The inhibition of cytochrome P450 2A13-catalyzed NNK metabolism by NAT, NAB and nicotine

Abstract

4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is considered to be the most carcinogenic of the four tobacco-specific nitrosamines (TSNAs) and it needs to be metabolically activated to exert its carcinogenic effect on humans. For the simultaneous intake of NNK and other compounds with similar molecular structures in the context of tobacco smoke, whether (,)--nitrosoanatabine (NAT), (,)--nitrosoanabasine (NAB) and nicotine contribute to the inhibitory potency of the cytochrome P450 (CYP) enzyme-catalyzed NNK metabolism or not needs to be investigated. In the study, 4-oxo-4-(3-pyridyl) butanal (OPB), 4-hydroxy-1-(3-pyridyl)-1-butanone (HPB) and 4-oxo-4-(3-pyridyl) butanoic acid (OPBA) were established as the products of the CYP2A13-catalyzed NNK metabolism and the kinetic parameters were calculated from the Michaelis-Menten equation. Addition of NAT, NAB or nicotine resulted in a competitive inhibition for the NNK metabolism catalyzed by CYP2A13. The inhibition constant values were calculated to be 0.21 μM (NAT), 0.23 μM (NAB) and 8.51 μM (nicotine) for OPB formation; 0.71 μM (NAT), 0.87 μM (NAB) and 25.01 μM (nicotine) for HPB formation and 0.36 μM (NAT), 0.50 μM (NAB) and 6.57 μM (nicotine) for OPBA formation, respectively. In addition, the study of the transformation of the three metabolites revealed OPB was not only an end product but also an intermediate product of the CYP2A13-catalyzed NNK metabolism. These results suggest that structurally similar tobacco constituents with weak or no carcinogenicity influence the metabolic activation of NNK, which interferes with its carcinogenicity to some extent.

Nicotine Inhibits the Cytotoxicity and Genotoxicity of NNK Mediated by CYP2A13 in BEAS-2B Cells

“…Tobacco is a potent multisite carcinogen with a substantial worldwide impact [ 18 ]. Both nicotine and NNK are important components in cigarette smoke that contribute to cancer development [ 19 , 20 ], and nicotine has been reported to inhibit the metabolism of NNK by inhibiting CYP2A13 [ 21 , 22 ], so it is of great significance to investigate the effect of CYP2A13-mediated nicotine on the cytotoxicity and genotoxicity of NNK on lung cells. Therefore, based on the constructed B-2A13 cells, the toxic effects of NNK alone and NNK combined with nicotine were studied in this study.…”

Genetic and Enzymatic Characteristics of CYP2A13 in Relation to Lung Damage

“…Tobacco smoke is known to contain thousands of unique compounds; some of them, namely (R,S)-N-nitrosoanatabine (NAT), (R,S)-N-nitrosoanabasine (NAB), and nicotine, inhibited the CYP2A13-mediated metabolism of NNK [24]. Similar inhibitory behavior expressed other two tobacco constituents, namely 1-methyl-4-(3-pyridinyl) pyrrole (betanicotyrine) and (-)-menthol, which were found to be CYP2A13 inhibitors with K(i) 0.17 µM and 8.2 µM, respectively [25].…”

The Multifarious Link between Cytochrome P450s and Cancer

“…This elevated level of CYP2A13 bioactivation and the expression of CYP2A13 in the human liver indicate that CYP2A13 could play a major part in NNK activation, which is more effective in CYP2A13 in vitro metabolism than CYP2A6 [57]. Many different CYP2A subfamily members can effectively activate several carcinogenic nitrosamines in vitro [58]. These P450s play a significant role in NNK metabolic activation in vivo in the human liver and mouse lung.…”