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 ≈ 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.