Cytochrome P450 isozymes catalyzing the hepatic microsomal oxidation of 9-anthraldehyde to 9-anthracene carboxylic acid in adult male rats

Cytochromes P450 catalyze the reduction of α,β-unsaturated aldehydes

The metabolism of α,β-unsaturated aldehydes, e.g., 4-hydroxynonenal, involves oxidation to carboxylic acids, reduction to alcohols, and glutathionylation to eventually form mercapturide conjugates. Recently, we demonstrated that P450s can oxidize aldehydes to carboxylic acids, a reaction previously thought to involve aldehyde dehydrogenase. When recombinant cytochrome P450 3A4 was incubated with 4-hydroxynonenal, O(2), and NADPH, several products were produced, including 1,4-dihydroxynonene (DHN), 4-hydroxy-2-nonenoic acid (HNA), and an unknown metabolite. Several P450s catalyzed the reduction reaction in the order (human) P450 2B6 ≅ P450 3A4 > P450 1A2 > P450 2J2 > (mouse) P450 2c29. Other P450s did not catalyze the reduction reaction (human P450 2E1 and rabbit P450 2B4). Metabolism by isolated rat hepatocytes showed that HNA formation was inhibited by cyanamide, while DHN formation was not affected. Troleandomycin increased HNA production 1.6-fold while inhibiting DHN formation, suggesting that P450 3A11 is a major enzyme involved in rat hepatic clearance of 4-HNE. A fluorescent assay was developed using 9-anthracenealdehyde to measure both reactions. Feeding mice a diet containing t-butylated hydroxyanisole increased the level of both activities with hepatic microsomal fractions but not proportionally. Miconazole (0.5 mM) was a potent inhibitor of these microsomal reduction reactions, while phenytoin and α-naphthoflavone (both at 0.5 mM) were partial inhibitors, suggesting the role of multiple P450 enzymes. The oxidative metabolism of these aldehydes was inhibited >90% in an Ar or CO atmosphere, while the reductive reactions were not greatly affected. These results suggest that P450s are significant catalysts of the reduction of α,β-unsaturated aldehydes in the liver.

Differences in susceptibility to N-butyl-N-(4-hydroxybutyl)nitrosamine-induced urinary bladder carcinogenesis between SD/gShi rats with spontaneous hypospermatogenesis and SD/cShi rats with spontaneous hydronephrosis

Differences in susceptibility to N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN)-induced urinary bladder carcinogenesis between two substrains of male Sprague-Dawley rats were examined. One substrain was SD/gShi, which has spontaneous hypospermatogenesis, and the other was SD/cShi, which is a sister strain of SD/gShi, and has normal testis but spontaneous hydronephrosis. SD/gShi rats had a lower incidence of urinary bladder tumors and had lower 5-bromo-2'-deoxyuridine labeling indices in the urinary bladder epithelium than SD/cShi rats when BBN was given. SD/gShi rats had significantly lower urinary concentrations of N-butyl-N-(3-carboxypropyl)nitrosamine (BCPN), which is a metabolite and proximate carcinogen of BBN. In vitro analysis also showed significantly less BCPN formation, using an S9 mix derived from the liver and kidney, in SD/gShi rats than in SD/cShi rats. BCPN formation in vitro was markedly inhibited by non-selective cytochrome P450 (CYP) inhibitors, but not alcohol dehydrogenase inhibitor. However, analysis of CYP proteins including hepatic CYP1A1/2, 2B1/2, 2E1, and 3A2 and renal CYP2E1 and 3A2 revealed no significant variation in levels in either tissue in the groups. There were also no significant intergroup differences in the mutagenicity of carcinogens, including heterocyclic amines and N-nitrosamines, activated by CYP1A1/2 and CYP2E1 and/or CYP2B1/2, respectively. These results suggest that SD/gShi rats are less susceptible to BBN, possibly because less BCPN is produced by CYP isoforms other than those investigated. A contribution of CYP4B1 to the strain difference is also possible.

