Mercurials and Dimercaptans: Synergism in the Induction of Chemoprotective Enzymes

The induction of NAD(P)H:quinone reductase (EC 1.6.99.2; QR) in Hepa 1c1c7 murine hepatoma cells provides a versatile quantitative model for measuring the potencies of inducers for Phase 2 detoxication enzymes. Since many inducers of these enzymes also protect animals and their cells against the toxic and neoplastic effects of carcinogens, understanding the mechanisms of induction of Phase 2 enzymes is important. Both HgCl₂ and 2,3-dimercaptopropanol (BAL) are inducers of QR in these cells, and paradoxically BAL (which is about 30 times less potent than HgCl₂ enhances the inducer potency of HgCl₂ substantially. This synergism depends on the presence of two thiol groups on adjacent carbon atoms. Since nonchelated mercury(II)—thiol compounds did not show synergism, the formation of very high affinity bidentate chelates appears to be essential for such synergism. A major mechanism for the augmentation of the inducer potency of mercury(II) by BAL is the more rapid cellular uptake and the accumulation of higher intracellular concentrations of mercury. It is also possible that BAL-mercury chelates are intrinsically more potent as inducers. Although equimolar mixtures of BAL and HgCl₂ and the synthetic chelate isolated from such mixtures were more potent inducers than HgCl₂ alone, the presence of excess BAL increased this inducer synergism even further. By chromatography we showed the reversible formation of higher order complexes between BAL and mercury(II). Such complexes are transported into cells more efficiently and appear to be more potent than free HgCl₂ or the chelate obtained from equimolar mixtures of BAL and HgCl₂. Depletion of intracellular glutathione levels by treatment of cells with buthionine sulfoximine (a glutathione synthetase inhibitor) enhanced the inducer potencies of HgCl₂, BAL, and their chelates, whereas elevation of intracellular glutathione levels by treatment with the ethyl ester of glutathione lowered the inducer potency of a BAL-mercury(II) chelate. Induction by BAL and by HgCl₂ and the synergism of inducer potency of their mixtures depend on the activation of the same genetic enhancer element (ARE or EpRE) that is part of the inducer mechanism of all other monofunctional inducers.