Inhibition of human cytochrome P450 2E1 and 2A6 by aldehydes: structure and activity relationships

Structure-activity relationship and in vitro inhibition of human cytochrome CYP2A6 and CYP2A13 by flavonoids

Cigarette smoking is one of the major risk factors of various diseases including respiratory diseases and lung cancer. While the liver-specific CYP2A6 is associated with the nicotine clearance and smoking addiction, the metabolic activation of the tobacco-specific nitrosamine by lung-specific CYP2A13 can lead to lung tumorigenesis.It has been reported that inhibition of CYP2A6 and CYP2A13 enzymes by flavonoids constituents could be an aids in smoking cessation. This study demonstrates the inhibition activity of kaempferol and myricetin and the structure-function relationship of these two flavonoids and previously isolated flavonoids from Vernonia cinerea and Pluchea indica against both enzymes.Kaempferol could inhibit CYP2A6 with K_ic value of 1.77 ± 0.47 µM while inhibit CYP2A13 with K_ic value of 0.12 ± 0.01 µM. Myricetin could inhibit CYP2A6 with K_ic value of 4.06 ± 0.52 µM while inhibit CYP2A13 with K_ic value of 1.88 ± 0.03 µM.Molecular docking indicated that CYP2A13 enzyme has strong hydrophobic interaction with ring B of flavonoids compared to CYP2A6 enzyme. The presence of the hydroxyl group at C3 position of ring C and the hydroxyl group at C5' of ring B affected inhibitory activity on both enzymes.

Aldehydes are the predominant forces inducing DNA damage and inhibiting DNA repair in tobacco smoke carcinogenesis

Tobacco smoke (TS) contains numerous cancer-causing agents, with polycyclic aromatic hydrocarbons (PAHs) and nitrosamines being most frequently cited as the major TS human cancer agents. Many lines of evidence seriously question this conclusion. To resolve this issue, we determined DNA adducts induced by the three major TS carcinogens: benzo(a)pyrene (BP), 4-(methylnitrosamine)-1-(3-pyridyl)-1-butanoe (NNK), and aldehydes in humans and mice. In mice, TS induces abundant aldehyde-induced γ-hydroxy-propano-deoxyguanosine (γ-OH-PdG) and α-methyl-γ-OH-PdG adducts in the lung and bladder, but not in the heart and liver. TS does not induce the BP- and NNK-DNA adducts in lung, heart, liver, and bladder. TS also reduces DNA repair activity and the abundance of repair proteins, XPC and OGG1/2, in lung tissues. These TS effects were greatly reduced by diet with polyphenols. We found that γ-OH-PdG and α-methyl-γ-OH-PdG are the major adducts formed in tobacco smokers' buccal cells as well as the normal lung tissues of tobacco-smoking lung cancer patients, but not in lung tissues of nonsmokers. However, the levels of BP- and NNK-DNA adducts are the same in lung tissues of smokers and nonsmokers. We found that while BP and NNK can induce BPDE-dG and O6-methyl-dG adducts in human lung and bladder epithelial cells, these inductions can be inhibited by acrolein. Acrolein also can reduce DNA repair activity and repair proteins. We propose a TS carcinogenesis paradigm. Aldehydes are major TS carcinogens exerting dominant effect: Aldehydes induce mutagenic PdG adducts, impair DNA repair functions, and inhibit many procarcinogens in TS from becoming DNA-damaging agents.