Metabolism by liver microsomes

The optical purities of the dihydrodiol metabolites formed in BaP metabolism by liver microsomes from Sprague-Dawley rats (1.14.15) are higher than those from liver microsomes from rats of Long-Evans strain (17). Repeated experiments in our laboratory using both rat strains indicate that small differences indeed exist (Table I). However, the percentages of R,R enantiomers are consistently higher than those reported by another laboratory (17). 

BA trans-3.4-dihvdrodiol cannot be separated from BA trans-8.9-dihydrodiol in several HPLC conditions (27-29). Quantification of BA trana-3,4-dihydrodiol by HPLC can only be accomplished after converting the 3,4-dihydrodiol to its diacetate (25.26). The BA trans-3.4-dihydrodiol formed in BA metabolism by liver microsomes from pheno-barbital-treated rats was determined to have a 3R,4R/3S,4S enantiomer ratio of 69 31 (30). Recently we have determined the optical purity of the BA trans-3.4-dihvdrodiol formed in the metabolism of BA by three liver microsomes prepared from untreated rats and rats that had been pretreated with an enzyme inducer. As shown in Table II, cytochrome P-450 isozymes contained in liver microsomes from 3-methylcholanthrene- or phenobarbital-treated rats had similar stereoselectivity toward the 3,4-double bond of BA. BA trans-3.4-dihydrodiol is formed via the 3,4-epoxide intermediate (31). 

Dantrolene is active orally, although its absorption is slow and incomplete. Its biological half-life (h/z) is 8.7 hours in adults. The drug is metabolized by liver microsomal enzymes and is eliminated in the urine and bUe. It is given IV when treating an attack of malignant hyperthermia. 

This preliminary work demonstrates that dietary oxidized S-amino acids do lead to altered BaP metabolism by liver microsomes under in vitro conditions of excess NADPH and O. ... 

Figure 11-17 shows the results of automated, unattended MRM and subsequent MS/MS analysis of buspirone, one of the four substrates to be rapidly metabolized by liver microsomes. A total of 42 MRMs were monitored in this particular MS/MS analysis. The MRMs pre-selected represented the common phase I oxidative metabohtes, including mono- and d-hydroxylation, A-oxide formation, and A-dealkylation, to name a few. For 21 of the MRM transitions, the mass corresponding to the expected oxidative metabolite was added to the mass of the precursor ion (e.g., buspirone MW = 386, mono-hydroxylated buspirone MW = 402), keeping the mass of the product ion of the MRM pair unchanged. For the remaining 21 MRM transitions, the mass of the expected... 

Abacavir has good oral systemic availability and penetrates the nervous system. It does not interfere with drugs that are metabolized by liver microsomal cytochrome P450 (1). It has no other significant drug interactions and can be administered without food restrictions. 

The test article is not metabolized by liver microsomes, or hepato-cytes This is indicated by the lack of either metabolite formation or parent disappearance in Studies 1 and 2. Hepatic metabolism is not involved in the metabolic clearance of the compound. There should be no concern with coadministered drugs that can alter drug-metabolizing enzyme activities. 

Endrin is metabolized by liver microsomal enzymes. In all species, oxidation of the methylene bridge in endrin to syn-, but mostly u //-12-hydro-xyendrin occurs, followed by dehydrogenation to 12-ketoendrin, a more toxic metabolite than the parent compound. Hydroxylated metabolites are conjugated as glucuronides and sulfates. 

Hall M, Parker DK, Grover PL, et al. 1990. Effects of 1-ethynylpyrene and related inhibitors of P450 isozymes upon benzo[a]pyrene metabolism by liver microsomes. Chem Biol Interact 76(2) 181-192. 

C-ABT-773 was metabolized by liver microsomes and hepatocytes from mouse, rat, dog, monkey, and human. The metabolic pathway was oxidation, and its profile was similar in all species to give A -desmethyl-ABT-773 (M-1) as the major metabolite (Fig. 11). The kinetics of ABT-773 metabolism were examined in four human liver microsomes over a drug concentration range of 1.5-50 pM. The metabolism of ABT-773 followed monophasic Michaelis-Menten kinetics with K = 22.3 xM and = 5.2 nmol/mg protein/min [108]. 

