Microsomal cytochrome dependent mixed-function oxidase

Vitamin E deficiency is also associated with impaired mitochondrial oxidative metabolism and impaired activity of microsomal cytochrome P450-dependent mixed-function oxidases, and hence the metabolism of xenobi-ofics. There is no evidence that vitamin E has any specific role in electron transport in mitochondria or microsomes. Again, changes in membrane lipids and oxidative damage presumably account for the observed metabolic abnormalities. 

Correia, M.A. and G.X Mannering (1973). Reduced diphosphopyridine nucleotide synergism of the reduced triphosphopyridine nucleotide dependent mixed function oxidase of hepatic microsomes. Role of the Type I drug binding site of cytochrome P-450. Mol. Pharmacol. 9, 470-485. 

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. 

An effect of ozone on lung microsomes has been suggested by morpholine studies that indicated alterations in the endoplasmic reticulum, Biochemical evidence of an effect on microsomal enzymes was originally obtained in the studies of Palmer et who demonstrated that ozone exposure (0.75-10 ppm for 3 h) resulted in a decrease in activity of Syrian hamster lung benzopyrene hydroxylase, a mixed-function oxidase that depends on cytochrome P-450. No changes in hepatic activities of this enzyme were observed, and the results were similar in animals in which high activities of benzopyrene hydroxylase had been induced. The maximal effect was not observed until a few days after the single ozone exposure. Palmer et also reported a decrease in rabbit tracheobronchial mucosal benzopyrene hydroxylase activity after exposure to similar ozone concentrations. 

Rat liver microsomes hydroxylate 5/8-cholestane-3a ,7a,12Q -triol at C-25 and C-26 both activities are dependent on cytochrome P450 and there is some evidence that different types of the latter are involved. A mitochondrial steroid 24-hydroxylase that accepts 3a,7a,12a-trihydroxy-5/3-cholestanoic acid has been extracted from rat liver apparently this is not a mixed-function oxidase although the presence of oxygen was obligatory for its action. Bile acids hydroxylated at C-23 have been formed from sodium cholate and deoxycholate in preparations from Viperinae species and a steroid-12ct-hydroxylase from liver microsomes has been studied.Sitosterol has been confirmed to be a precursor of C24 and C29 bile acids in mammalian liver, and here hydroxylation at C-26 precedes that at C-7. ° "... 

A large proportion of the phase 1 metabolism of drug molecules occurring in the liver appears to be dependent upon a group of mixed function oxidases found in liver microsomes. These enzymes require a supply of reduced NADP and molecular oxygen and contain a haemoprotein, cytochrome P-450. The reactions catalysed together with examples are given in the final section. 

Formation of sterols from squalene involves cyclization. First a microsomal mixed-function oxidase (squalene epoxidase) forms squalene-2,3-oxide in the presence of NADPH, FAD and O2 (there is no requirement for cytochrome P450 in this reaction). The cyclization of the oxide to lanosterol then takes place by a concerted reaction without the formation of any stable intermediates. This conversion, which has been described as the most complex known enzyme-catalysed reaction, depends on a cyclase with a molecular mass of only 90kDa. In plants and algae squalene-2,3-epoxide is cyclized to cycloartenol which is the precursor of stigmasterol whereas lanosterol is the precursor of cholesterol and ergosterol (Figure 7.19). 

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