Mixed-function oxidases cytochrome metabolism

In silico estimation of metabolism is still an area of intense study and development. Accurate prediction of intrinsic clearance is still not possible with the currently available methods [15]. Most of the progress in this area has been focused on the mixed function oxidase cytochrome P450 enzyme family. Advances in this area have been focused on three areas (1) prediction of the cytochrome P450 (CYP) enzyme isotype that is responsible for the major metabolism, (2) prediction of the chemical site of a molecule that is most likely to undergo biotransformation by oxidative metabolism, and (3) structure-based docking studies of CYP enzyme substrate complexes. ... 

Further biotransformations of A VPA involve both the liver microsomal cytochrome P-450 enzymes and the fatty acid 3-oxidation pathway (Fig. 32.28). The mixed-function oxidase system metabolizes the unsaturated metabolite to a y-butyrolactone derivative through a chemically reactive entity that is a suicide-substrate inhibitor of cytochrome P-450. The alkylation of the prosthetic haem by means of the radical occurs prior to the formation of the epoxide. Thus the epoxide is not involved in the cytochrome P-450 inhibition. 

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. 

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. 

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. 

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. 

Hepatic mixed-function oxidase activities demonstrated seasonal trends, with higher specific activities in the cold weather months in both populations with few differences in enzyme activities or cytochrome levels between the two populations. Metabolism of aldrin, dieldrin and DDT was similar between the two populations. R fish have larger relative liver size and, therefore, a greater potential for xenobiotic metabolism. However, biotransformation appears to be of minor importance in chlorinated alicyclic insecticide resistance in mosquitofish barriers to penetration appear to be of greater importance and an implied target site insensitivity appears to be the most important factor in resistance. 

In mammals the cytochrome P-U50 mediated monooxygenase or mixed function oxidase system involved in the elimination of lipophilic environmental contaminants and other foreign compounds, has been implicated in the carcinogen activation process. There are several distinct variants of cytochrome P-U50 in mammalian tissues and there may be more than one form of this ubiquitous cytochrome also in fish. The significance of this lies in the fact that different forms of cytochrome P-U50 result in different metabolite patterns, which in turn may reflect on the carcinogenicity or toxicity of compounds being metabolized. 

The plasma protein binding of tacrolimus is approximately 99%. Tacrolimus is bound mainly to albumin and alpha-1-acid glycoprotein and has a high level of association with erythrocytes. It is extensively metabolized by the mixed-function oxidase system, primarily the cytochrome P450 system (CYP3A). The disposition of tacrolimus from whole blood was biphasic with a terminal elimination half-life of 11.7 hours in liver transplant patients. 

Cimetidine binds to cytochrome P-450 hepatic mixed-function oxidase enzymes. This leads to enzyme inhibition and reduced metabolism of... 

This enzyme system, also known as the mixed function oxidase system, uses NADPH as a cofactor in the metabolism of ethanol (Figure 23-1, right) and consists primarily of cytochrome P450 2E1, 1A2, and 3A4 (see Chapter 4). 

Although first reported with the cytochrome(s) P-450 mixed function oxidases, it is now known that a number of the enzymes involved in the metabolism of foreign compounds are inducible. Thus, as well as the CYPs, NADPH cytochrome P-450 reductase, cytochrome b5, glucuronosyl transferases, epoxide hydrolases, and GSTs are also induced to various degrees. However, this discussion concentrates on the induction of the CYPs with mention of other enzymes where appropriate. 

Enzymes responsible for metabolism are located at various subcellular sites, for example the cytosol, mitochondria and smooth endoplasmic reticulum. However, it is enzymes derived from endoplasmic reticulum, called mixed function oxidases or monooxygenases , which have been most intensely studied in the past two or three decades. These enzyme systems, which utilize a family of haemoprotein cytochromes, or P-450 as terminal oxidases, require molecular oxygen and reduced nicotinamide adenine dinucleotide phosphate (NADPH) for activity. The overall stoichiometry of the reactions catalyzed by these enzymes is normally represented by equation (1). 

Pond et al (23) reported that the rate of metabolism of tolbutamide was decreased by chronic adminstration of certain drugs, they claimed that the tolbutamide half-life was increased by chronic adminstration of sulphaphenazole (9.5 hrs to 28.6 hrs), phenylbutazone (7.9 hrs to 23.1 hrs), and oxyphenbutazone (8.1 hrs to 30.2 hrs). The rate of elimination of tolbutamide was decreased within one to two hours after a single dose of sulphaphenazole and the half-life was increased from 9.2 hrs to 25.7 hrs. In contrast, phenylbutazone and oxyphenbutazone, adminstered as a single dose 800 mg have no immediate effect on tolbutamide elimination. It is suggested that phenylbutazone and oxyphenbutazone act by inducing a form of a cytochrome P-450 with low activity for tolbutamide hydroxylation, whereare, sulphaphenazole acts by direct inhibition of the microsomol mixed function oxidase system. 

Oxidation is by far the most important Phase I metabolic reaction. One of the main enzyme systems involved in the oxidation of xenobiotics appears to be the so called mixed function oxidases or monooxygenases, which are found mainly in the smooth endoplasmic reticulum of the liver but also occur, to a lesser extent, in other tissues. These enzymes tend to be nonspecific, catalysing the metabolism of a wide variety of compounds (Table 9.2). Two common mixed function oxidase systems are the cytochrome P-450 (CYP-450) and the flavin monoxygenase (FMO) systems (Appendix 12). The overall oxidations of these systems take place in a series of oxidative and reductive steps, each step being catalysed by a specific enzyme. Many of these steps require the presence of molecular oxygen and either NADH or NADPH as co-enzymes. 

There are several known effects of BHA treatment which have been proposed to reduce the levels of reactive metabolites of BP which bind to DNA. It has been demonstrated that BHA feeding alters the microsomal mixed-function oxidase system in mice and rats (7,17-19). Several studies suggest that BHA treatment does not decrease the amounts of BP-7, 8-diol formed (7,20,21) whereas there is some indirect evidence that BHA treatment alters the metabolism of BP-7,8-diol (3,20) An induction by BHA of an lsozyme(s) of cytochrome P-450, which has kinetics of metabolism of BP-7,8-diol different than that of the constitutive isozyme(s), could account for the BHA-induced shifts in the dose-response curve for BPDEI-DNA adduct levels (15). 

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