1993), and the interactions among cytochromes P-450 in the endoplasmic reticulum (Alston et al., 1991). See Chapter 28 for methods of using photoreactive heterobifunctional crosslinkers to study protein interactions. 

Because NO synthases belong to the same superfamily of enzymes as cytochrome P-450, they are able to produce not only nitric oxide (although it is undoubtedly their main function) but also other free radicals, first of all, superoxide. In 1992, Pou et al. [148] showed that brain nitric oxide synthase (NOS I) produced superoxide identified as a DMPO—OOH adduct in a calcium- or calmodulin-dependent manner. This finding was confirmed in numerous studies for all three isoforms of NO synthase. Although the structures of all the three NO oxidase... 

The formation of nitric oxide in microsomes results in the inhibition of microsomal reductase activity. It has been found that the inhibitory effect of nitric oxide mainly depend on the interaction with cytochrome P-450. NO reversibly reacts with P-450 isoforms to form the P-450-NO complex, but at the same time it irreversibly inactivates the cytochrome P-450 via the modification of its thiol residues [64]. Incubation of microsomes with nitric oxide causes the inhibition of 20-HETE formation from arachidonic acid [65], the generation of reactive oxygen species [66], and the release of catalytically active iron from ferritin [67],... 

Funae, Y., Okada, S., Isoforms of cytochrome P-450 on organic nitrate-derived nitric oxide release in human heart vessels. FEBS Lett. 452 (1999), p. 165-169... 

Shin, J.G., Soukhova, N. and Flockhart, D.A. (1999) Effect of antipsychotic drugs on human liver cytochrome P-450 (CYP) isoforms in vitro preferential inhibition of CYP2 D6. Drug Metabolism and Disposition, 27 (9), 1078-1084. 

Metabolism - Linezolid is primarily metabolized by oxidation of the morpholine ring, which results in 2 inactive metabolites. Linezolid is not detectably metabolized by human cytochrome P-450 and it does not inhibit the activities of clinically significant human CYP isoforms. 

Kobayashi K., T. Ishizuka, N. Shimada, Y. Yoshimura, K. Kamijima, and K. Chiba (1999). Sertraline N-demethylation is catalyzed by multiple isoforms of human cytochrome P-450 in vitro. Drug Metabolism and Disposition 27 763-766. 

Oxidation of ethanol. Although the major metabolic pathway for alcohols such as ethanol is oxidation catalyzed by alcohol dehydrogenase (see below), ethanol can also be metabolized by cytochrome P-450. The product, ethanal, is the same as produced by alcohol dehydrogenase. The isoform of cytochrome P-450 is CYP2E1. The mechanism may involve a hydroxylation to an unstable intermediate, which loses water to yield ethanal. Alternatively, a radical mechanism could be responsible. The importance of this route of metabolism for ethanol is that it is inducible (see chap. 5), assuming more importance after repeated exposure to ethanol such as in alcoholics and regular drinkers. 

The most important enzyme involved in bio transformation is cytochrome P-450, which catalyzes many phase 1 reactions. This enzyme is located primarily in the SER (microsomal fraction) of the cell and is especially abundant in liver cells. Cytochrome P-450 primarily catalyzes oxidation reactions and consists of many isoforms (isozymes). These isoenzymes have overlapping substrate specificities. The most important subfamily in humans is CYP3A4, although there is considerable variation in CYP3A4 expression between individuals. 

In vitro. The activity or absolute level of enzymes such as cytochrome P-450 and glucuronosyl transferase can be measured in cells, tissue fractions, or subcellular fractions (e.g., microsomes) and compared with those from control animals. The activity is measured by using a particular substrate for each of the isoforms of the enzyme (e.g., cytochrome P-450 or UDPGT) of interest. The total level of cytochrome P-450 could be determined by spectrophotometry using standard methods (e.g., carbon monoxide binding and difference spectra). Alternatively, the level of protein can be determined by gel electrophoresis and Western blotting, and this would allow the separation of different isoforms. 

Metabolic activation is catalyzed by cytochrome P-450, and the particular isoform (2E1, 1A2, or 3A4) depends on the dose. 

CYP3A4 is the most important isoform of cytochrome P-450 in humans. 

Porter TD, Coon MJ. Cytochrome P-450. Multiplicity of isoforms, substrates, and catalytic and regulatory mechanisms. J Biol Chem 1991 266 13469-13472. 

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