The effect of inhibitors of multixenobiotic resistance mechanism on the production of mutagens by Dreissena polymorpha in waters spiked with premutagens

It has been shown previously that accumulation of 2-aminofluorene (AF) or benzo(a)pyrene (BaP) in aquatic invertebrates enhanced in the presence of inhibitors of multi-xenobiotic resistance (MXR) mechanism. In this work we investigated the consequence of the enhanced accumulation of AF or BaP on the production of their direct mutagenic metabolites. The production of mutagens has been measured by the detection of mutagens excreted into the exposure-water. The exposure of the freshwater mussel Dreissena polymorpha to water spiked with AF (55 mu M) for 1 and 2 h resulted in a time-dependent production of direct mutagenic metabolites in the hexane extracts of the water, as detected by Salmonella typhimurium TA 98 in the Ames test without activation. The exposure of mussels to AF at concentrations from 5.5 to 55.0 mu M (the maximal solubility) resulted in a dose-dependent increase of AF mutagens excreted into the exposure-water. When mussels were exposed to water spiked with 55.0 mu M of AF in the presence of model MXR-inhibitors verapamil (20 mu M) or cyclosporin A (10 mu M), the excretion of direct AF-mutagens into the exposure-waters was increased by 2- or 5-fold, respectively. Such increase in the excretion of direct mutagens could not be demonstrated after exposure of D. polymorpha to BaP (15 mu M), either alone or in the presence of verapamil or cyclosporin A. The explanation of this selectivity was found to be based on the potential of mussel postmitochondrial fraction (PMF) to bioactivate exclusively the aromatic amines (AF), but not polycyclic hydrocarbons (BaP), by its methimazole-sensitive FAD-dependent monooxygenase. Thus, the D. polymorpha PMF failed to bioactivate BaP into its mutagenic products because it lacks the cytochrome P450 alpha-naphthoflavone-sensitive monooxygenase activity. The number of direct mutagens produced by mussel from AF spiked in Sava River water (collected downstream from the inlet of municipal waste waters, known to possess high concentration of chemosensitizers), was 2-fold higher than was the number of mutagens produced from AF spiked in waters known to possess low concentration of chemosensitizers (the Sava River water collected upstream from the inlet of waste waters, or the water from the Lake Jarun, city of Zagreb). Thus, the inhibitors of MXR-defence mechanism present in environment reveal the genotoxicating potential. (C) 1999 Elsevier Science B.V.;AII rights reserved.