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J. Gen. Physiol.,
Volume 113, Number 3, March 1, 1999 491-503
From the * Department of Physiology, Boston University School of Medicine, Boston, Massachusetts 02118; and The retinal analogue Department of Ophthalmology, Medical University of South Carolina, Charleston, South Carolina 29425
-ionone was used to investigate possible physiological effects of the noncovalent interaction between rod opsin and its chromophore 11-cis retinal. Isolated salamander rod photoreceptors
were exposed to bright light that bleached a significant fraction of their pigment, were allowed to recover to a
steady state, and then were exposed to -ionone. Our experiments show that in bleach-adapted rods -ionone
causes a decrease in light sensitivity and dark current and an acceleration of the dim flash photoresponse and the
rate constants of guanylyl cyclase and cGMP phosphodiesterase. Together, these observations indicate that in
bleach-adapted rods -ionone activates phototransduction in the dark. Control experiments showed no effect of
-ionone in either fully dark-adapted or background light-adapted cells, indicating direct interaction of -ionone
with the free opsin produced by bleaching. We speculate that -ionone binds specifically in the chromophore
pocket of opsin to produce a complex that is more catalytically potent than free opsin alone. We hypothesize that
a similar reaction may occur in the intact retina during pigment regeneration. We propose a model of rod pigment regeneration in which binding of 11-cis retinal to opsin leads to activation of the complex accompanied by a
decrease in light sensitivity. The subsequent covalent attachment of retinal to opsin completely inactivates opsin
and leads to the recovery of sensitivity. Our findings resolve the conflict between biochemical and physiological data concerning the effect of the occupancy of the chromophore binding site on the catalytic potency of opsin.
We show that binding of -ionone to rod opsin produces effects opposite to its previously described effects on
cone opsin. We propose that this distinction is due to a fundamental difference in the interaction of rod and cone
opsins with retinal, which may have implications for the different physiology of the two types of photoreceptors.
-ionone;
visual pigments;
bleaching adaptation
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