Oxidase liver microsomes

Saeman, M.C. and J.E. Casida (1984). Metribuzin metabolites in mammals and liver microsomal oxidase systems Identification, synthesis, and reactions. J. Agric. Food Chem., 32 749-755. 

In man, hydrolysis of the thiobarbiturate ring of thiopeiital does not occur [58], and biotransformation by the liver microsomal oxidase system appears to be the main route of elimination from the body [59]. However, in spite of the many studies of thiobarbiturate metabolism, the fate of the majority of the administered dose is yet to be identified [58]. What is clear is that less than 0.5% of the dose is excreted as unchanged thiopental [35,60]. 

Ritter CL, Malejka-Giganti D. 1982. Mixed function oxidase in the mammary gland and liver microsomes of lactating rats. Biochem Pharmacol 31 239-247. 

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. 

Arinc, E. and A. Sen. 1994. Effects of in vivo benzo[a]pyrene treatment on liver microsomal mixed-function oxidase activities of gilthead seabream (Sparus aurata). Comp. Biochem. Physiol. 107C 405-414. 

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. 

Gorsky LD, Koop DR, Coon MJ. On the stoichiometry of the oxidase and monooxygenase reactions catalyzed by liver microsomal cytochrome P-450. Products of oxygen reduction. J Biol Chem 1984 259(11) 6812-6817. 

Stevens JT, Greene FE, Stitzel RE, et al. 1973. Effects of anticholinesterase insecticides on mouse and rat liver microsomal mixed function oxidase. In Deichmann W.B., Ed. Pesticides and the environment A continuing controversy. Proceedings of the 8th Inter-American Conference on Toxicology and Occupational Medicine, University of Miami School of Medicine, 489-501. 

Molinate Metabolism in Carp Hepatic Mixed-function Oxidase System. Incubation of molinate with carp liver microsomes produced four major organosoluble metabolites (molinate sulfoxide, 3- and 4-hydroxy molinate, and keto HMI). Parameters affecting... 

Figure 3. Mutagenic activities of the promutagens cis- and Xrms-diallate and sulfallate, the proximate mutagen cis-diallate sulfoxide, and the ultimate mutagen 2-chloroacrolein, assayed with S. typhimurium strain TA 100 sensitive to base-pair substitution mutagens. The diallate isomers and sulfallate are not mutagenic without the S9 mix. S9 mix refers to a microsomal oxidase system prepared from rat liver and appropriate cofactors. The methodology is detailed in Refs. 6, 22, and 29.

Kostyuk VA, Potapovich Al. 1991. Damage of rat liver microsomal mixed function oxidase system by carbon tetrachloride in wVo study with selective inhibitor of lipid peroxidation. Biochemistry International 25 349- 353. 

Cyclophosphamide, probably one of the most frequently used anti-cancer drugs, is a pro-drug. It can be given orally as well as intravenously. It is converted by the liver microsomal cytochrome P450 mixed-function oxidase system to its active forms... 

Pichard L, Eabre I, Fabre G, et al. Cyclosporin A drug interactions. Screening for inducers and inhibitors of cytochrome P-450 (cyclosporin A oxidase) in primary cultures of human hepatocytes and in liver microsomes. Drug Metab Dispos 1990 18(5) 595-606. 

A volatile alkylating metabolite was formed in a mouse-liver microsomal system. The primary metabolite formed in vitro by mixed function oxidases is 2-bromoethylene oxide, which rearranges to 2-bromoacetaldehyde. 

Hydrazine is metabolized by rat liver microsomal enzymes to unknown products, ultimately yielding molecular nitrogen (Timbrell et al., 1982 Jeimer Timbrell, 1995). This was dependent upon oxygen and NADPH and was increased by NADH in the presence of NADPH. Hydrazine metabolism was 20-70% lower in human microsomes prepared from three individuals compared with rats. Hydrazine is also metabolized by rat liver mitochondria, but the monoamine oxidase inhibitors clorgyline and pargyline do not significantly decrease this activity (Jenner Timbrell, 1995). 

Vinylidene fluoride is taken up rapidly via the pulmonary route in rats, but at equilibrium the mean concentration (by volume) in rats was only 23% of that in the gaseous phase. Metabolism proceeded very slowly and was saturable at exposure concentrations of about 260 mg/m Its maximum rate was 1% that of vinyl chloride and less than 20% that of vinyl fluoride there has been a report of an increase in the urinary excretion of fluoride in exposed rats. No alkylating intermediate was demonstrated after passage through a mouse-liver microsomal system. However, vinylidene fluoride inhibits mixed-function oxidase activity in vitro and, like similar halogenated compounds that are transformed to reactive metabolites, it alters rat intermediary metabolism, leading to acetone exhalation (lARC, 1986). 

The reactivity of the individual O—P insecticides is determined by the magnitude of the electrophilic character of the phosphorus atom, the strength of the bond P—X, and the steric effects of the substituents. The electrophilic nature of the central P atom is determined by the relative positions of the shared electron pairs, between atoms bonded to phosphorus, and is a function of the relative electronegativities of the two atoms in each bond (P, 2.1 O, 3.5 S, 2.5 N, 3.0 and C, 2.5). Therefore, it is clear that in phosphate esters (P=0) the phosphoms is much more electrophilic and these are more reactive than phosphorothioate esters (P=S). The latter generally are so stable as to be relatively unreactive with AChE. They owe their biological activity to in vivo oxidation by a microsomal oxidase, a reaction that takes place in insect gut and fat body tissues and in the mammalian liver. A typical example is the oxidation of parathion (61) to paraoxon [311 -5-5] (110). 

Cyclophosphamide in its parent form does not have direct cytotoxic effects, and it must be activated to cytotoxic forms by microsomal enzymes (Figure 55-5). The liver microsomal cytochrome P450 mixed-function oxidase system converts cyclophosphamide to 4-hydroxycyclophosphamide, which is in equilibrium with aldophosphamide. These active metabolites are believed to be delivered by the bloodstream to both tumor and normal tissue, where nonenzymatic cleavage of aldophosphamide to the cytotoxic forms—phosphoramide mustard and acrolein—occurs. The liver appears to be protected through the enzymatic formation of the inactive metabolites 4-ketocyclophosphamide and carboxyphosphamide. 

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