Human Rhodopsin

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Science  31 Jan 1958:
Vol. 127, Issue 3292, pp. 222-249
DOI: 10.1126/science.127.3292.222


Human rhodopsin in aqueous solution has λmax. of 493 mµ and is lower in the spectrum than the rhodopsins of all other known vertebrates, with the exception of certain deep-sea fishes. Its molar extinction is 40,000 ± 800. Like other rhodopsins, it bleaches to a mixture of opsin and all-trans retinene and is resynthesized by incubating opsin with neo-b (11-cis) retinene. The regenerated rhodopsin has the same λmax. as the extracted pigment; this is due, therefore, not to an unusual retinene but to a characteristic human opsin. The regeneration in solution from opsin and neo-b retinene is a second-order reaction with a half-time, at 29.5° C, of about 2.5 minutes. This is much faster than the synthesis of rhodopsin in the living human eye, and faster than human rod dark-adaptation; the rate of both processes in vivo must be limited by reactions which precede the union of neo-b retinene with opsin, the final step in rhodopsin synthesis. In the rods, rhodopsin is virtually in the solid state—highly oriented in close relation with other highly oriented molecules. In this situation its spectrum is displaced toward the red (λmax 500 mµ) and is narrower than in solution. For light entering the rods axially, rhodopsin has also a considerably increased extinction, some 1.5 times higher than when randomly oriented. The spectrum of rhodopsin in rods agrees well in form and position with the spectral sensitivity of human rod vision, measured at the retinal surface.