Rapid hydrogen ion uptake of rod outer segments and rhodopsin solutions on illumination.

1. Flash illumination of a suspension of frog rod outer segments or rhodopsin solution in contact with a platinum electrode produces a rapidly developing negative displacement of potential of the electrode (with respect to a reversible electrode).2. The amplitude of the potential change varies inversely with the H(+) buffering capacity of the medium. It is inferred that the response is due to an uptake of H(+) by the rod outer segments or rhodopsin, with the platinum surface acting as a pH electrode.3. Determination of the action spectrum shows that the response depends on the absorption of light by rhodopsin.4. In frog rods one acid-binding group with a pK of about 7.9 is produced for each molecule of rhodopsin bleached, consistent with a rhodopsin concentration in frog rods of 1.7 mM.5. It is suggested that the time course of the response with rhodopsin solutions reflects the kinetics of the conversion of metarhodopsin I to metarhodopsin II.6. A slower time course of voltage change observed for suspensions of outer segments is attributable to the time required for the diffusion of H(+) buffer out of the rods.