Reactions of the Sulfhydryl Groups of Membrane-Bound Bovine Rhodopsin

Reactions of the sulfhydryl groups of bovine rhodopsin in rod outer segment membranes have been investigated using 4,4′-dithiopyridine. This reagent is uncharged at neutral pH and rapidly equilibrates across phospholipid bilayers. Membrane-bound rhodopsin has two kinetically distinguishable sulfhydryl groups reactive to the reagent, this stoichiometry being unchanged by bleaching provided the sulfhydryl reactions themselves are carried out in the dark. The rates of the reactions, however, are substantially increased by bleaching. Irradiation of bleached membranes, either with white light or wavelengths in the neighborhood of 475 nm, results in an increase in the number of reactive sulfhydryls relative to that found for bleached membranes in the dark. A component of the light-driven reaction is dependent on the Ca²⁺ content of the medium.