The Light-induced Proton Uptake in Bovine Retinal Outer Segment Fragments

Fragments of bovine retinal outer segments (ROS) containing intact photoreceptor discs took up protons from an aqueous medium upon illumination. Proton uptake in the light required the presence in the ROS of unbleached rhodopsin. After light exposure had bleached the visual pigment, it no longer absorbed light and proton uptake did not occur. The action spectrum of the proton uptake conformed to the unbleached minus bleached difference spectrum of the ROS. Moreover, reconstitution of the visual pigment by incubating the bleached ROS in darkness with 11-cis retinal resulted in recovery of the light-induced proton uptake. In the presence of Triton X-100, the ROS membranes became dispersed, but the light-induced proton uptake persisted below about pH 6. However, at higher pH release of protons from the dispersed ROS was observed. The stoichiometric ratio between protons taken up or released and rhodopsin bleached could be experimentally changed from more than 5 taken up to about 3 released in Triton X-100. This finding does not provide support for a proposal offered by previous investigators that a light-induced pH increase was attributable to uptake of exactly 1 proton per molecule of visual pigment bleached.