QUANTITATIVE STRUCTURE-TOXICITY RELATIONSHIPS AND VOLUME FRACTION ANALYSES FOR SELECTED ESTERS

The acute toxicity of aliphatic and aromatic mono and diesters in two eucaryotic organisms was compared. The test systems were the static 2-d Tetrahymena pyriformis 50% population growth impairment (IGC(50)(-1)) assay, and the flow-through 4-d Pimephales promelas 50% mortality (LC(50)(-1)) assay. In ciliates, esters act via the nonpolar narcosis mechanism of toxic action. This was indicated by: the high quality 1-octanol/water partition coefficient (log K-ow) dependent quantitative structure-activity relationship (QSAR), log IGC(50)(-1) = 0.79 (log K-ow) - 1.93, n = 15, r(2) = 0.945, s = 0.22, f = 222.37 Pr > f = 0.0001); volume fraction (V-f) (0.8e-02); and ''a'' coefficient (0.3) which are not different from other nonpolar narcotics. In vivo hydrolysis in Tetrahymena appears to be insignificant. However, in fish, presumably because of more active esterases, in vivo hydrolysis is significant and leads to greater toxicity of esters than observed for nonpolar narcotics. Moreover, it leads to a unique high quality QSAR, log LC(50)(-1) = 0.64 (log K-ow) - 0.64, n = 14, r(2) = 0.945, s = 0.22, f = 207.08, Pr > f = 0.0001). Due to in vivo hydrolysis, a nonreducing concentration gradient is formed between water and fish. Therefore, the fish take up more toxicant as compared to a situation that leads to thermodynamic equilibrium. Additional information about the mechanism of ester toxicity in fish was gained by applying corrections for hydrolysis in volume fraction analyses. The corrected V-f (0.6e-02) is very close to the one found for nonpolar narcotics (0.7e-02). These analyses suggest that esters which hydrolyze to an acid and aliphatic alcohol act as nonpolar narcotics. Moreover, the mechanism of toxic action of esters that yield a phenol upon hydrolysis is mixed and represents polar and nonpolar narcoses.