Carbonyl reduction of timiperone in human liver cytosol

Carbonyl reduction of bupropion in human liver

Bupropion is metabolized extensively in humans by oxidative and reductive processes. CYP2B6 mediates oxidation of bupropion to hydroxybupropion, but the enzyme(s) catalyzing carbonyl reduction of bupropion to erythro- and threohydrobupropion in human liver is unknown. The objective of this study was to examine the enzyme kinetics of bupropion reduction in human liver. In human liver cytosol, the reduction of bupropion to erythro-and threohydrobupropion was NADPH dependent with Cl(int) values of 0.08 and 0.60 µL·min(-1)mg(-1) protein, respectively. Bupropion reduction in liver microsomes was also NADPH dependent with Cl(int) values of 10.4 and 280 µL·min(-1)mg(-1) protein, respectively. Formation of erythro-and threohydrobupropion in microsomes exceeded that in cytosol by 70 and 170 fold, respectively. Menadione, an inhibitor of cytosolic carbonyl reducing enzymes (e.g. CBRs), inhibited erythro-and threohydrobupropion formation in cytosol with IC(50) of 30 and 54 µM, respectively. In microsomes 18β-glycyrrhetinic acid, an inhibitor of microsomal carbonyl reductases (e.g. 11β-HSDs), inhibited their formation with IC(50) of 25 and 26 nM, respectively. Our findings, in agreement with recent human placental studies, show that carbonyl reducing enzymes in hepatic microsomes are significant players in bupropion reduction. Contrary to past studies, we found that threohydrobupropion (not hydroxybupropion) is the major microsomal generated hepatic metabolite of bupropion.

CBR1 and CBR3 pharmacogenetics and their influence on doxorubicin disposition in Asian breast cancer patients

The present study aimed to identify polymorphic genes encoding carbonyl reductases (CBR1, CBR3) and investigate their influence on doxorubicin disposition in Asian breast cancer patients (n = 62). Doxorubicin (60 mg/m(2)) was administered every 3 weeks for four to six cycles and the pharmacokinetic parameters were estimated using non-compartmental analysis (WinNonlin). The Mann-Whitney U-test was used to assess genotypic-phenotypic correlations. Five CBR1 (-48G>A, c.219G>C, c.627C>T, c.693G>A, +967G>A) and CBR3 (c.11G>A, c.255C>T, c.279C>T, c.606G>A, c.730G>A) polymorphisms were identified. The CBR1 D2 diplotypes were characterized by the presence of at least one variant allele at the c.627C>T and +967G>A loci. Patients in the CBR1 D1 diplotype group had significantly higher clearance (CL) normalized to body surface area (BSA) (CL/BSA[L/h/m(2)]: median 25.09; range 16.44-55.66) and significantly lower exposure levels; area under curve (AUC(0-infinity)/dose/BSA [h/m(5)]; median 15.08; range 6.18-38.03) of doxorubicin compared with patients belonging to the CBR1 D2 diplotype group (CL/BSA[L/h/m(2)]; median 20.88; range 8.68-31.79, P = 0.014; and AUC(0-infinity)/dose/BSA[h/m(5)]; median 21.35; range 9.82-67.17, P = 0.007 respectively). No significant influence of CBR3 polymorphisms on the pharmacokinetics of doxorubicin were observed in Asian cancer patients. The present exploratory study shows that CBR1 D2 diplotypes correlate with significantly higher exposure levels of doxorubicin, suggesting the possibility of lowered intracellular conversion to doxorubicinol in these patients. Further evaluation of carbonyl reductase polymorphisms in influencing the treatment efficacy of doxorubicin-based chemotherapy in Asian cancer patients are warranted.

The aldo-keto reductase superfamily and its role in drug metabolism and detoxification

The aldo-keto reductase (AKR) superfamily comprises enzymes that catalyze redox transformations involved in biosynthesis, intermediary metabolism, and detoxification. Substrates of AKRs include glucose, steroids, glycosylation end-products, lipid peroxidation products, and environmental pollutants. These proteins adopt a (beta/alpha)(8) barrel structural motif interrupted by a number of extraneous loops and helixes that vary between proteins and bring structural identity to individual families. The human AKR family differs from the rodent families. Due to their broad substrate specificity, AKRs play an important role in the phase II detoxification of a large number of pharmaceuticals, drugs, and xenobiotics.

