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

The biochemistry of drug metabolism--an introduction: Part 2. Redox reactions and their enzymes

This review continues a general presentation of the metabolism of drugs and other xenobiotics started in a recent issue of Chemistry & Biodiversity. This Part 2 presents the numerous oxidoreductases involved, their nomenclature, relevant biochemical properties, catalytic mechanisms, and the very diverse reactions they catalyze. Many medicinally, environmentally, and toxicologically relevant examples are presented and discussed. Cytochromes P450 occupy a majority of the pages of Part 2, but a large number of relevant oxidoreductases are also considered, e.g., flavin-containing monooxygenases, amine oxidases, molybdenum hydroxylases, peroxidases, and the innumerable dehydrogenases/reductases.

Aldo-keto reductases and bioactivation/detoxication

Aldo-keto reductases (AKRs) are soluble NAD(P)(H) oxidoreductases that primarily reduce aldehydes and ketones to primary and secondary alcohols, respectively. The ten known human AKR enzymes can turnover a vast range of substrates, including drugs, carcinogens, and reactive aldehydes. They play central roles in the metabolism of these agents, and this can lead to either their bioactivation or detoxication. AKRs are Phase I drug metabolizing enzymes for a variety of carbonyl-containing drugs and are implicated in cancer chemotherapeutic drug resistance. They are involved in tobacco-carcinogenesis because they activate polycyclic aromatic trans-dihydrodiols to yield reactive and redox active o-quinones, but they also catalyze the detoxication of nicotine derived nitrosamino ketones. They also detoxify reactive aldehydes formed from exogenous toxicants, e.g., aflatoxin, endogenous toxicants, and those formed from the breakdown of lipid peroxides. AKRs are stress-regulated genes and play a central role in the cellular response to osmotic, electrophilic, and oxidative stress.

Stereoselective reduction of 4-benzoylpyridine in the heart of vertebrates

The stereoselectivity in the reduction of 4-benzoylpyridine (4-BP) was examined in the cytosolic fractions from the heart of 9 vertebrates (pig, rabbit, guinea pig, rat, mouse, chicken, soft-shelled turtle, frog and flounder). 4-BP was stereoselectively reduced to S(-)-alpha-phenyl-4-pyridylmethanol [S(-)-PPOL] in the cytosolic fractions from the heart of pig, rabbit and guinea pig. However, of mammalian heart cytsol tested, only rat heart cytosol had little ability to reduce stereoselectively 4-BP. In an attempt to elucidate this reason, amino acid sequence of rat heart carbonyl reductase (RatHCR) was deduced from the cloned cDNA and compared with that of pig heart carbonyl reductase (PigHCR), which shows a high stereoselectivity in the reduction of 4-BP to S(-)-PPOL. RatHCR showed a high identity with PigHCR in amino acid sequence. Furthermore, recombinant RatHCR was confirmed to reduce stereoselectively 4-BP to S(-)-PPOL with a high optical purity comparable to recombinant PigHCR. It is possible that in the cytosolic fraction from the heart of rat, constitutive reductase other than RatHCR counteracts the stereoselective reduction of 4-BP to S(-)-PPOL, by catalyzing the reduction of 4-BP to the R(+)-enantiomer.

Ritonavir has minimal impact on the pharmacokinetic disposition of a single dose of bupropion administered to human volunteers

A drug-drug interaction study was conducted to determine whether ritonavir (200 mg; 4 doses over 2 days) alters the pharmacokinetic disposition of bupropion (75 mg; once) coadministered to 7 healthy volunteers in a placebo-controlled 2-way crossover study. Serum samples collected from 0 to 24 hours after bupropion administration were assayed for concentrations of bupropion and metabolites (hydroxybupropion, threohydrobupropion, and erythrohydrobupropion). Derived pharmacokinetic parameters were compared between placebo/bupropion and ritonavir/bupropion trials by paired t test. The effect of ritonavir on most pharmacokinetic parameters was minimal (<20% mean change). The only parameters that showed a statistically significant effect were threohydrobupropion area under the blood concentration curve (14% +/- 5% decrease, mean +/- SE; P = .04) and erythrohydrobupropion time-to-maximal serum concentration (161% +/- 92% increase, P = .03), suggesting that ritonavir may inhibit the carbonyl reductase enzyme responsible for formation of these metabolites. These findings indicate that short-term ritonavir dosing has only minimal impact on the pharmacokinetic disposition of a single dose of bupropion in healthy volunteers.

Higher activity of polymorphic NAD(P)H:quinone oxidoreductase in liver cytosols from blacks compared to whites

In human liver, the two-electron reduction of quinone compounds, such as menadione is catalyzed by cytosolic carbonyl reductase (CBR) and NAD(P)H:quinone oxidoreductase (NQO1) activities. We assessed the relative contributions of CBR and NQO1 activities to the total menadione reducing capacity in liver cytosols from black (n=31) and white donors (n=63). Maximal menadione reductase activities did not differ between black (13.0+/-5.0 nmol/min mg), and white donors (11.4+/-6.6 nmol/min mg; p=0.208). In addition, both groups presented similar levels of CBR activities (CBR(blacks)=10.9+/-4.1 nmol/min mg) versus CBR(whites)=10.5+/-5.8 nmol/min mg; p=0.708). In contrast, blacks showed higher NQO1 activities (two-fold) than whites (NQO1(blacks)=2.1+/-3.0 nmol/min mg versus NQO1(whites)=0.9+/-1.6 nmol/min mg, p<0.01). To further explore this disparity, we tested whether NQO1 activity was associated with the common NQO1(*)2 genetic polymorphism by using paired DNA samples for genotyping. Cytosolic NQO1 activities differed significantly by NQO1 genotype status in whites (NQO1(whites[NQO1*1/*1])=1.3+/-1.7 nmol/min mg versus NQO1(whites[NQO1*1/*2+NQO1*2/*2])=0.5+/-0.7 nmol/min mg, p<0.01), but not in blacks (NQO1(blacks[NQO1*1/*1])=2.6+/-3.4 nmol/min mg versus NQO1(blacks[NQO1*1/*2])=1.1+/-1.2 nmol/min mg, p=0.134). Our findings pinpoint the presence of significant interethnic differences in polymorphic hepatic NQO1 activity.

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.