Mutagenicity of Primary and Secondary Carcinogens Altered by Normal and Induced Hepatic Microsomes 1

Abstract

To establish the role of the activity of the microsomal biotransformation system in chemical carcinogenesis, the mutagenic effect of primary (ultimate) and secondary (potential) carcinogens after exposure to isolated hepatic microsomes was studied. The principal enzyme system, suggested to be active in the metabolism of carcinogenic compounds, is the nonspecific cytochrome P-450 dependent mixed-function oxidase (1). This microsomal enzyme system is inducible by many substrates, including some environmental contaminants (2). The involvement of the microsomal enzymes in carcinogenesis is suggested by the observed increased carcinogen biotransformation in animals (3) and isolated cells (4) following exposure to inducing substrates. Most carcinogenic compounds are mutagenic for microorganisms (5). The primary are themselves mutagenic, the secondary are inactive until metabolized to a mutagenic form (6). Metabolism of primary carcinogens may reduce mutagenic and carcinogenic properties. Both types of metabolic conversion, activation and deactivation, have been studied by host-mediated assay, which tests the ability of laboratory animals to alter the mutagenicity of carcinogens (6). Similar activity also has been demonstrated in the 30,000g supernatant of mice liver homogenates (7).

To demonstrate the precise relationship between the activity of the microsomal enzyme system and the biological activity of carcinogens, an assay is required in which both activities can be measured. For this purpose the ability of isolated normal or induced microsomes to alter the mutagenic activity of two carcinogens, dimethylnitrosamine (DMN) (8) and N-methyl-N′-nitro-N-nitrosoguanidine (MNNG) (9) was studied. These compounds were chosen as examples of primary (MNNG) and secondary (DMN) carcinogens.

Methods. Hepatic microsomes were isolated from male Swiss-Webster mice untreated or pretreated four days before sacrifice from a single ip injection of 500 mg/kg of the polychlorinated biphenyls (PCB), Arochlor 1254, an inducer of the microsomal enzyme system (10) and a wide-spread polluting agent (1l).