Photochemical functionality of rhodopsin-phospholipid recombinant membranes

Purified rhodopsin was incorporated into phospholipid bilayers to give recombinant membranes. The photochemical functionality in these systems was examined by low-temperature spectroscopy and by kinetic spectrophotometry. Changes in the absorption spectra of glycerol-water mixtures of rhodopsin-egg phosphatidylcholine and rhodopsin-asolectin recombinants were monitored after the sample was cooled to -196 degrees C, presented with light of wavelength greater than 440 nm, and then warmed gradually to room temperature. Absorption characteristics indicative of the spectral intermediates prelumirhodopsin, lumirhodopsin, metarhodopsin I, and metarhodopsin II were observed. The kinetics of the metarhodopsin I -o metarhodopsin II transition in these recombinants was studied by flash photolytic observation of the decay of meta I and the formation of meta II. Recombinants prepared from unsaturated phospholipids, e.g., asolectin, egg phosphatidylcholine, egg phosphatidylethanolamine, and dioleoylphosphatidylcholine, showed first-order kinetics for the transition with rates comparable to that of rod outer segment membranes. Recombinants prepared from saturated phosphatidylcholines have a retarded rate of conversion from meta I to meta II and are considered to be nonfunctional. The photochemical functionality of rhodopsin-phospholipid recombinants is dependent upon the presence of phospholipid unsaturation and the fluidity of the phospholipid hydrocarbon chains, and is independent of the polar head group of the phospholipid.