Mutation Research 541 (2003) 123–136
Possibility of the involvement of 9H-pyrido[3,4-b]indole
(norharman) in carcinogenesis via inhibition of cytochrome
P450-related activities and intercalation to DNA
Hiroaki Nii∗
Laboratory of Radiochemistry, Gifu Pharmaceutical University, 5-6-1 Mitahorahigashi, Gifu 502-8585, Japan
Received 7 March 2003; received in revised form 1 August 2003; accepted 1 August 2003
Abstract
This study investigated the inhibitory effect of 9H-pyrido[3,4-b]indole (norharman), one of the naturally occurring -
carbolines, on cytochrome P450 (CYP)-related activities and the relationship between its inhibitory effect, its intercalation to
DNA, and its comutagenic effect. Norharman reduced the mutagenicities of heterocyclic amines (HCAs) containing 2-amino-6-
methyldipyrido[1,2-a:3′,2′-d]imidazole (Glu-P-1), aflatoxin B1, benzo[a]pyrene (BP), and some nitrosamines in the pres-
ence of 10l liver S9 (20.9g protein/ml) from polychlorinated biphenyl-treated rats. Norharman inhibited microsomal
CYP-related enzyme activities and CO-binding to the CYP heme (50% inhibitory concentration (IC50), 0.07–6.4 g/ml).
It also inhibited the formation of 3-hydroxyamino-6-methyldipyrido[1,2-a:3′,2′-d]imidazole (N-OH-Glu-P-1) and was a
noncompetitive-inhibitor of CYP1A-related activities, while it enhanced the direct mutagenicity of N-OH-Glu-P-1 (50%
effective concentration, 25.0g/ml) and inhibited topo I activity (IC50, 31.0g/ml). In the presence of norharman, S9 up to
100l incrementally enhanced the mutagenicities of HCAs, BP and dimethylnitrosamine. These data clarified that norharman
acts as an inhibitor of the CYP-mediated biotransformation of Glu-P-1 via inhibition of O2-binding to CYP heme, and its
inhibition of CYP enzymes occurs at much lower concentration than that for its intercalation to DNA. It is indicated that
norharman’s inhibitory effect on CYP results in the inhibition of excess metabolism by S9 and this is more likely the mecha-
nism for comutagenic action than the intercalation. Norharman’s