Multiplicity of mammalian reductases for xenobiotic carbonyl compounds

A variety of carbonyl compounds are present in foods, environmental pollutants, and drugs. These xenobiotic carbonyl compounds are metabolized into the corresponding alcohols by many mammalian NAD(P)H-dependent reductases, which belong to the short-chain dehydrogenase/reductase (SDR) and aldo-keto reductase superfamilies. Recent genomic analysis, cDNA isolation and characterization of the recombinant enzymes suggested that, in humans, the six members of each of the two superfamilies, i.e., total of 12 enzymes, are involved in the reductive metabolism of xenobiotic carbonyl compounds. They comprise three types of carbonyl reductase, dehydrogenase/reductase (SDR family) member 4, 11beta-hydroxysteroid dehydrogenase type 1, L-xylulose reductase, two types of aflatoxin B1 aldehyde reductase, 20alpha-hydroxysteroid dehydrogenase, and three types of 3alpha-hydroxysteroid dehydrogenase. Accumulating data on the human enzymes provide new insights into their roles in cellular and molecular reactions including xenobiotic metabolism. On the other hand, mice and rats lack the gene for a protein corresponding to human 3alpha-hydroxysteroid dehydrogenase type 3, but instead possess additional five or six genes encoding proteins that are structurally related to human hydroxysteroid dehydrogenases. Characterization of the additional enzymes suggested their involvement in species-specific biological events and species differences in the metabolism of xenobiotic carbonyl compounds.

Measurement of xenobiotic carbonyl reduction in human liver fractions

Carbonyl reducing enzymes are involved in the metabolism of endogenous as well as xenobiotic molecules. Enzymes that catalyze the reversible oxidoreduction of aldehyde and ketone moieties include alcohol dehydrogenases, aldo-keto reductases, quinone reductases, and short-chain dehydrogenases/reductases. These enzymes differ with respect to subcellular location, cofactor dependence, and susceptibility to chemical inhibitors. Thus, it is possible to assess the relative contributions of these enzyme systems in the hepatic metabolism of a particular xenobiotic through simple in vitro experiments with commercially available reagents. The approaches described in this unit assume the availability of analytical procedures for measuring the parent compound and metabolites, such as HPLC with radiochemical, UV, or MS detection. Thus, the purpose of this unit is to outline methods for the study of the enzymatic carbonyl reduction of a drug development candidate or other xenobiotic molecule of interest.

Human Carbonyl Reduction Pathways and a Strategy for Their Study In Vitro

Carbonyl reduction plays a significant role in physiological processes throughout the body. Although much is known about endogenous carbonyl metabolism, much less is known about the roles of carbonyl-reducing enzymes in xenobiotic metabolism. Multiple pathways exist in humans for metabolizing carbonyl moieties of xenobiotics to their corresponding alcohols, readying these molecules for subsequent conjugation and/or excretion. When exploring carbonyl reduction clearance pathways for a drug development candidate, it is possible to assess the relative contributions of these enzymes due to their differences in subcellular locations, cofactor dependence, and inhibitor profiles. In addition, the contributions of these enzymes may be explored by varying incubation conditions, such as pH. Presently, individual isoforms of carbonyl-reducing enzymes are not widely available, either in recombinant or purified form. However, it is possible to study carbonyl reduction clearance pathways from simple experiments with commercially available reagents. This article provides an overview of carbonyl-reducing enzymes, including some kinetic data for substrates and inhibitors. In addition, an experimental strategy for the study of these enzymes in vitro is presented.

Carbonyl reduction of the potential cytostatic drugs benfluron and 3,9-dimethoxybenfluron in human in vitro

Benfluron (B, [5-(2-N-oxo-2-N',N"-dimethylaminoethoxy)-7-oxo-7H-benzo[c]fluorene]) is a potential benzo[c]fluorene antineoplastic agent with high activity against a broad spectrum of experimental tumors in vitro and in vivo. The structure of B has been modified to repress its rapid deactivation through carbonyl reduction on C7. 3,9-Dimethoxybenfluron (D, [3,9-dimethoxy-5-(2-N-oxo-2-N',N"-dimethylaminoethoxy)-7-oxo-7H-benzo[c]fluorene]) is one of the B derivatives developed. The present paper was designed to compare the C7 carbonyl reduction of B and D in microsomes, cytosol and hepatocytes from human liver. Two purified human enzymes, microsomal 11beta-hydroxysteroid dehydrogenase 1 (11beta-HSD 1) and cytosolic carbonyl reductase, were tested if they are responsible for B and D carbonyl reduction in the respective fractions. Indeed, carbonyl reduction of D in comparison to that of B was 4 and 6-10 times less extensive in human liver microsomes and cytosol, respectively. Moreover, about 10-20 times higher amounts of dihydro B than dihydro D were detected in primary culture of human hepatocytes. 11beta-HSD 1 was shown to be able to reduce B and D. For this enzyme, about 10 times higher rates of carbonyl reduction were observed for B than for D. Likewise, CR participates in B and D carbonyl reduction, although smaller amounts of both reduced metabolites were detected. In summary, carbonyl reduction of D was significantly less extensive than that of B in all in vitro experiments. This lower rate of D inactivation was especially pronounced in hepatocytes which represent a close to in vivo situation. Our results clearly demonstrate that dimethoxy substitution protects the carbonyl group of the benzo[c]fluorene moiety against the deactivation by microsomal and cytosolic reductases. Detailed knowledge on the participating enzymes may serve as a basis for the co-application of specific inhibitors in chemotherapy to further improve the pharmacokinetics of benzo[c]fluorene derivatives.

