Zinc-induced decrease of the thermal stability and regeneration of rhodopsin

Zinc is present at high concentrations in the photoreceptor cells of the retina where it has been proposed to play a role in the visual phototransduction process. In order to obtain more information about this role, the study of the effect of zinc on several properties of the visual photoreceptor rhodopsin has been investigated. A specific effect of Zn2+ on the thermal stability of rhodopsin, obtained from bovine retinas and solubilized in dodecyl maltoside detergent, in the dark is reported. The thermal stability of rhodopsin in its ground state (dark state) is clearly reduced with increasing Zn2+ concentrations (0-50 mum Zn2+). The thermal bleaching process is accelerated in the presence of Zn2+ with k rate constants, at 55 degreesC, of 0.028 +/- 0.002 min(-1) (0 mum Zn2+) and 0.056 +/- 0.003 min(-1) (50 muM Zn2+), corresponding to t(1/2) values of 24.4 +/- 1.6 min and 11.8 +/- 0.1 min, respectively. Thermodynamic parameters derived from Arrhenius plots show a significant E-a increase at 50 muM Zn2+ for the process, with DeltaG(double dagger) decrease and increase in DeltaH(double dagger) and DeltaS(double dagger) possibly reflecting conformational rearrangements and reordering of water molecules. The stability of the metarhodopsin IT intermediate is also decreased and changes in the metarhodopsin II decay pathway are also detected. The extent of rhodopsin regeneration in vitro is also reduced by zinc. These effects, specific for zinc, are also seen for rhodopsin in native disc membranes, and may be relevant to the suggested role of Zn2+ in normal and pathological retinal function.