P-450-Oxidase

L-Tyrosine metabohsm and catecholamine biosynthesis occur largely in the brain, central nervous tissue, and endocrine system, which have large pools of L-ascorbic acid (128). Catecholamine, a neurotransmitter, is the precursor in the formation of dopamine, which is converted to noradrenaline and adrenaline. The precise role of ascorbic acid has not been completely understood. Ascorbic acid has important biochemical functions with various hydroxylase enzymes in steroid, dmg, andhpid metabohsm. The cytochrome P-450 oxidase catalyzes the conversion of cholesterol to bUe acids and the detoxification process of aromatic dmgs and other xenobiotics, eg, carcinogens, poUutants, and pesticides, in the body (129). The effects of L-ascorbic acid on histamine metabohsm related to scurvy and anaphylactic shock have been investigated (130). Another ceUular reaction involving ascorbic acid is the conversion of folate to tetrahydrofolate. Ascorbic acid has many biochemical functions which affect the immune system of the body (131). 

Ichikawa, Y., Yamano, T., and Fujishima, H. Relationship between the interconversion of c34ochrome P-450 and P-420 and its activities in hydroxylations and demethylations by P-450 oxidase systems, Biochim. Biophys. Acta, 171 32-46, 1969. 

Aromatic EC5-EC9 Fraction. As indicated by studies with humans and animals exposed to individual BTEXs, compounds in this fraction may be expected to be metabolized via cytochrome P-450 oxidases, either at carbons in the aromatic ring or in alkyl side groups, to metabolic intermediates that can be conjugated with glucuronides, sulfates, glutathione, or amino acids (e.g.,... 

Aliphatic EC5-EC8 Fraction. Examination of urinary metabolites in humans and rats after exposure to /7-hexane indicates that hydrocarbons in this fraction may be oxidatively metabolized via cytochrome P-450 oxidases to several alcohol, ketone, and carboxylic acid derivatives. Based on... 

Serotonin is metabolized to bufotenin, and much of this is excreted in the urine as a glucuronide conjugate [56]. Vindoline and related alkaloids of Catharanthus roseus are extensively metabolized by mammals through the actions of esterases, peroxidases, and P-450 oxidases [57]. Oxidation of vindoline and vinblastine occurs in human serum, catalyzed by ceruloplasmin [58], The major metabolite of yohimbine is 11-hydroxyyohimbine, but the 10-hydroxy derivative is also produced [59], Strychnine injected into rats gives as its major metabolite its 21,22-epoxide, but other epoxidized and hydroxy derivatives are also prodced [60]. Harman is hydroxylated at C-6 by mice [61]. 

Shah S, Xue Q, Tang L, Carney JR, Betlach M, McDaniel R (2000) Cloning, characterization and heterologous expression of a polyketide synthase and P-450 oxidase involved in the biosynthesis of the antibiotic oleandomycin. J Antibiotic 53 502-508... 

Cytochrome P-450 oxidases are involved in host-plant utilization by these Sonoran Desert Drosophila species (Frank and Fogleman, 1992). 

El Autunmaline cytochrome P-450 oxidase E2 0-Methylandrocymbine methyl transferase E3 0-Methylandrocymbine cytochrome P450 oxidase [yJeO E4 A-Formyldemecolcine deformylase E5 Colchicine A-acetyl transferase... 

Many examples of microbial hydroxylation of sterols/steroids have been reported. These hydroxylations usually involve mixed function oxidases which utilise molecular oxygen and cytochrome P-450. The reaction can be represented by ... 

Attempts to diminish the overall metabolism of trichloroethylene might be useful (e.g., hypothermia, mixed-function oxidase inhibitors, competitive inhibitors of trichloroethylene metabolism [i.e., P-450 substrates]), if instituted soon enough after trichloroethylene exposure. Catecholamines (especially beta agonists) act in concert with trichloroethylene, increasing the risk of cardiac arrhythmias. Hence, catecholamines should be administered to patients only in the lowest efficacious doses and for certain limited presentations of trichloroethylene poisoning. Ethanol should also be avoided because concurrent exposure to trichloroethylene and ethanol can cause vasodilation and malaise and may potentiate central nervous system depression at high dosage levels of either compound. 

CYP, cytochrome P-450 isoenzyme MAO, monoamine oxidase. Data from references 59-65. 

BUN, blood urea nitrogen CBC, complete blood cell count CNS, central nervous system CYP, cytochrome P-450 isoenzyme LFT, liver function test MAO, monoamine oxidase QTc, Q-T interval corrected for heart rate SCr, serum creatinine TMP-SMX, trimethoprim-sulfamethoxazole. 

Isoenzymes Structurally related enzyme proteins that catalyse very similar or identical reactions (e.g., monoamine oxidase A and B, cytochrome P-450). 

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. 

A potentially powerful probe for sorting out the contribution of hydroperoxide-dependent and mixed-function oxidase-dependent polycyclic hydrocarbon oxidation is stereochemistry. Figure 9 summarizes the stereochemical differences in epoxidation of ( )-BP-7,8-dihydrodiol by hydroperoxide-dependent and mixed-function oxidase-dependent pathways (31,55,56). The (-)-enantiomer of BP-7,8-dihydrodiol is converted primarily to the (+)-anti-diol epoxide by both pathways whereas the (+)-enantiomer of BP-7,8-dihydrodiol is converted primarily to the (-)-anti-diol epoxide by hydroperoxide-dependent oxidation and to the (+)-syn-diol epoxide by mixed-function oxidases. The stereochemical course of oxidation by cytochrome P-450 isoenzymes was first elucidated for the methycholanthrene-inducible form but we have detected the same stereochemical profile using rat liver microsomes from control, phenobarbital-, or methyl-cholanthrene-induced animals (32). The only difference between the microsomal preparations is the rate of oxidation. 

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. 

Nature has had a long time to perfect her inorganic chemistry, and understanding the mechanisms of enzymatic action can suggest ways for the inorganic chemist to develop corresponding in vitro catalysts. We shall show how knowledge of cytochrome oxidase and cytochrome P-450 has led to such possibilities. 

How does nature prevent the release of hydrogen peroxide during the cytochrome oxidase-mediated four-electron reduction of dioxygen It would appear that cytochrome oxidase behaves in the same manner as other heme proteins which utilize hydrogen peroxide, such as catalase and peroxidase (vide infra), in that once a ferric peroxide complex is formed the oxygen-oxygen bond is broken with the release of water and the formation of an oxo iron(IV) complex which is subsequently reduced to the ferrous aquo state (12). Indeed, this same sequence of events accounts for the means by which oxygen is activated by cytochromes P-450. 

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