Abstract
Quinones are widely distributed substances of often potential toxicological significance. On the other hand, cyclic AMP is known to promote a cell-survival response and to retard apoptosis [Berridge, Tan and Hilton (1993) Exp. Hematol. 21, 269-276]. Therefore the effects of quinones on adenylate cyclase were tested. Adenylate cyclase is rapidly inhibited by quinones, with IC50 values of 40-45 microM for p-benzoquinone (BQ) or 200 microM for dichlorophenol-indophenol (DCIP), with 2-substituted quinones being inactive. Membrane solubilization decreases the IC50 values for BQ and DCIP to 18 microM and 40 microM respectively. The inhibition is not affected by GTP, GDP or analogues, or by cholera and pertussis toxins; therefore it is not mediated by a G-protein or the activation of a defined receptor. Further, the inhibition stoichiometrically competes with forskolin activation of adenylate cyclase, equimolar concentrations of quinone and forskolin restoring the enzyme activity to its basal value. Reduction of BQ with sodium dithionite stoichiometrically prevents the inhibition of adenylate cyclase; in turn, oxidation of hydroquinone with ferricyanide fully restores it, indicating that the oxidized state of the quinone is required for inhibition. In addition, BQ is cytotoxic in vivo on HepG2, a human hepatocellular carcinoma cell line, but the effect can be prevented with forskolin. In plasma membranes, BQ tightly binds only one major and two minor proteins; these BQ-binding proteins were purified by means of labelling with [14C]BQ followed by PAGE under native conditions. Together these observations indicate that the action of quinone can be traced to targeting a limited number of proteins at the plasma membrane in a highly selective way and to affecting key enzymes such as adenylate cyclase.