Combined genotoxicity of chlorinated products from tyrosine and benzophenone-4

The toxicity of disinfection by-products (DBPs) from a single precursor was studied intensively. Here we examined the genotoxicity when two precursors (tyrosine (Tyr) and benzophenone-4 (BP-4)) were chlorinated together and separately. We sought to examine whether the genotoxicity of the mixture (GCM) could be estimated from the sum of the genotoxicities of the individual precursors (GCI), which were chlorinated separately. We determined the genotoxicity using the SOS/umu test. The results revealed that GCM was not identical to GCI. The difference in genotoxicity between GCM and GCI (G_Δ) was observed to decrease with increasing pH. GCM was higher than GCI (G_Δ>0) at pH 5.0-6.1, and lower than GCI (G_Δ<0) at pH 6.3-8.0. We found that nitrogen-containing DBPs played a dominant role in determining GCM and GCI. We propose that the total organic nitrogen (TON) ratio, TON_{(chlorinatedmixture)}/TON_{(thesumofchlorinatedindividuals)}, is useful to estimate G_Δ.