Abstract
We found by microspectrophotometry that β‐ionone partitioned into the outer segment membranes of salamander green‐sensitive (GS) and blue‐sensitive (BS) rods with greater uptake into BS rods. Electrophysiology showed that BS rod rhodopsin, but not that of GS rods, was activated and bleached by β‐ionone. But biochemical assays using rhodopsins formed from recombinant salamander GS rod and BS rod opsins and 11‐cis retinal (A1) showed little activation by β‐ionone. Since salamander photoreceptors normally contain both A1 and 3‐dehydro 11‐cis retinal (A2)‐based visual pigments, we hypothesized that only the latter activated pharmacologically. We recorded BS rods whose native chromophore had been partially replaced by A1. Lowering the amount of A2 rhodopsin decreased pharmacological activation. Because β‐ionone does not occur naturally in the retina, we also examined the effects of vitamin A on rod activity. In contrast to β‐ionone, vitamin A activated rhodopsin in both GS and BS rods, and also sensitized rods to UV light, presumably by resonance energy transfer. Thus the different effects of retinoids on rods argue for the existence of allosteric binding sites on rhodopsins.Research Support: NEI EY011358 and Lions of Massachusetts
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