Alcohol Mixed function oxidase

Orme-Johnson, W.H. and D.M. Ziegler (1965). Alcohol mixed function oxidase activity of mammalian liver micoromes. Biochem. Biophys. Res. Commun. 21, 78-82. 

Many other cases of carbon tetrachloride-induced hepatic and/or renal injury associated with ethanol ingestion have been described in the medical literature (Durden and Chipman 1967 Guild et al. 1958 Jennings 1955 Lamson et al. 1928 Markham 1967 Tracey and Sherlock 1968). These clinical reports establish that occasional or frequent ingestion of alcoholic beverages can increase the danger from relatively moderate carbon tetrachloride exposure. As ethanol is known to induce microsomal mixed-function oxidase activity in man (Rubin and Lieber 1968), the mechanism of potentiation may involve ethanol-induced enhancement of the metabolic activation of carbon tetrachloride. 

In addition to alcohol dehydrogenase, ethanol can be oxidized to acetaldehyde by the microsomal mixed-function oxidase system (cytochrome P450 2 El), as illustrated in Figure 35.1. Although this microsomal ethanol-oxidizing system probably has minor impor-... 

Alkenes. Alkenes are, in general, metaholically stable. The majority of alkene-containing drugs do not exhibit significant rapid metabolism at the double bond. There are some isolated examples of alkene-containing compounds that undergo epoxidation, catalyzed by mixed-function oxidase, or that add water across the double bond to give an alcohol. 

For foreign compounds, the majority of oxidation reactions are catalyzed by monooxygenase enzymes, which are part of the mixed function oxidase (MFO) system and are found in the SER (and also known as microsomal enzymes). Other enzymes involved in the oxidation of xenobiotics are found in other organelles such as the mitochondria and the cytosol. Thus, amine oxidases located in the mitochondria, xanthine oxidase, alcohol dehydrogenase in the cytosol, the prostaglandin synthetase system, and various other peroxidases may all be involved in the oxidation of foreign compounds. 

For most drugs, oxidative biotransformation is performed primarily by the mixed-function oxidase enzyme system, which is present predominantly in the smooth endoplasmic reticulum of the liver. This system comprises (1) the enzyme NADPH cytochrome P450 reductase (2) cytochrome P450, a family of heme-containing proteins that catalyze a variety of oxidative and reductive reactions and (3) a phospholipid bilayer that facilitates interaction between the two proteins. Important exceptions to this rule are ethyl alcohol and caffeine, which are oxidatively metabolized by enzymes primarily present in the soluble, cytosolic fraction of the liver. 

Heptane is converted to hydroxy derivatives (e.g., alcohol) by the cytochrome P450 mixed function oxidase system before being converted to keto forms. It may then be conjugated to the glucuronide and subsequently excreted. 

Factors that inhibit or alter the activity of the mixed function oxidase enzymes may increase the risk from exposure to the indicator compounds in the aromatic EC5-EC9 fraction (the BTEXs), the aromatic EC>16-EC35 fraction (the carcinogenic PAHs in this fraction) and a constituent of the aliphatic I < C5IiCH fraction (//-hexane). For example, concurrent alcohol consumption may increase the risk of central nervous system depression from the BTEXs, ototoxicity from toluene, and hematotoxicity from benzene. Acetone exposure may increase the risk of peripheral neuropathy of n-hexane. People who take haloperidol, acetaminophen, or aspirin, or who have a nutritionally inadequate diet, may also be more susceptible to the toxicity of these agents. ATSDR (1995f) noted that a substantial percentage of children consume less than the recommended dietary allowances of certain nutrients. 

AZRI, S. and RENTON, K.W. (1991) Factors involved in the depression of hepatic mixed function oxidase during infection with Listeria monocytogenes. Int. J.Immunopharmacol., 13, 197. HOYIJMPA, A.M. and SCHENKER, S. (1982) Major drug interactions effect of liver disease, alcohol and malnutrition. Rev. Med., 33, 113. 

Like the acyl-CoA desaturases (Chapter 7), the 1 -alkyl desaturase exhibits the typical requirements of a microsomal mixed-function oxidase. Molecular oxygen, a reduced pyridine nucleotide, cytochrome b, cytochrome reductase, and a terminal desaturase protein that is sensitive to cyanide are all required. The precise reaction mechanism responsible for the biosynthesis of ethanolamine plasmalogens is unknown, but it is clear from an investigation with a tritiated fatty alcohol that only the 15 and 25 (erythro)-labeled hydrogens are lost during the formation of the alk-l -enyl moiety of ethanolamine plasmalogens. 

Appreciable quantities of alkanes are ingested by animals as constituents of plant material and also by man as artificial additives (e.g. mineral oils) of foodstuffs or as components of natural or artificially produced oils and fats. Very little is known about the details of the metabolic turnover of such compounds. As in micro-organisms, it is likely that alkanes are detoxified in animals and plants by a mixed-function oxidase which forms the alcohols that may be further metabolized or excreted in the urine as conjugates, such as glucoronides or sulphates. Most of the compounds reported were characterized (GC-MS) after extraction from urine fractions that had been incubated with the appropriate enzyme to cleave the conjugate. Some biotransformations of alkanes by animal systems (mainly in vivo) are summarized in Figure 7. 

A few facts are also known concerning the metabolism of alkanes by plants. Cuticular waxes typically contain secondary alcohols, ketones and ) -diketones in addition to alkanes and the former have the skeletons as in Figure 8, where functionalization is near the centre of the molecule and the chain lengths of the alcohols and ketones are closely similar to those of the major n-alkanes present in any particular species. Tracer evidence indicates that the secondary alcohol and ketone are formed in sequence by oxidation of the corresponding n-alkane by a mixed function oxidase which is inhibited by chelating agents. Thus in Brassica species, the 14- or 15-hydroxy- and oxo-derivatives of the n-C29 alkane were thus formed both in vivo and in cell-free extracts ". ... 

The mechanistic details involved in these reactions remain obscure. However, the limited amount of available evidence suggests that a mixed function oxidase is responsible for the conversion of alkanes to secondary alcohols in B. oleracea. The O2 requirement for the conversion of exogenous [G- H]nonacosane to both secondary alcohols and ketones (Kolattukudy et al., 1973a ), as well as the incorporation of from O2 in the ketone (Kolattukudy, 1970a), support this hypothesis. Furthermore, this conversion was inhibited by metal ion chelators such as phenanthroline, and this inhibition was reversed by Fe ". In any case, without a cell-free preparation catalyzing the hydroxylation of hydrocarbons, the components involved in this reaction and the mechanistic details cannot be elucidated. 

The enzymatic method for determination of cholesterol Is based on that of Klose et al. [15], as modified by Leon and Stasiw [16]. It involves the use of cholesterol stearase to hydrolyse the cholesterol esters In serum to free cholesterol, which Is oxidized to HzCte that In turn forms a qulnonelmlne dye. The reaction is quantitative, so the concentration of the dye formed Is directly proportional to that of cholesterol In the sample. Figure 14.4 Illustrates the function of the SMAC channel used for the determination, in which the reagent stream (cholesterol oxidase, cholesterol stearase, peroxidase, phenol and 4-aminophenazone), the sample and some air are aspirated by the pump and, after mixing in reactor Ri, are Incubated for 4 min in a bath at 37 C, after which the dye Is extracted Into alcohol and sent to the detector, where Its absorbance Is measured at 525 nm. The aqueous phase from the extraction and the cell waste are aspirated by pump P2. The method has fewer and less serious interferences than Its non-enzymatic counterpart. 

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