Metarhodopsin III formation and decay kinetics: Comparison of bovine and human rhodopsin

Absorbance spectra were recorded at various times after photolysis of hypotonically washed membrane suspensions of human and bovine rhodopsin (pH 7 or 8, T = 20 or 37°C). Global data analysis showed that at 20°C, metarhodopsin III formation was the only process occurring from 1 to 30 min following photolysis. At pH 8, a significant amount of human metarhodopsin I was present before metarhodopsin III formed. At 37°C, two decay processes were seen, formation of metarhodopsin III (lifetime 113 ± 3 sec bovine, 93 ± 2 sec human) and formation of a species with the properties of n-retinylidine opsin (lifetime 900 ± 150 sec bovine, 2000 ± 350 sec human). A larger fraction of human rhodopsin (~ 50%) is converted to metarhodopsin III than is the case for bovine rhodopsin (~ 35%).