Cytochrome P-450 enzyme system

Because LCEC had its initial impact in neurochemical analysis, it is not, surprising that many of the early enzyme-linked electrochemical methods are of neurologically important enzymes. Many of the enzymes involved in catecholamine metabolism have been determined by electrochemical means. Phenylalanine hydroxylase activity has been determined by el trochemicaUy monitoring the conversion of tetrahydro-biopterin to dihydrobiopterin Another monooxygenase, tyrosine hydroxylase, has been determined by detecting the DOPA produced by the enzymatic reaction Formation of DOPA has also been monitored electrochemically to determine the activity of L-aromatic amino acid decarboxylase Other enzymes involved in catecholamine metabolism which have been determined electrochemically include dopamine-p-hydroxylase phenylethanolamine-N-methyltransferase and catechol-O-methyltransferase . Electrochemical detection of DOPA has also been used to determine the activity of y-glutamyltranspeptidase The cytochrome P-450 enzyme system has been studied by observing the conversion of benzene to phenol and subsequently to hydroquinone and catechol... 

The majority of antiretroviral medications are metabolized by the cytochrome P-450 enzyme system (CYP). Therefore, it is important to review patient medication profiles for drugs that may interact with antiretroviral drugs. 

Recent work in our laboratories has confirmed the existence of a similar pathway in the oxidation of vindoline in mammals (777). The availability of compounds such as 59 as analytical standards, along with published mass spectral and NMR spectral properties of this compound, served to facilitate identification of metabolites formed in mammalian liver microsome incubations. Two compounds are produced during incubations with mouse liver microsome preparations 17-deacetylvindoline, and the dihydrovindoline ether dimer 59. Both compounds were isolated and completely characterized by spectral comparison to authentic standards. This work emphasizes the prospective value of microbial and enzymatic transformation studies in predicting pathways of metabolism in mammalian systems. This work would also suggest the involvement of cytochrome P-450 enzyme system(s) in the oxidation process. Whether the first steps involve direct introduction of molecular oxygen at position 3 of vindoline or an initial abstraction of electrons, as in Scheme 15, remains unknown. The establishment of a metabolic pathway in mammals, identical to those found in Strep-tomycetes, with copper oxidases and peroxidases again confirms the prospective value of the microbial models of mammalian metabolism concept. 

The measurement of the ethoxyresorufin-O-deethylase (EROD) activity is another sensitive parameter to detect the effects of paper mill industrial effluents on living organisms in the receiving waters. The EROD activity is a measure of the activity of the cytochrome P-450 enzyme system, which plays a central role in the transformation and elimination of xenobiotics. Increased EROD activity has been shown as far as 40 km from pulp mills, and EROD induction in fish caused by pulp mill effluents remains after biological treatment [60]. It is specified that EROD activity and erythrocytic nuclear abnormalities are induced by abietic and dehydroabietic acid [60]. 

Oxidation Carbons Adjacent to sp Atoms. Carbon atoms that are situated adjacent to imine, carbonyl, or aromatic groups are frequently oxidized. This reaction appears to be catalyzed by the cytochrome P-450 enzyme system. Typically, a hydroxyl group is attached to the carbon as part of the oxidation process. 

Cyclosporin is metabolised by the hepatic cytochrome P-450 enzyme system, and enzyme induction by phenobarbital, phenytoin, carbamazepine, or rifampicin will drastically increase the clearance of cyclosporin. Concurrent administration of these drugs has caused rejection of transplanted organs. Conversely, the use of enzyme inhibitors, such as erythromycin or the azole antifungal agents, e.g. ketoconazole, will increase the blood concentrations of cyclosporin leading to an increased risk of toxic side effects. 

The cytochrome P-450 enzyme system, responsible for the cleavage of Maillard compounds, is highly sensitive to pH change, maximum activity being at about pH 7.2. A small change of the pH in either direction decreases considerably the activity of the enzyme system. 

Frerichs et al. [128] developed and validated a method for the quantitation of omeprazole and hydroxyomeprazole from one 250 [A sample of human plasma using HPLC coupled to tandem mass spectrometry. The method was validated for a daily working range of 0.4-100 ng/ml, with limits of detection between 2 and 15 pg/ml. The interassay variation was less than 15% for all analytes at four control concentrations and the samples were stable for three freeze-thaw cycles under the analysis conditions and 24 h in the postpreparative analysis matrix. The method was used to analyze samples in support of clinical studies probing the activity of the cytochrome P-450 enzyme system. 

In humans, CQ is metabolized in the liver, mostly by oxidation via the cytochrome P-450 enzyme system, leading to /V-desethylchloroquine and then didesethylchloro-quine [151]. Successive dealkylation of the side chain ultimately produces the 7-chloro-4-aminoquinoline. In comparison to CQ, the metabolism of FQ was... 

The renal cytochrome P-450 enzyme system is involved in oxidative reactions in which an atom of molecular oxygen is inserted in an organic molecule. The flavoprotein NADPH-cytochrome P-450 reductase is an essential component of the mixed-function oxidase systems (MFO). Microsomal membranes appear to be particularly subject to attack by reactive oxygen radicals due to their high content of unsaturated fatty acids and the presence of the cytochrome P-450 system [40]. Cephaloridine-induced peroxidation of membrane lipids is decreased by the cytochrome P-450 inhibitor cobalt chloride [31], suggesting a role for a cytochrome P-450 reductase in the P-lactam-induced generation of reactive oxygen species and subsequent peroxidation products. 

