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Isolated retinas and rod outer segments from frogs (Rana pipiens) were exposed to light that produced axially uniform total bleaches of rhodopsin. Using fluorescence video microscopy, it was shown that the formation and equilibrium distribution of all-trans-retinol, the final chromophore product of rhodopsin bleaching is axially uniform. This result shows that the rate and amount of oxidoreductase-mediated reduction of all-trans-retinal to all-trans-retinol is not affected by the relative age of the disk membranes to which the enzymes are bound. Therefore previously reported axial differences in regeneration of rhodopsin and recovery of photocurrent after exposure to bright light probably are not due to axial differences in the formation of rhodopsin photoproducts. In addition, measurements on individual rod cells show that there is no significant redistribution of retinol for up to 2 hr following localized partial bleaches of rhodopsin. This raises the perplexing question of how retinol is shuttled between disk membranes and the pigment epithelium during visual pigment regeneration following substantial bleaches.
Starch-gel electrophoresis of rat ocular tissues shows two anodic isoenzymes of alcohol dehydrogenase (ADH), designated as ADH-1 and ADH-2, ADH-1 is characteristic of the ocular tissues, and corresponds to more than 95% of all ADH activity in the eye. The well known cathodic forms of rat liver ADH, that we named ADH-3, are not observed in the ocular tissues. ADH-1 is detected in retina, pigment epithelium-choroid, ocular fluid, and cornea but not in the lens. The cornea exhibits the highest ADH activity [200 +/- 59 milliunits (munits) mg-1] followed by the pigment epithelium-choroid (11 +/- 7 munits mg-1). Activity in the retina is very small (0.6 +/- 0.2 munit mg-1) and represents only 0.6% of the total activity in the eye. Most of the rat ocular ADH is localized in the cornea (68%) where it could play a significant role in the detoxication of the alcohols of a broad range of structures. Purified ADH-1 shows a low Km for retinol oxidation (20 microM) and for retinal reduction (30 microM) indicating that this isoenzyme may have a function in the metabolism of retinoids. Ethanol competitively inhibits retinol oxidation, but with a very high apparent inhibition constant (0.6 M) demonstrating that the inhibitory effect is not significantly at the usual concentrations found in the blood during ethanol intoxication.
Retinoid transport and chromophore exchange have been investigated in cephalopods using autoradiographic and radiobiochemical techniques. In dark adapted retinas, [3H]-retinoid is concentrated in myeloid bodies present in the photoreceptor inner segments and is bound to the photopigment retinochrome. In retinas exposed to light, there is a shift in the distribution of [3H]-retinoid. The rhabdomes become more heavily labeled than the inner segments, and rhodopsin labeling exceeds that of retinochrome. In animals returned to the dark, another shift in retinoid distribution occurs and the inner segments are again more labeled than the rhabdomes. In these animals [3H]-retinoid is bound primarily to retinochrome. Exposure to light seems to activate a transport mechanism that results in the redistribution of retinoid between the inner segments and rhabdomes and chromophore exchange among the photopigments.
Rat retinas were labeled either by intravitreal injection of [14C]leucine or by incubation with [3H]-leucine or [35S]-methionine. Subcellular fractions were prepared on linear sucrose gradients and rhodopsin was extracted with detergent and purified by chromatography on ConA-Sepharose. A fraction enriched in rough endoplasmic reticulum (RER) and substantially free of rod outer segments (ROS) was found to contain a light-sensitive protein exhibiting the properties of rhodopsin on ConA-Sepharose or Agarose chromatography and on SDS-polyacrylamide gel electrophoresis, as well as immunologically. Intravitreal injection of [3H]-retinol also labeled the rhodopsin in the RER under conditions in which the rhodopsin in the ROS was not heavily labeled. Thus the chromophore appears to be attached to opsin shortly after the apoprotein is translated in the RER.
The cephalopod retina contains two photopigments that are spatially separated within the photoreceptors; rhodopsin, localized in the light-sensitive rhabdoms, and retinochrome, present in the myeloid bodies of the photoreceptor inner segments. In the light, the chromophore of retinochrome, all-trans retinal, is photoisomerized to 11-cis to form metaretinochrome. Metaretinochrome is believed to serve as a store for 11-cis retinal used in the regeneration or biosynthesis of rhodopsin. Previous studies suggest that a soluble retinal binding protein (RALBP) serves as a shuttle between retinochrome and rhodopsin, and, in the dark, may transport chromophore from the myeloid bodies to the rhabdoms. Our study supports this hypothesis and demonstrates that RALBP is in the correct cellular locations to function as a shuttle. Dark- and light-adapted octopus retinas were labeled with anti-RALBP using immunofluorescence and immunogold techniques. Our results showed that RALBP was distributed differently in the dark- and light-adapted retinas. Our most significant observation was that myeloid bodies from light-adapted retinas were more heavily labeled by anti-RALBP than myeloid bodies in dark-adapted retinas. The rhabdomeres, interphotoreceptor matrix, and inner limiting membrane were also labeled in both light and dark conditions. Based on these results and evidence from previous biochemical studies, we conclude that in the dark RALBP leaves the myeloid bodies and transports 11-cis retinal to the rhabdoms where chromophore exchange with metarhodopsin may occur.
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