Quantum Efficiency and Photosensitivity of the Rhodopsin ⇌ Metarhodopsin Conversion in Crayfish Photoreceptors

Photosensitivity (K λ ) of a visual pigment is the product of the molecular absorption coefficient (α λ ) and the quantum efficiency for photoconversion (γ). Among the invertebrates, many visual pigments are stable not only in the rhodopsin (R) conformation but also as the photoproduct, metarhodopsin (M), We here employ a method for determining the photosensitivities of the two stable pigments of a rhodopsin‐metarhodopsin pair, using kinetic analysis of fluorescence from metarhodopsin combined with measurements of spectral absorption made before and after saturation at the isosbestic wavelength of the pigment pair. A curve fitting technique, in which a theoretical function is scaled for best fit to the measured absorption spectrum of the photosteady‐state mixture, yields values for the photosensitivity of rhodopsin at λ. max , the ratio of quantum efficiencies for rhodopsin—metarhodopsin interconversion, and the fractional composition of the steady‐state mixture. With knowledge of the molecular extinction coefficient, the absolute values of quantum efficiency can be calculated. For crayfish ( Orconectes, Procambarus ) rhodopsin, measured in isolated rhabdoms, K max = 1.05 x 10 ‐16 cm 2 at 535 nm with >7λR→M0.69. These values are similar to the photosensitivity and quantum efficiency of bleaching of vertebrate rhodopsins in digitonin solution (Dartnall, 1972). For the metarhodopsin, K max = 1.02 x 10 ‐16 cm 2 at 510 nm, and λ M‐R = 0.49.