The identity of metarhodopsin III

A fast-scanning dichroic microspectrophotometer was used to trace products of rhodopsin photolysis (metarhodopsins I/II/III and later) in structurally intact amphibian rod outer segments (ROSs) and metabolically active rods. The instrument allows the recording of absorbance spectra with a time resolution better than 1 s, and to discriminate between products with similar absorbance spectra that differ with respect to the orientation of their chromophore in the photoreceptor membrane. We demonstrate that metarhodopsin III is in a pH-reversible equilibrium with metarhodopsin II and that the metarhodopsin III chromophore is orientated with respect to the membrane plane even more strictly than the 11-cis retinal in "dark" rhodopsin. This indicates that all-trans retinal in metarhodopsin III is still attached to its native binding site on opsin. The kinetic scheme of the decay of metarhodopsins is presented in which metarhodopsin III lies in a shunt pathway from metarhodopsin II to retinal. Formation of metarhodopsin III was detected at bleaches as low as approximately 3%, contrary to previous reports that it is not formed at below 10% bleaches. Another product that is spectrally similar to metarhodopsin III, termed P440, appears at later stages of photolysis as the result of the decay of metarhodopsin II and metarhodopsin III. The chromophoric group in P440 is orientated preferentially across the disk membrane. The final product(s) in isolated ROS, where the reduction of retinal to retinol is blocked, consists of a mixture of a free retinal and retinal possibly attached to different binding sites in the membrane. In metabolically active rods the later products are quickly converted to retinol. We conclude that metarhodopsin III represents a specific conformational state of metarhodopsin where the chromophoric binding site is still occupied by all-trans retinal. Hence, the formation and decay of metarhodopsin III may be limiting for the rate of rhodopsin regeneration and photoreceptor dark adaptation.