PARENT SESSION
41 - Physiological and Molecular Mechanisms of Toxicity
8:00 AM to 6:30 PM, Tuesday, 14 May 2002
Exhibition Area

(41-30) Chemical mixture toxcity: a mechanistic approach.

Dawson, Douglas^*,1, Rinaldi, Andrea², Poch, Gerald³, ¹ Ashland University, Ashland, OH, USA² University of Cagliari, Monserrato, Italy³ University of Graz, Graz, Austria

ABSTRACT- Studies examining the mechanistic basis of chemical mixture toxicity are needed to better assess the combined effects of toxicants in the environment. Here combined effects examination involved biochemical study of agent reactivity with a model cofactor (mLTQ) of lysyl oxidase (LO) and a frog embryo mixture toxicity assay. LTQ is needed for proper LO function in a connective tissue cross-linking reaction. Reactivity of mLTQ with individual osteolathyrogens, chemicals which inhibit cross-linking, was assessed via UV-VIS spectrophotometry. The 96-h toxicity assay determined combined effects of osteolathyrogen mixtures, with toxicity manifested as lesions in the notochord of preserved tadpoles. Single chemical dose-response curves (DRCs) were used to calculate theoretical mixture DRCs for two mechanistic combined effects models: dose-addition and independence. Experimental mixture DRCs were compared with theoretical curves for each model using the -squared goodness-of-fit test. Spectrophotometry revealed adduct formation at 350 nm when semicarbazide (SC), thiosemicarbazide (TSC), and, to a lesser extent, aminoacetonitrile (AAN) were individually incubated with mLTQ. This coincided with progressive bleaching of the mLTQ peak at 504 nm. When -aminopropionitrile (APN) was incubated with mLTQ there was no appreciable change in the spectrum in the absence of prolonged incubation. Toxicity data indicated combined effects consistent with dose-addition (0.1< p <0.8)for the AAN:SC, SC:TSC and AAN:SC:TSC mixtures; a combined effect expected for chemicals working by the same mechanism. Slope and EC50 additivity quotients (experimental/additive) for these mixtures were 0.93-1.14 (dose-addition = 1.0) The combined effect produced by APN:SC and APN:SC:TSC mixtures was significantly different from dose-addition (p <0.001). Additivity quotients for these mixtures were 0.74-0.89; indicating greater-than dose-additive effects. The results suggest a different mechanism of osteolathyrism for APN, and exemplify a potentially useful mechanistic approach to predicting chemical mixture toxicity in the environment.

Key words: uv-vis spectrophotometry, Xenopus laevis, osteolathyrogens, cross-linking