Mixed function oxidases, P450 dependent

The answers are 34-g, 35-a, 36-d. (Katzung, pp 53—56J There are four major components to the mixed-function oxidase system (1) cytochrome P450, (2) NAD PH, or reduced nicotinamide adenine dinucleotide phosphate, (3) NAD PH—cytochrome P450 reductase, and (4) molecular oxygen. The figure that follows shows the catalytic cycle for the reactions dependent upon cytochrome P450. 

Phase I renders xenobiotics more polar. Oxidation is the most important process of phase I. It is carried out in the endoplasmic reticulum in many tissues by monooxygenases that contain cytochrome P450 (P450-dependent mixed function oxidases) as electron carrier. These enzymes have evolved in the past billion years in response to plant secondary metabolites. There are a number of P450 gene families. 

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. 

The flavin-dependent mixed-function oxidases include amine N-oxidases and a variety of S-oxidases. They provide an alternative to cytochrome P450-dependent enzymes in the metabolism of xenobiotics. 

The hepatic enzymes responsible for carrying out a-car-bnn hydroxylation reactions arc the cytochrome P-4S0 mixed-function oxidases. The N-hydroxylation or N-oxida-tion reactions, however, appear to be catalyzed not only by cylochrome P-4S0 but also by a second class of hepatic mixed-function oxidases called timine oxidases (sometimes called N oxidases) These enzymes are NADPH-dependent flavoproteins and do not contain cytochrome P450." They require NADPH and molecular oxygen 10 cany out N-oxidation. 

Clofibrate at a concentration of 0.5 mmol in culture medium maintained the cytochrome P-450 content of rat hepatocytes for up to 96 h. This effect was associated with a marked induction of lauric acid hydroxylation whereas little effect was observed on the metabolism of three other cytochrome p450 dependent mixed function oxidase substrates. 

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. ° "... 

Lipoxygenase Malate dehydrogenase Metallo-endopeptidase N-Methyl transferase Monoamine oxidase Mixed-function oxidase (cytochrome P450 dependent) NADH2 diaphorase NADPH2 diaphorase Neutral endopeptidase 24.11 Nitro oxide synthase Nitro reductase 5 -Nucleotidase Peroxidase... 

Chen, Y. L., Florentin, I., Batt, A. M., Ferrari, L., Giroud, J. P., and Chauvelot-Moachon, L. (1992). Effects of interleukin-6 on cytochrome P450-dependent mixed-function oxidases in the rat. Biochem. Pharmacol. 44, 137-148. 

We will discuss the carcinogenic properties of nitrosoamines in spite of the fact that it seems to be a problem particularly related to N-nitroso derivatives of secondary amines. In 1956, Magee and Barnes found that rats fed with N-nitrosodimethylamine developed hepatic tumors. Nitrosoamines cause alkylation of DNA, as suggested first by Druckrey et al. (1967) and Druckrey (1973). They postulated the pathway shown in (4-10), originally for 7V-nitrosodimethylamine, but likely to be valid for all dialkyl- and cycloalkylamines with at least one H-atom bonded to one of the C(a)-atoms. The nitrosoamine is metabolized by a cytochrome P450-dependent, so-called mixed-function oxidase. This enzyme catalyzes the hydroxylation of the C(a)-atom... 

Boyd, M. R. Evidence for the Clara cell as a site of cytochrome P450-dependent mixed-function oxidase activity in lung. Nature, 1977, 269, 713-715. 

Sortinl decomposes in plant growth media where one of the products resembles the effects of BSO, a GSH biosynthesis inhibitor. This indicates that a series of observed Sortinl effects might be due to an oxidized cellular environment that it creates. It could be that Sortinl or an active decomposition product sequesters GSH or inhibits GSH production. This may induce the transcription of cytochrome P450-dependent monooxygenases and mixed function oxidases, as shown in the transcription profile after Sortinl treatment. Reduced GSH level then affects flavonoid transport. 

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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