Assessing the effect of N-oxidation on the mutagenicity of 1-pyrazolines using the Ames assay.

-Nitrosamines are well known as environmental carcinogens. We have reported that -nitroso--methylbutylamine was oxidized by Fe-Cu-HO to 5-methyl-5-nitro-1-pyrazoline, a direct-acting -oxide. 1-Pyrazolines have not been reported to exhibit genotoxicity. In this study, we investigated the effect of -oxidation on the mutagenicity of 1-pyrazolines using the Ames assay. The mutagenicity of 5-alkyl-5-nitro-1-pyrazoline 1-oxide (1a; methyl, 1b; ethyl), the -oxide isomer (3-alkyl-3-nitro-1-pyrazoline 1-oxide; 2a; methyl, 2b; ethyl), and the corresponding nonoxides (3-alkyl-3-nitro-1-pyrazoline; 3a; methyl, 3b; ethyl) was assayed in TA1535 and WP2. The ratios of mutagenic potency in TA1535 versus WP2 were compared with those of -alkylnitrosoureas. To predict the reaction site on the pyrazolines with nucleophiles, the electron density of the pyrazolines was obtained by theoretical calculations. The pyrazolines were mutagenic in TA1535 and WP2. The ratio of TA1535 to WP2 1a (87:13) or 1b (90:10) was similar to that of -ethyl--nitrosourea (70:30). In contrast, the mutagenic ratio of 2a (22:78) or 2b (52:48) was similar to that of -propyl--nitrosourea (48:52) or -butyl--nitrosourea (14:86). The ratio of 3a (53:47) or 3b (54:46) was similar to that of -propyl--nitrosourea or -butyl--nitrosourea. The pyrazolines exhibit genotoxicity, and the mutagenic potency of the 1-pyrazolines is influenced by -oxidation. We estimated that the mutagenicity of 1a or 1b was caused by DNA ethylation, and the isomers or the nonoxides were mutagenic via formation of alkylated DNA, which contains an alkyl chain longer than the propyl.