Rhodopsin-to-metarhodopsin II transition triggers amplified changes in cytosol ATP and ADP in intact retinal rod outer segments.

We have observed rapid, light-initiated changes in unbound cytosol ATP and ADP during the rhodopsin-to-metarhodopsin II transition in intact rod outer segments (ROS). Upon illumination of the ROS, ATP is rapidly removed from the unbound phase of ROS, accompanied by the concomitant release of ADP into the cytosol. The exchange process involves decreases of approximately equal to 0.5 mM ATP in ROS cytosol ATP content in response to a saturating flash. At levels of light well below saturation (less than 0.001% bleach), the process is highly amplified, with a decrease in cytosol ATP of approximately equal to 2,000 ATP molecules per absorbed photon per ROS. Rapid time-resolution techniques reveal that cytosol ATP content decreases rapidly, within 250 msec of a saturating flash. Bleaching rhodopsin to metarhodopsin II results in a decrease in cytosol ATP, accompanied by an increase in cytosol ADP, whereas photoreversal of metarhodopsin II by a blue flash reverses the process, increasing ATP concentration to its control level in the dark. The photoreversibility of the ATP decrease during the rhodopsin-to-metarhodopsin II transition establishes a direct link between the state of an early intermediate of photolyzed rhodopsin and the state of a nucleoside triphosphate in intact ROS. The results are consistent with a light-activated exchange of unbound ATP for bound ADP, and we propose, therefore, an ATP/ADP amplification cycle in which metarhodopsin II catalyzes the exchange of ATP for ADP on a nucleotide binding protein.