Effect of digitonin on the rhodopsin meta I-meta II equilibrium.

Absorbance difference spectra were recorded from 10 micros to 540 ms after photoexcitation of sonicated suspensions of hypotonically washed bovine rod outer segments with varying amounts of the detergent digitonin added (0 to 2%) at 20 degrees C. Metarhodopsin I480 and metarhodopsin II displayed the expected anomalous pH dependence at pH 6 and 8 (i.e. opposite to that expected from direct protonation of the chromophore Schiff base). However, increasing levels of digitonin eliminated the pH dependence of the equilibrium, and at 2% digitonin the pH 6 and pH 8 data were both similar to the data collected at pH 8 without digitonin. Addition of 0.5% azolectin restored approximately 50% of the anomalous pH dependence at pH 6 in the 2% digitonin sample. The possibility that digitonin induced large-scale aggregation of rhodopsin in the disk membrane that could be reversed by azolectin was tested using time-resolved linear dichroism. Those results showed that even 0.3% digitonin disrupted the membrane, and no large aggregates were detectable under any conditions. Thus, digitonin reduces the activity of a component of the disk membrane required for metarhodopsin II formation, and that deficiency can be compensated for by azolectin.