Interplay between hydroxylamine, metarhodopsin II and GTP-binding protein in bovine photoreceptor membranes

The decay reactions of metarhodopsin II and the dissociation of the complex between rhodopsin (in the metarhodopsin II state) and the GTP-binding protein (G-protein) (in its inactive, GDP-binding form) have been compared at various concentrations of hydroxylamine. The reactions of the chromophore were measured by absorption changes in the visible range, the complex dissociation by changes in the near-infrared scattering. An additional monitor of the complex was given by the G-protein-dependent equilibrium between metarhodopsin I and metarhodopsin II. For all measurements, fragments of isolated bovine rod outer segments in suspension were used. In the absence of hydroxylamine, the rhodopsin-G-protein complex dissociated within 20-30 min at room temperature. The presence of hydroxylamine greatly accelerated (e.g., 5-fold at 1 mM NH2OH) the dissociation. Under all conditions, the free, dissociated G-protein can reassociate to metarhodopsin II produced by subsequent bleaching. Dissociation of the metarhodopsin II-G-protein complex required the decay of photoproducts with a maximal absorbance of 380 nm, but was not affected by the simultaneous presence of metarhodopsin III or metarhodopsin III - like photoproducts with a maximal absorbance between 450 and 470 nm. Despite the acceleration of metarhodopsin II-G-protein dissociation by NH2OH, metarhodopsin II-G-protein was relatively stabilized as compared to free metarhodopsin II. The ratio of the decay rates of free metarhodopsin II and metarhodopsin III-G-protein was increased as much as 10-fold in the presence of 25 mM NH2OH. The results indicate a mutual interdependence of retinal, opsin and G-protein.