Rat liver, microsomal preparation

In contrast to the metabolism of BA and BaP, the 5,6-dihydrodiols formed in the metabolism of DMBA by liver microsomes from untreated, phenobarbital-treated, and 3-methylcholanthrene-treated rats are found to have 5R,6R/5S,6S enantiomer ratios of 11 89, 6 94, and 5 95, respectively (7.49 and Table II). The enantiomeric contents of the dihydrodiols were determined by a CSP-HPLC method (7.43). The 5,6-epoxide formed in the metabolism of DMBA by liver microsomes from 3MC-treated rats was found to contain predominantly (>97%) the 5R,6S-enantiomer which is converted by microsomal epoxide hydrolase-catalyzed hydration predominantly (>95%) at the R-center (C-5 position, see Figure 3) to yield the 5S,6S-dihydrodiol (49). In the metabolism of 12-methyl-BA, the 5S,6S-dihydrodiol was also found to be the major enantiomer formed (50) and this stereoselective reaction is similar to the reactions catalyzed by rat liver microsomes prepared with different enzyme inducers (unpublished results). Labeling studies using molecular oxygen-18 indicate that 5R,68-epoxide is the precursor of the 5S,6S-dihydrodiol formed in the metabolism of 12-methyl-BA (51). 

The 8,9- and 10,11-dihydrodiols formed in the metabolism of BA and DMBA respectively are all highly enriched (>90%) in R,R enantiomers (Table III). Labeling experiments using molecular oxygen-18 in the in vitro metabolism of the respective parent compounds and subsequent mass spectral analyses of dihydrodiol metabolites and their acid-catalyzed dehydration products indicated that microsomal epoxide hydrolase-catalyzed hydration reactions occurred exclusively at the nonbenzylic carbons of the metabolically formed epoxide intermediates (unpublished results). These findings indicate that the 8,9- and 10,11-epoxide intermediates, formed in the metabolism of BA and DMBA respectively, contain predominantly the 8R,9S and 10S,11R enantiomer, respectively. These stereoselective epoxidation reactions are relatively insensitive to the cytochrome P-450 isozyme contents of different rat liver microsomal preparations (Table III). 

With UDP-glucuronic acid as the variable substrate and bilirubin at constant concentration, Michaelis-Menten kinetics were obeyed (A2, H2, HIO, P5, V2, W12). Rat liver microsomal preparations treated in different ways yielded apparent Km for UDP-glucuronic acid of 0.37-0.70 mM (A2, H2, HIO, V2), with albumin-bound bilirubin as the aglycon a higher value (1.66 mAf) was found in a carrier-free system (W12). These values probably do not represent Km for UDP-glucuronic acid at saturation with bilirubin (P5, V6). Under certain conditions of activation, cell extracts from livers of newborn and adult rats, wdicn tested with o-amino-... 

Zhao and Lou [164] studied the metabolism of omeprazole to its two major metabolites, hydroxyomeprazole and omeprazole sulfone, in rat liver microsomes by a reversed-phase HPLC assay. The formation of metabolites of omeprazole depended on incubation time, substrate concentration, microsomal protein concentration, and was found to be optimal at pH 7.4. The Pmax and Km of omeprazole hydroxylation in the rat liver microsomal preparation were 2033 nmol /(min mg protein), and 46.8 ymol/l, respectively. The effects of seven drugs on omeprazole metabolism were tested. Mephenytoin, five benzodiazepines and pava-verine caused inhibition of omeprazole metabolism. 

No such enhancement by retinyl acetate was observed when 2-fluoren-amine activation was carried out by rat liver microsomes prepared from S9 (Figure 2). 

The synthesis [111] and examination of 1-p-toluenesulfonyl-6,7,8,9-lelrahydro-N,N-di- -propyl-lH-bcnz [g]indol-7-amine (TPBIA in Fig. 5) for behavioral effects in rats related to interactions with central dopamine receptors showed remarkable antioxidant activity. Because TPBIA has increased lipophilicity, penetrating the blood-brain barrier in a considerable degree was expected and it was found that it completely inhibits the peroxidation of rat liver microsome preparations [112]. 

The source of enzyme is rat liver microsomes prepared by standard techniques. 

Codeine-7/3,8/3-epoxide (codeine-7,8-oxide) (140) has been isolated (see Refs. 244,448) as a codeine metabolite from a rat liver microsomal preparation and identified by gc/ms (MID) andhplc. It had two times the analgesic potency of codeine in the rat/450 ... 

Correia MA, Krowech G, Caldera-Munoz P, et al. Morphine metabolism revisited. II. Isolation and chemical characterization of a glutathionylmorphine adduct from rat liver microsomal preparations. Chem Biol Interact. 1984 51(l) 13-24. 

The following (hydroxylated) compounds have been successfully conjugated to fatty acids with the in vitro CoA-fortifled rat liver microsomal preparations N-(l-phenylcyclohexyl)-4-hydroxy-plperldlne and N-(l-phenyl-4-hydroxycyclohexyl)piperidine (20),... 

For Instance, when 1-naphthyl glucoside (7) was administered orally to rats It was quickly absorbed and 17% of the dose was eliminated unchanged (1 ). Similarly, the major glucoside metabolite (8) of oxamyl Is relatively resistant to degradation In the rat both In vivo following oral administration and In vitro In rat liver microsomal preparations. In the former, at least 30% of the dose was recovered as the glucoside In the urine (13). A similar result was obtained with the glucoside of N-(3-chloro-4-hydroxymethylphenyl)... 

Kassahun, K., Baillie, T. A. Cytochrome P-450-mediated dehydrogenation of 2-n-propyl-2(E)-pentenoic acid, a pharmacologically active metabolite of valproic acid in rat liver microsomal preparations. Drug Metab. Dispos. 1993, 21, 242-248. 

FIGURE 10.2 Combined mass chromatograms of rat liver microsomal preparations of the... 

Shefer, S., Gheng, F. W., Hauser, S., Batta, A. K., and Salen, G., Regulation ofbile acid synthesis. Measurement of cholesterol 7 -hydroxylase activity in rat liver microsomal preparations in the absence of endogenous cholesterol. J. Lipid Res. 22, 532-536... 

The possible role of cytochrome P450 as an enzyme that can hydroxylate tropane alkaloids has been examined in vitro by use of a rat liver microsomal preparation. 3a-Tigloyloxytropane (40) was converted into meteloidine (41). The reaction was dependent on NADPH and was inhibited by carbon monoxide, thus suggesting that cytochrome P450 was the enzyme responsible. 

Neal, G.E., and Colley, P.E., 1978, Some high-performance liquid-chromatographic studies of the metabolism of aflatoxins by rat liver microsomal preparations, Biochem. J., 174 839. 

The rate of hydrolysis of various organic phosphates by rat liver microsome preparations incubated at pH 6.7 expressed in relation to G-6-P hydndyste, which is arbitrarily ex sr sed as 100. 

Baker, J.K. Yarber, R.H. Hufford, C.D. Lee, I.-S. ElSohly, H.N. McChesney, J.D. Thermospray mass spectroscopy/high performance liquid chromatographic identification of the metabolites formed from arteether using a rat liver microsome preparation, Biorned.Environ.Mass Spectrom., 1989,18,337-351. 

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