Major Cytochrome P450 Enzymes Responsible for Microsomal Aldehyde Oxygenation of 11-Oxo-Δ8-tetrahydrocannabinol and 9-Anthraldehyde in Human Liver

Hepatic microsomes from human liver catalyzed oxidation of the allyl aldehydes such as 11-oxo-Delta(8)-tetrahydrocannabinol and 9-anthraldehyde to the corresponding carboxylic acid metabolites. The oxygenation mechanism was confirmed by GC-MS that molecular oxygen was exclusively incorporated into Delta(8)-tetrahydrocannabinol-11-oic acid and 9-anthracene carboxylic acid formed under oxygen-18 gas. The microsomal aldehyde oxygenase (named MALDO) activities of 11-oxo-Delta(8)-tetrahydrocannabinol and 9-anthraldehyde were significantly inhibited by the antibody against CYP2C and CYP3A, respectively. MALDO activity for 11-oxo-Delta(8)-tetrahydrocannabinol was significantly inhibited by sulfaphenazole whereas that for 9-anthraldehyde was markedly inhibited by troleandomycin, but not by sulfaphenazole. CYP2C9 expressed in human B-lymphoblastoid cells catalyzed efficiently the MALDO activity for 11-oxo-Delta(8)-tetrahydrocannabinol (10.1 nmol/min/nmol P450), while the catalytic activities of other human CYPs expressed in the cells were lesser extents. In MALDO activity for 9-anthraldehyde, CYP3A4 expressed in the cells had the highest catalytic activity (7.72 nmol/min/nmol P450). These results indicate that CYP2C9 and CYP3A4 are major enzymes responsible for the MALDO activity in human liver for 11-oxo-Delta(8)-tetrahydrocannabinol and 9-anthraldehyde, respectively.

Recent advances in the metabolism of cannabinoids

This review describes recent advances in the metabolism of cannabinoids. Cannabidiol was metabolized to cannabielsoin, 6 beta-hydroxymethyl-delta 9-tetrahydrocannabinol and an oxepine derivative through epoxide intermediates by hepatic microsomal enzymes containing cytochrome P450 of animals. Cannabidiol inactivated cytochrome P450 UT-2 (CYP2C11) not equal to in male rats and a member of 3A subfamily in mouse liver. These inactivations may be very important because serious drug-drug interactions will occur in the case that cannabidiol is co-administered with drugs which are metabolized mainly by the enzyme system containing these P450 isozymes. A member of cytochrome P450 belonging to 2C subfamily was the major isozymes responsible for the cannabinoid metabolism in many experimental animals and that of 3A subfamily made some contribution to the metabolism of cannabinoids by human hepatic microsomes. Microsomal aldehyde oxygenase, a particular isozyme of cytochrome P450 catalyzing the oxidation of 11-oxo-tetrahydrocannabinol to tetrahydrocannabinol-11-oic acid, was found for the first time by the authors. Cytochrome P450 MUT-2 (CYP2C29) is the major isozyme responsible for the microsomal aldehyde oxygenase activity in mouse hepatic microsomes.

Metabolism of delta 9-tetrahydrocannabinol by cytochrome P450 isozymes purified from hepatic microsomes of monkeys

The most abundant metabolite of delta 9-tetrahydrocannabinol (delta 9-THC) formed with hepatic microsomes of monkeys was 11-OH-delta 9-THC, followed by 8 alpha-OH-, 8 beta-OH- and 3'-OH-delta 9-THCs. Two cytochrome P450 isozymes, P450RM-A and P450JM-C, were purified from monkey hepatic microsomes and found to have a molecular weight of 51,000. In the reconstituted system, the activities of P450RM-A towards formation of 11-OH-, 8 alpha-OH-, 8 beta-OH- and 3'-OH-delta 9-THCs were 19-, 40-, 22- and 10-fold higher, respectively, than the corresponding activities of the hepatic microsomes. The activity of P450JM-C towards formation of 3'-OH-delta 9-THC was 10-fold higher than that of P450RM-A, while the activities of both isozymes for 11- and 8 alpha-hydroxylation were not so much different and the 8 beta-hydroxylation activity was 14-fold higher in P450RM-A than in P450JM-C. Antibodies against P450RM-A and P450JM-C markedly inhibited the microsomal formation of 11-OH- and 8 alpha-OH-delta 9-THCs, and 3'-OH-delta 9-THC, respectively. These results suggest that P450RM-A and P450JM-C are major isozymes responsible for the formation of 11-OH- and 8 alpha-OH-delta 9-THCs, and 3'-OH-delta 9-THC, respectively, in monkeys.