MODULATION OF THE METARHODOPSIN-I METARHODOPSIN-II EQUILIBRIUM OF BOVINE RHODOPSIN BY IONIC-STRENGTH - EVIDENCE FOR A SURFACE-CHARGE EFFECT/

The effects of ionic strength on formation and decay of metarhodopsin II (MII), the active photointermediate of bovine rhodopsin, were studi ed in the native membrane environment by means of ultraviolet/visible and Fourier-transform infrared (FTIR) spectroscopy. By increasing the concentration of KCl in the range from hypotonic to 4 M, the apparent pK(a) of the metarhodopsin I(MI)/MII equilibrium is shifted by approxi mately pH three, in favor of the MII intermediate. In addition, the ap parent rate of MII formation is enhanced by an increase in ionic stren gth (about twofold in the presence of 2 M KCl). MII decay is independe nt of the salt concentration. Attenuated-total-reflectance/FTIR data s how that the high-salt conditions have no effect on the rigidity of th e membrane matrix and do not induce: structural changes in the interme diates themselves. Different salts were tested for their ability to sh ift the MI/MII equilibrium; however, no clear ion dependence was obser ved. We interpret these results as an indication for direct involvemen t of the cytosolic surface charge in the regulation of the photochemic al activity of bovine rhodopsin.