The formaldehyde metabolic detoxification enzyme systems and molecular cytotoxic mechanism in isolated rat hepatocytes

The toxicity and carcinogenicity of formaldehyde (HCHO) has been attributed to its ability to form adducts with DNA and proteins. A marked decrease in mitochondrial membrane potential and inhibition of mitochondrial respiration that was accompanied by reactive oxygen species formation occurred when isolated rat hepatocytes were incubated with low concentrations of HCHO in a dose-dependent manner. Hepatocyte GSH was also depleted by HCHO in a dose-dependent manner. At higher HCHO concentrations, lipid peroxidation ensued followed by cell death. Cytotoxicity studies were conducted in which isolated hepatocytes exposed to HCHO were treated with inhibitors of HCHO metabolising enzymes. There was a marked increase in HCHO cytotoxicity when either alcohol dehydrogenase or aldehyde dehydrogenase was inhibited. Inhibition of GSH-dependent HCHO dehydrogenase activity by prior depletion of GSH markedly increased hepatocyte susceptibility to HCHO. In each case, cytotoxicity was dose-dependent and corresponded with a decrease in hepatocyte HCHO metabolism and increased lipid peroxidation. Antioxidants and iron chelators protected against HCHO cytotoxicity. Cytotoxicity was also prevented, when cyclosporine or carnitine was added to prevent the opening of the mitochondrial permeability transition pore which further suggests that HCHO targets the mitochondria. Thus, HCHO-metabolising gene polymorphisms would be expected to have toxicological consequences on an individual's susceptibility to HCHO toxicity and carcinogenesis.