The cytochrome P-450 enzyme system is comprised of multiple isoenzymes that are inducible to varying degrees [32]. These two characteristics are major determinants of metabohc pathways and rates. Induction can be caused by exposure to one or more of a large... 

Differences in biotransformation capacities have been identified for at least two distinct groups poor metabolizers and extensive metabolizers. Poor metabolizers demonstrate a deficiency in one or more pathways of the cytochrome P/450 enzyme system. Since the metabolism of antidepressants and neuroleptics depends on this system (see appendix A), poor metabolizers may be at risk for complications of therapy. For certain tricyclic antidepressants and neuroleptics, it has been established that these individuals will demonstrate increased serum levels and exaggerated medication response. Additionally, they wiU be more susceptible to side effects. Although data is limited on the SSRIs, similar patterns are evident. 

Drug interactions with hawthorn are theoretically possible with cardioactive medications, but have not been documented (2). In addition, the flavonoid constituents have been shown to have inhibitory and inducible effects on the cytochrome P-450 enzyme system, making other drug interactions possible (20). However, an in vivo study of a potential pharmacokinetic interaction of digoxin and hawthorn demonstrated that concurrent administration had no effect on digoxin pharmacokinetics, suggesting that the two could be safely administered together from a pharmacokinetic point of view (21). However, one must be mindful of additive effects and a potential pharmacodynamic interaction. 

Lotlikar PD, Baldy WJ Jr, Dwyer EN. 1975. Dimethylnitrosamine demethylation by reconstituted liver microsomal cytochrome P-450 enzyme system. Biochem J 152 705-708. 

The cytochrome P-450 enzyme system that has been characterized structurally is the form cytochrome P-450cam, which catalyzes the oxidation of the monoter-... 

It is now recognized that the majority of drug-drug interactions are mediated by two intimately related mediators, the cytochrome P-450 enzyme system and... 

Most phase I processes are catalyzed by the cytochrome P-450 enzyme system associated with the endoplasmic reticulum of the cell and occurring most abundantly in the livers of vertebrates. 

In users of Ge preparations, particularly high accumulations of this element were found in spleen, renal cortex, brain, and skeletal muscle (Nagata et al. 1985). Organic Ge preparations are biotransformed by the liver microsomal cytochrome P-450 enzyme system, probably through a C-hydroxylation mechanism (Prough et al. 

Because phenobarbital induces the microsomal cytochrome P-450 enzyme system responsible for MBOCA hydroxylation (Chen et al. 1991), phenobarbital treatment might increase the breakdown of MBOCA and speed its removal from the body. However, because MBOCA s metabolites are more toxic than the unmetabolized moiety, this intervention must be used with caution. Such treatment could also affect MBOCA adduct formation. While phenobarbital did not increase in wVo hemoglobin adduct formation in one study in rats (Chen et al. 1991), Cheever et al. (1991) found an increase in... 

Mechanism. Numerous studies on the mechanism of IPO toxicity have supported the view that tissue damage by the compound is due to a highly reactive, alkylating metabolite(s) (Figure 2)(12). In vitro experiments demonstrated that this metabolic activation is catalyzed by a cytochrome P-450 enzyme system which is located in the endoplasmic reticulum of target cells(lO). This metaboliteCs) forms covalent bonds with cellular macromolecules, and it causes cell death (necrosis). The amount of cellular necrosis (measured by microscopic examination of the respective tissues 24 hours after exposure to the toxin) and the extent of protein alkylation (assayed by employing or H-IPO and measuring the amount of label... 

With dioxygen as the oxidant the cytochrome P-450 enzymic system and its synthetic models require two electrons from an exogeneous source for their monooxygenase activity. These are used for reducing one 0-atom of the 0 to water. This suggests the possibility of... 

Animal and human susceptibility to carbon tetrachloride hepatotoxicity is dependent on many different factors. There is substantial interspecies variation in carbon tetrachloride induced hepatotoxicity in animals due to differences in metabolic pathways among species. Based on animal models, hepatotoxicity in humans is most likely mediated from the trichloromethyl radical formed from the metabohsm of carbon tetrachloride by hepatic cytochrome p 450 2E1 Animal studies suggest differential hepatotoxicity based upon the animal s age and gender, with greater toxicity demonstrated in adult rats compared to newborns, and males compared with females. Cytochrome p-450 enzyme systems are present in the human fetus suggesting a potential for in utero fiver toxicity. Human gender differences in the metabolism of carbon tetrachloride have not been demonstrated despite potential sex steroid influences on the cytochrome p-450 system. ... 

Typically, lipid-soIuble toxicants and protoxicants are converted by Phase I reactions to species that are more polar and water-soluble and more easily eliminated from the body through urine (Figure 2.14) compared to the species from which they are made. This usually occurs through attachment of an -OH group. A Phase I reaction is generally catalyzed by the cytochrome P-450 enzyme system associated with cellular endoplasmic reticulum, which occurs most abundantly in the liver of vertebrates. A typical Phase I reaction is the production of phenol from benzene ... 

FIGURE 2.14 Illustration of Phase I reactions in which the cytochrome P-450 enzyme system attaches a functional group, typically -OH. The Phase I reaction product is generally more water-soluble and is amenable to Phase II reactions in which a conjugating agent is attached. 

Current interest in the use of cytochrome P-450 enzyme systems for commercial syntheses or biosensor development is restrained not only by the need for expensive cofactors, but also by the limited supplies of isolated and purified enzyme which until a few years ago was known to be produced by only mammalian cells. Now, bacterial sources of P-450 offer the possibility to produce larger quantities of the active enzyme. The known bacterial sources of cytochrome P-450 has been summarized by Sligar... 

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