Relationship between acute toxicity of (bis)aziridinylbenzoquinones and cellular pyridine nucleotides

The toxicity of aziridinylbenzoquinones may occur by a number of mechanisms, including oxidative stress caused by redox cycling and the activation of the aziridine groups. Isolated hepatocytes were used to assess the relationship between the redox status of NADP(H) associated with oxidative stress, the level of NAD(H) closely linked with DNA repair and the cytotoxicity of three 2,5-bis(aziridinyl)-1,4-benzoquinones (BABQ). Exposure of hepatocytes to the BABQ TW13 (200 microM) and TW25 (100 microM), which are able to arylate and to redox cycle, resulted in increased intracellular NADP+ from < 0.3 nmol/mg protein to 1.5 nmol/mg protein within 60 min. The increase in intracellular NADP+ was followed by the onset of cell death by 180 min. In contrast, exposure to lower concentrations of TW13 (100 microM), TW25 (50 microM) and carboquone (100-200 microM) (which neither arylates nor redox cycles via a one-electron reduction) resulted in a less pronounced (< 1.0 nmol/mg) increase in NADP+ and there was no evidence of cell death within the 180 min incubation. BABQ had a concentration dependent effect on intracellular NAD+. Exposure of hepatocytes to TW13 (200 microM) and TW25 (100 microM) resulted in a decrease in intracellular NAD+ from > 2.7 to < 1.0 nmol/mg protein within 60 min. At concentrations of the BABQ where the level of NAD+ remained > 1.0 nmol/mg protein after 30 min, the hepatocytes remained viable at 180 min. These changes in intracellular pyridine nucleotides suggests two mechanisms may be involved in BABQ cytotoxicity. At high concentrations, aziridinylbenzoquines may cause cytotoxicity via oxidative stress following redox cycling. At lower concentrations however, the predominant pyridine nucleotide change is a prolonged depletion of NAD+, suggesting extensive DNA damage which may lead to delayed cell death.