The development of easy-to-use assays for determining theophylline blood levels afforded a handle on maintenance of effective but nontoxic levels. The relatively good availability of such assays in the United States probably contributed to the historical preference for theophylline treatment by U.S. physicians. Carefiil titration of the dose must be done on a patient-by-patient basis because individual rates of metabolism vary widely. Most ( 85%) of an oral dose of theophj line is metabolized by liver microsomal enzymes. As a result many drugs, eg, cimetidine [51481-61-9], anticonvulsants, or conditions, eg, fever, cigarette smoking, liver disease, which affect liver function alter theophylline blood levels. 

A large number of barbiturates are metabolized by liver microsomes. Isoniazid is quickly metabolized in Japanese race to the extent of 86.7%, whereas approximately half of it (44.9%) in American and Canadian whites. This disparity is due to the genetic differences in the said races. 

Endoxan has proved to be one of the most selective of alkylating agents, and undoubtedly owes its activity to activation in vivo. The actual mechanism of activation is not as predicted. The initial activation step is due to metabolism by liver microsomes. 

Until recently the evidence involving NADPH cytochrome c reductase (a ilavoprotein) in the cytochrome P-450 catalysed reactions was indirect The concept was first based on the fact that cytochrome c blocked drug metabolism, presumably by diverting electrons from NADPH cytochrome c reductase in micro-somes to cytochrome oxidase in the mitochondria which invariably contaminate microsomal preparations. The view was more firmly established by the finding that on treatment of liver microsomes with various concentrations of steapsin, retention of the NADPH cytochrome c reductase in microsomes paralleled the retention of cytochrome P-450 reductase and aniline hydroxylase. The best evidence implicating the flavoprotein was recently obtained by Omura, who showed that a specific antibody for NADPH cytochrome c reductase inhibited both cytochrome P-450 reduction and drug metabolism by liver microsomes. 

The enantiomers of ketamine exhibit differences in pharmacological activities and in metabolism. Kharasch and Labroo [27] studied the in vitro metabolism by liver microsomes of the isolated isomers of racemate. In a separate experiment, they determined the concentrations of the main metabolite NK. This secondary amino compound was treated with PFPA for derivatization. The hexadeuterated NK was used as the internal standard. 

Among common in vitro metabolizing systems, liver microsomes and liver S9 fractions are used more often in metabolite synthesis than other systems. The majority of drug metabolism is mediated by CYPs [8]. Liver microsomes contain a high concentration of CYPs and other... 

Table I. Optical Purity of the Dihydrodiol Metabolites Formed in the Metabolism of Benzo[a]pyrene by Liver Microsomes from Untreated, Phenobarbital (PB)-, 3-Methylcholanthrene (3MC)-, and Polychlorinated Biphenyls (PCBs, Aroclor 1254)-Treated Rats...

It was recently reported that. >97% of BaP 4,5-epoxide metabolically formed from the metabolism of BaP in a reconstituted enzyme system containing purified cytochrome P-450c (P-448) is the 4S,5R enantiomer (24). The epoxide was determined by formation, separation and quantification of the diastereomeric trans-addition products of glutathione. Recently a BaP 4,5-epoxide was isolated from a metabolite mixture obtained from the metabolism of BaP by liver microsomes from 3-methylcholanthrene-treated Sprague-Dawley rats in the presence of the epoxide hydrolase inhibitor 3,3,3-trichloropropylene oxide, and was found to contain a 4S,5R/4R,5S enantiomer ratio of 94 6 (Chiu et. al., unpublished results). However, the content of the 4S,5R enantiomer was <60% when liver microsomes from untreated and phenobarbital-treated rats were used as the enzyme sources. Because BaP 4R,5S-epoxide is also hydrated predominantly to 4R,5R-dihydro-... 

