Submitochondrial particles as toxicity biosensors of chlorophenols

An in vitro bioassay procedure was used to investigate the toxic action of chlorophenols on mitochondrial respiratory parameters. The toxicity of these compounds was evaluated by determining their effects on the energy-coupled reverse electron transfer (RET) in submitochondrial particles (SMPs) from beef heart mitochondria. The bioassay procedure is based on the spectrophotometric recording of the effects of toxicants on the rate of NAD(+) reduction, induced by ATP and succinate at the first site level of the respiratory chain. The toxicity end point was expressed as the toxicant concentration that causes 50% inhibition of NAD(+) reduction rate (EC50). The ED50 values determined for the 14 tested chlorophenols ranged from 17 mg/L for 2-chlorophenol to 0.081 mg/L for pentachlorophenol, indicating a general trend of increasing toxicity with increasing chlorine substitution. Among chlorophenol isomers, which have the same number of chlorine atoms, a lesser toxicity was associated with ortho-substituted chlorophenols, whereas meta-substituted chlorophenols were much more toxic. The EC50 values were compared with the toxicity data for a variety of bioassays, by means of linear regression analysis. High degrees of correlation obtained with toxicity tests involving different freshwater species demonstrate the ability of SMPs to reproduce the toxic effects of the tested compounds upon aquatic organisms. This supports the assessment that the respiratory chain is the main target of this class of toxicants. Results obtained with chlorophenols and, in previous studies, with other environmental contaminants confirm the suitability of the SMP bioassay as a prescreening or complementary short-term test for monitoring aquatic toxicity