Fluorescence of crayfish metarhodopsin studied in single rhabdoms

Isolated photoreceptor organelles (rhabdoms) from eyes of crayfish (Procambarus. Orconectes) were examined on a microscope system designed for quantitative measurements of fluorescent. Although fully dark-adapted rhabdoms are nonfluorescent or very weakly fluorescent, an increasing emission appears on exposure to light. Over the 30-fold range of intensities studied, the rate of the appearance of this fluorescence is identical to the rate of formation of metarhodopsin from rhodopsin. Furthermore, the excitation spectra for the observed emission are similar to the absorption spectra of crayfish metarhodopsin at both neutral and acid pH. Finally, the amount of fluorescence observed in rhabdoms previously irradiated with selected wavelengths of light is proportional to the amount of metarhodopsin present in the photosteady state established by the prior irradiation. The emission therefore originates from crayfish metarhodopsin. Fluorescence emission peaks at 670 nm at neutral pH. The quantum efficiency is 1.6 +/- 0.4 X 10(-3). Although emission from other rhodopsin photoproducts has previously been noted, this is the first description of fluorescence from the metarhodopsin chromophore site.