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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 EC50 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

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The effects on mitochondrial respiratory parameters of linear alkylbenzene sulfonates (LAS), nonylphenol polyethoxylates (NPEO), and some of their biotransformation products, namely sulfophenyl carboxylates (SPCs), nonylphenol (NP), and nonylphenoxy acetic acid (NPlEC), were recorded by using the in vitro response of submitochondrial particles (SMP) from beef heart. The toxicity of these compounds was estimated by determining their effects on the energy-coupled reverse electron transfer (RET), which IS induced by ATP and succinate at the first site level of the respiratory chain and reduces exogenous NAD+ to NADH. The toxicity of the substances, expressed as the toxicant concentration decreasing the reduction rate of NAD+ to an extent of 50% (EC50), ranged from 0.61 mg/L for a commercial LAS mixture to 18,000 mg/L for individual SPCs; from 1.3 mg/L for NPEO, with an average of 10 ethoxy units, to 8.2 and 1.8 mg/L for NPlEC and NP, respectively. These results were related to the molecular structure of each compound class and compared with the toxicity values obtained by a variety of biological systems currently used for toxicity testing. The acute toxicity data have demonstrated that (a) the SMP bioassay is suitable for reproducing the toxicological response of whole organisms, such as fishes and invertebrates, to the tested chemicals; and (b) the hydrophobic moiety of these compounds plays a significant role in eliciting their toxic effects. From a toxicological standpoint, attention must be paid to the occurrence in natural waters of residual LAS, whereas in the case of NPEO both unaltered surfactant and all biotransformation products need to be identified and quantified. Keywords-Surfactant toxicity Submitochondrial particles Nonylphenol polyethoxylates Nonylphenol 10. Abel, P.D. 1974. Toxicity of synthetic detergents to fish and 1 1 . Schoeberl, P. 1989. Principles of LAS biodegradation. Tenside 11:26-31. aquatic invertebrates. J. Fish Biol. 6:279-298. Det. 26:86-94.