Interactions of nitrilotriacetic acid (NTA) with Cr(VI) compounds in the induction of gene mutations in cultured mammalian cells

Abstract

Data concerning the mutagenicity of NTA are contradictory but the weight of evidence indicates that, although NTA is not a very strong mutagen, it is not completely lacking genotoxicity. With regard to the production of gene mutations in mammalian cell systems, until now NTA was found inactive in mouse lymphoma cells cultured in vitro (induction of mutations at the TK locus), but active in human cells cultured in vitro (induction of resistance to the diphteria toxin). We used the V79 Chinese hamster cell line to detect the induction of 6-thioguanine resistance, due to mutation at the HGPRT locus, with direct (methylmethanesulphonate) and indirect (dimethylnitrosamine) mutagens as positive controls. NTA was tested within a range of 10 −4 − 1.5×10 −2 M concentrations: although it was cytotoxic above the 10 −2 M dose, it did not increase the frequency of mutations at any of the tested concentrations, independently of metabolic activation (rat liver S-9 mix system). On the other hand, NTA is known to solubilize heavy metals and, therefore, to increase their genotoxicity. Here we found that an insoluble Cr(VI) compound, lead chromate (PbCrO 4), was not cytotoxic nor mutagenic on V79 cells, probably because it is taken up by the cells very slowly, whereas the presence of NTA (2.5×10 −3 M, in water) elicited a direct cytotoxicity and mutagenicity, which was dose-dependent within a range of 10 −6 − 10 −4 M PbCrO 4. This effect was due to solubilization by NTA of chromate anion, as determined by comparing spectrophotometric determinations of Cr(VI) in PbCrO 4 treatment solutions with a mutagenicity titration curve obtained with a completely soluble Cr(VI) salt (potassium dichromate, K 2Cr 2O 7).