The contribution of a sensitizing pigment to the photosensitivity spectra of fly rhodopsin and metarhodopsin.

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Abstract
Photoreceptors of the fly compound eye have high sensitivity in the UV as well as the visible spectral range. UV sensitivity arises from a photostable pigment acting as a sensitizer for rhodopsin. Because the sensitizing pigment cannot be bleached, the classical determination of the photosensitivity spectrum from measurements of the difference spectrum of the pigment cannot be applied. A new method was used to determine the photosensitivity spectra of rhodopsin and metarhodopsin in the UV spectral range. The method is based on the fact that the invertebrate visual pigment is a bistable one, in which rhodopsin and metarhodopsin are photointerconvertible. The pigment changes were measured by a fast electrical potential, called the M potential, which arises from activation of metarhodopsin. The photosensitivity spectrum of rhodopsin and metarhodopsin was calculated using 2 kinds of experimentally measured spectra: the relaxation and the photoequilibrium spectra. The relaxation represents the wavelength dependence of the rate of approach of the pigment molecules to photoequilibrium. This spectrum is the weighted sum of the photosensitivity spectra of rhodopsin and metarhodopsin. The photoequilibrium spectrum measures the fraction of metarhodopsin (or rhodopsin) in photoequilibrium which is reached in the steady state for application of various wavelengths of light. Although the photosensitivity spectra of rhodopsin and metarhodopsin are very different in the visible, they show strict coincidence in the UV region. The photostable pigment apparently acts as a sensitizer for rhodopsin and metarhodopsin.