Table II. Enantiomeric Composition of the 3,4- and 5,6-Dihydrodiols Formed in the Metabolism of BA and DMBA by Liver Microsomes from Rats of the Sprague-Dawley Strain...

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 finding that BaP is highly stereoselectively metabolized to dihydrodiols and bay-region 7,8-dihydrodiol-9,10-epoxides (summarized in Figure 1) by liver microsomes from 3-methylcholanthrene-treated rats led Jerina et a . (48) to propose a model of the substrate binding site for cytochrome P-450c (Figure 7) which... 

Gill, S.J. 1980. In vitro metabolism of carbofuran by liver microsomes of the padifield fish Trichogaster pectoralis. Bull. Environ. Contam. Toxicol. 25 697-701. 

Sikka, H.C., J.P. Rutkowski, and C. Kandaswami. 1990. Comparative metabolism of benzo[a]pyrene by liver microsomes from brown bullhead and carp. Aquat. Toxicol. 16 101-112. 

Figure 7 shows the major metabolic pathway of triprolidine hydrochloride as determined in a study of 14C labelled triprolidine hydrochloride in guinea pigs.6 Triprolidine is converted to metabolite I by liver microsomes. Metabolite I is converted to metabolite II which is the major metabolite found in guinea pig urine. 

Nastainczyk W, Ahr HJ, Ullrich V. 1982b. The reductive metabolism of halogenated alkanes by liver microsomal cytochrome P450. Biochem Pharmacol 31 391-396. 

This compound requires metabolic activation by liver microsomes to yield highly mutagenic derivatives in the Ames test204. In addition, IQ is a multipotent animal carcinogen that is metabolized by prostaglandin-H synthase205 and the hepatic cytochrome P-450 system204. 

Holder, G., Yagi, H., Dansette, P., Jerina, D. M., Levin, W., Lu, A. Y. H., and Conney, A. H. Effects of inducers and epoxide hydrase on the metabolism of benzo(a)pyrene by liver microsomes and a reconstituted system Analysis by high pressure liquid chromatography. Proc. Natl. Acad. Sci. U.S.A. (1974) 71 4356-4360. 

Tweedie DJ, Burke MD. (1987). Metabolism of the beta-carbolines, harmine and harmol, by liver microsomes from phenobarbitone- or 3-methylcholanthrenetreated mice. Identification and quantitation of two novel harmine metabolites. Drug Metab Dispos. 15(1) 74-81. 

Clearance of nicotine is decreased in the elderly (age >65) compared to adults (Molander et al. 2001). Total clearance was lower by 23%, and renal clearance lower by 49% in the elderly compared to yonng adults. Lower nicotine metabolism in the elderly may be contribnted to by rednced liver blood flow, since no decrease in CYP2A6 protein levels or nicotine metabolism in liver microsomes due to age has been detected (Messina et al. 1997). No differences in steady-state nicotine plasma levels or estimated plasma clearance valnes were detected in three age gronps (18-39, 40-59, and 60-69 years) nsing patches with the same nicotine content (Gonrlay and Benowitz 1996). The volnme of distribntion of nicotine is lower in older snbjects due to a decrease in lean body mass (Molander et al. 2001). 

In vitro data show that both etonogestrel and ethinyl estradiol are metabolized in liver microsomes by the cytochrome P450 3A4 isoenzyme. Etonogestrel and ethinyl estradiol are primarily eliminated in urine, bile, and feces. 

A variety of other therapeutic compounds have mechanisms that probably involve alkylation. Procarbazine and dacarbazine are the two most successful agents in this group. Dacarbazine is a synthetic compound that functions as an alkylating agent, following metabolic activation by liver microsomal enzymes, to yield diazomethane. It has been used successfully in the treatment of sarcomas. 

The amide linkage of amide local anesthetics is hydrolyzed by liver microsomal cytochrome P450 isozymes. There is considerable variation in the rate of liver metabolism of individual amide compounds, with prilocaine (fastest)... 

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