Regeneration of rhodopsin in frog rod outer segments.

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Abstract
Bleaching/regeneration cycles were performed in perfused frog [Rana esculenta] retina while the optical transmittance at suitable wave-lengths was measured continuously. Rhodopsin was identified from its spectral absorbance, its photosensitivity and from the kinetics of its regeneration. In the absence of the pigment epithelium regeneration was complete when .ltoreq. 2.5% of the rhodopsin initially present was bleached. The cycles could be repeated to a total of regenerated rhodopsin exceeding that explicable on the utilization of stored chromophores. The rate of regeneration was fast, with 0.12 min-1 rate constant, following 1st order reaction kinetics. Under these conditions, the cycle did not seem to involve stages beyond metarhodopsin II. With the moderate bleaching intensities used, half-time 53 min, the Bunsen-Roscoe law was obeyed up to 15 min, indicating a capacity for the photoproducts to be accommodated in situ for subsequent regeneration. Only substantial bleaches which exceed that capacity, result in hydrolyzed chromophores. These surplus chromophores became esterified and were temporarily taken up by the pigment epithelium to be re-entered into the visual cycle as fast as they could be processed by the regenerative machinery of the rod outer segments.