Comparative evaluation of the combined osteolathyritic effects of two nitrile combinations on Xenopus embryos

Two nitrile combinations, beta-aminopropionitrile (beta APN) with aminoacetonitrile (AAN) and beta APN with beta APN (as a sham combination), were evaluated using the frog embryo mixture toxicity assay to determine their combined osteolathyritic effects and to compare the results with theoretical effects for two combined effects models. In separate tests each nitrile was tested with copper sulfate to determine the importance of copper in osteolathyrogen-induced disruption of connective tissue cross-linking. Frog embryos (Xenopus laevis) were exposed for 96 h, with daily solution removal and replacement. Preserved tadpoles were evaluated for osteolathyritic lesions. For the nitrile:nitrile combinations, the chi(2) goodness-of-fit test was used to compare the resulting mixture-response curves to theoretical curves for dose-addition and independence. For beta APN with AAN, the combined osteolathyritic effect for five of the seven mixture curves generated was greater than expected for each of the combined effects models. For beta APN with beta APN, the combined effect for all seven mixture curves was consistent with dose-addition, the combined effect expected for chemicals inducing toxicity by the same mechanism. For the nitrile:copper combinations, the EC50 for beta APN-induced osteolathyrism was increased two- to threefold (i.e. made less toxic) by co-administration with copper sulfate, while the EC50 for AAN-induced osteolathyrism was unchanged. The results are consistent with the idea that beta APN and AAN induce osteolathyrism, at least in part, by different mechanisms. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.