Rhodopsin-phospholipid interactions: dependence of rate of the meta I to meta II transition on the level of associated disk phospholipid.

Solubilization of retinal rod outer segment disk membranes in octyl glucoside was employed to prepare rhodopsin samples with varying amounts of associated disk phospholipid. Flash photolysis studies were carried out on these samples to determine the dependence of the meta I to meta II transition kinetics on the level of associated phospholipid. The rate constant for the formation of meta II increased from 6.9 X 10(3) to 19.5 X 10(3) s-1 as the molar ratio of phospholipid per rhodopsin fell from 35 to 5. The activation free energy for this process had a linear dependence on the level of phospholipid, with a slope of 24 cal/mol of rhodopsin-associated phospholipid. A variety of evidence suggests that rhodopsin undergoes a reversible conformation change during the meta I to meta II transition. No evidence was found for an enhanced effect on the activation free energy for this conformation change at the level of associated phospholipid which corresponds to the formation of a phospholipid boundary layer around rhodopsin.