Carbonyl reduction of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) by cytosolic enzymes in human liver and lung

The tobacco specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a potent pulmonary carcinogen, independent of the route and type of administration. There are competing metabolic activation and detoxification pathways. NNK is activated by alpha-hydroxylation at either the methyl or methylene carbonyl adjacent to the N-nitroso group to yield intermediates that methylate and pyridyloxobutylate DNA. Detoxification of NNK in humans usually occurs via carbonyl reduction to its hydroxy product NNAL, which undergoes glucuronosylation and final excretion. In vitro studies on NNK metabolism have usually been performed with tissue homogenates, microsomal fractions and/or purified microsomal enzymes, but cytosolic metabolism of NNK has been ignored until today. The results of this study demonstrate that cytosolic fractions of human liver and lung also participate in NNK metabolism. We provide evidence that a substantial degree of NNK carbonyl reduction occurs by cytosolic enzymes and that these enzymes may contribute to NNK detoxification in human liver and lung. The relative contribution of cytosolic vs. microsomal NNK carbonyl reduction is nearly identical in liver, whereas it is more than 3-fold higher in lung microsomes compared to lung cytosol. The inhibition profile suggested that mainly carbonyl reductase (EC 1.1.1.184) was active in cytosol of both organs. The expression of carbonyl reductase mRNA in liver and lung was proven by reverse transcription-(RT)-PCR. In conclusion, the results of this study provide the first data on cytosolic enzymes participating in NNK detoxification in human liver and lung.

Carbonyl reduction of the tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) in cytosol of mouse liver and lung

The tobacco specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a strong lung carcinogen in all species tested. To elicit its tumorigenic effects, NNK requires metabolic activation which is supposed to occur via alpha-hydroxylation by cytochrome P450 enzymes. Carbonyl reduction to 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) followed by glucuronosylation is considered to be the main detoxification pathway in humans. Therefore, NNK carbonyl reducing activity is crucial for NNK inactivation since it initiates the final excretion of this lung carcinogen. Until the present work, studies on NNK metabolism have focused exclusively on microsomal fractions, and several cytochrome P450 enzymes have been shown to be involved in alpha-hydroxylation of NNK. In addition, 11beta-hydroxysteroid dehydrogenase type 1(11beta-HSD1) which is located in the endoplasmic reticulum of the cell has been identified to catalyze the carbonyl reduction of NNK in microsomes. In this study, we provide evidence that carbonyl reduction of NNK does also take place in the cytosolic fraction of mouse liver and lung, and that cytosolic carbonyl reductase contributes to the detoxification of NNK. At a fixed substrate concentration of 1 mM NNK, the specific activity of cytosolic NNAL formation amounts to 72% (liver) and 28% (lung) compared with that in the respective microsomal fractions. Although considerable NNK carbonyl reduction occurred with NADH, the preferred cosubstrate in cytosol is either NADPH or an NADPH-regenerating system. Due to the inhibitor sensitivity to menadione, ethacrynic acid, dicoumarol and quercitrin, it is concluded that carbonyl reductase (EC 1.1.1.184) is mainly responsible for NNAL formation in liver and lung cytosol. The expression of cytosolic carbonyl reductase and microsomal 11beta-HSD1 was established on the mRNA level by reverse transcription-PCR in both liver and lung. Enzyme kinetic studies revealed a nonsaturable Michaelis-Menten kinetic of NNK carbonyl reduction in cytosol. Possibly some other cytosolic NNK carbonyl reducing enzymes are also involved in NNAL formation. In conclusion, this is the first report to show that carbonyl reduction of NNK does occur in cytosol. Further studies with purified enzyme preparations are needed to explore the detailed contribution of the cytosolic enzymes participating in the final elimination of this lung carcinogen.

Plasma concentrations of timiperone and its reduced metabolite in the patients on timiperone

Plasma levels of timiperone (TIM) and reduced timiperone (RTIM) were determined in 10 schizophrenic patients who were treated with TIM. Activities of ketone reductase in red blood cells (RBC) were assayed using TIM and haloperidol (HAL) as substrates. Plasma levels of TIM and RTIM ranged from 3.2 ng/mL to 9.5 ng/mL and from 1.7 ng/mL to 7.6 ng/mL, respectively, and approximately four-fold inter-individual variations in RTIM/TIM (0.37-1.43, 0.67 +/- 0.25) were observed. There were significant and positive correlations between plasma levels of TIM or RTIM with the daily doses of TIM (mg/kg body weight); TIM (ng/mL)= 19.5 x daily dose of TIM (mg/kg) + 1.3, r = 0.79, n = 18, P < 0.01; RTIM (ng/mL) = 13.1 x daily dose of TIM (mg/kg) + 0.7, r = 0.73, n = 18, P < 0.001). Given 0.3 mg/kg of TIM, a plasma level of TIM as 7.15 ng/mL was predicted, which is approximately 60% that of HAL. The activity of TIM reductase in RBC was determined as approximately 70% of HAL reductase in RBC, although the correlation between the activities of TIM reductase and HAL reductase in RBC was high (r = 0.98, n = 10, P < 0.001).