Metabolic activation of the potent carcinogen dibenzo[a,h]anthracene by cDNA-expressed human cytochromes P450.

The metabolic activation of the potent carcinogen dibenzo[a,h]anthracene (DB[a,h]A) was investigated with recombinant human cytochrome P450 enzymes 1A2, 2B6, 2C8, 2C9, 2E1, 3A3, 3A4, and 3A5 expressed in hepatoma G2 cells and with 14 different human liver microsomes. Three dihydrodiols, three phenols, and one diphenol were formed and separated by high-performance liquid chromatography and identified by UV absorption and mass spectra. Of all P450s tested, 1A2 and 2C9 were the most active and 2B6 was moderately active in the rate of total DB[a,h]A metabolism (2.5- to 12-fold greater activity than that for other P450s). The trans-3,4-dihydrodiol, generally recognized as a precursor of the ultimate carcinogenic 3,4-diol-1,2-epoxides, was produced most actively by 2C9, then 1A2 and 2B6. The values of enzymatic kinetics (K(m) and V(max)) indicated that 2C9 had the highest catalytic efficiency (V(max)/K(m) = 9.7) in the formation of 3,4-dihydrodiol, in contrast to 1A2 (5.9) and 2B6 (4.4). 1A2 had the highest activity toward production of the 1,2-dihydrodiol, which is considered to be a weakly carcinogenic metabolite. Although specific activities of human liver microsomes in overall metabolism of DB[a,h]A markedly differed between individuals, metabolic patterns were observed similar to that generated from 1A2. Since human 1A1, a predominant enzyme for metabolism of polycyclic aromatic hydrocarbons, is not significantly expressed in the liver, hepatic microsomal 2C9, 1A2, and 2B6 all probably contribute to the metabolic activation of DB[a,h]A.