Biochemical aspects of the visual process. XXX. Distribution of stereospecific retinol dehydrogenase activities in subcellular fractions of bovine retina and pigment epithelium.

The distribution of stereospecific retinol dehydrogenase activities and of lysosomal and microsomal enzymes in subcellular fractions of bovine retinas and pigment epithelium were studied. Whereas the highest total and specific activities of all-trans retinol dehydrogenase are found in the rod outer segments of the retina, the highest total and specific activities of 11-cis retinol dehydrogenase are found in the microsome fraction of the pigment epithelium. Whereas the outer segment enzyme would serve to interconvert retinaldehyde and retinol during the photolytic cycle, the pigment epithelium enzyme possibly functions to return 11-cis retinaldehyde to the pool of vitamin A compounds needed during the rod renewal process.     

The roles of vitamin E and unsaturated fatty acids in the visual process

Relatively high proportions of long-chain, polyunsaturated fatty acids seem to be required in rod photoreceptor membranes in order to provide the precise microenvironment for the proper function of the visual pigment rhodopsin. At the same time, such high levels of lipid unsaturation put the photoreceptor membranes at a high risk for autoxidation. The antioxidant vitamin E which can minimize autoxidation of polyunsaturated fatty acids is found in rather high concentrations in the outer segment membranes. Dietary deficiency in vitamin E induces disintegration of rod outer segment membranes, probably by increasing autoxidation. Also, it greatly accelerates the accumulation of aging pigments in the retinal pigment epithelium, probably because these lipofuscin granules do indeed represent the end products of lipid peroxidation. Vitamin E supplements, up to threefold normal levels, appear to provide no significant protection of the retina from light damage produced either by short but acute or by long-term, low level exposures to light. This is not consistent with current theories which implicate lipid peroxidation in the destruction of rod outer segments in light damaged retinas; more work is needed before any relation between retinal light damage and vitamin E levels can be assessed. Surprisingly, the amount of lipofuscin granule accumulation in the retinal pigment epithelium is influenced dramatically by dietary levels of vitamin A. Even retinas lacking a source of polyunsaturated fatty acids from rod outer segments still may accumulate massive lipofuscin if dietary vitamin A is provided. Perhaps vitamin A, which has such a dynamic relationship with the retinal pigment epithelium, becomes oxidized, and then contributes to the formation of a lipofuscin-like pigment. Centrophenoxine, a drug claimed to be effective in reversing the accumulation of age-related lipofuscin in the central nervous system, has no obvious effect in the eye or uterus in removing the lipofuscin granules induced by vitamin E deficiency. Microperoxisomes are abundant in the retinal pigment epithelium, and may be associated with rapid lipid turnover and/or utilization of lipid soluble vitamins. Their potential roles, however, need further documentation and clarification. Recently developed techniques and new discoveries in lipid research open the way for many fruitful studies on the interactions and precise roles of lipids and lipid-soluble vitamins in vision.     

Visual cycle in the mammalian eye. Retinoid-binding proteins and the distribution of 11-cis retinoids

This work was designed to provide an insight into the mammalian visual cycle by investigating the possible function of retinoid-binding proteins in this system, and the distribution and type of 11-cis retinoids present in the interphotoreceptor matrix and the cytosols of the retinal pigment epithelium and retina. The total retinol and retinal in the soluble fractions from these three compartments was 8% (3.31 nmol/eye) of the retinyl palmitate and stearate stored in the pigment epithelium membrane fractions (39 nmol/eye). Only small amounts of retinoids were detected in the rod outer segment cytosol. The insoluble fractions also contained retinol, nearly all of which was found in the retina. The retinoids in the soluble fractions appeared to be bound to cellular retinol-binding protein (CRBP), cellular retinal-binding protein (CRA1BP) and interstitial retinol-binding protein (IRBP, a high-Mr glycoprotein). Using immunospecific precipitation, immunoblot and immunocytochemical techniques it was demonstrated that IRBP was localized in the interphotoreceptor matrix and was synthesized and secreted by the retina, a process that did not require the protein to be glycosylated. The amount of retinol bound to IRBP increased if the eyes were exposed to light, when it was estimated that the protein carried up to 30% of its full capacity for all-trans retinol. In addition to all-trans retinol, IRBP carried smaller amounts of 11-cis retinol. The proportion of 11-cis retinol was frequently higher in eyes that had been protected from illumination, suggesting that IRBP plays a role in rhodopsin regeneration during dark-adaptation. Additionally, endogenous 11-cis retinoids in the retina and RPE cytosols were bound to an Mr 33,000 protein tentatively identified as CRA1BP. The 11-cis retinoid in the retina cytosol was mainly in the form of retinol, while in the RPE cytosol it was mainly in the form of retinal. Substantial amounts of 11-cis retinol were also found in the insoluble (membrane) fraction from the retina. It is suggested that in the mammalian retina 11-cis retinol is generated from all-trans retinol (possibly in the Muller cells). Lack of an 11-cis retinol oxidoreductase in the retina prevents it from being utilized for rhodopsin regeneration until it has been transported to the pigment epithelium, where it is converted to 11-cis retinal and returned to the rod outer segments. It is also suggested that IRBP may be implicated in the transport of retinoids between the rod outer segments, the Muller cells and the pigment epithelium.(ABSTRACT TRUNCATED AT 400 WORDS)     

Rod outer segment membranes: Good acceptors for retinoids?

The exchange of all-trans retinoids (retinal, retinol, retinylpalmitate) between PC-vesicles, PC-vesicles and liver microsomes or PC-vesicles and rod outer segment membranes is investigated using 11,12³H labeled compounds. In the first two systems, retinal and retinol exchange rapidly, retinyl acetate slowly and retinyl palmitate not at all. Rod outer segment membranes however take up relatively small amounts of retinoids (retinylpalmitate < retinol < retinal) and rapidly lose 60–90% of their label in the presence of PC-vesicles. E.G. retinoids clearly favour the PC-vesicle membrane. Apparently, rod outer segment membranes have a much lower affinity for retinoids than other artificial or natural membranes investigated so far.     

Dark adaptation and the retinoid cycle of vision

Following exposure of our eye to very intense illumination, we experience a greatly elevated visual threshold, that takes tens of minutes to return completely to normal. The slowness of this phenomenon of "dark adaptation" has been studied for many decades, yet is still not fully understood. Here we review the biochemical and physical processes involved in eliminating the products of light absorption from the photoreceptor outer segment, in recycling the released retinoid to its original isomeric form as 11-cis retinal, and in regenerating the visual pigment rhodopsin. Then we analyse the time-course of three aspects of human dark adaptation: the recovery of psychophysical threshold, the recovery of rod photoreceptor circulating current, and the regeneration of rhodopsin. We begin with normal human subjects, and then analyse the recovery in several retinal disorders, including Oguchi disease, vitamin A deficiency, fundus albipunctatus, Bothnia dystrophy and Stargardt disease. We review a large body of evidence showing that the time-course of human dark adaptation and pigment regeneration is determined by the local concentration of 11-cis retinal, and that after a large bleach the recovery is limited by the rate at which 11-cis retinal is delivered to opsin in the bleached rod outer segments. We present a mathematical model that successfully describes a wide range of results in human and other mammals. The theoretical analysis provides a simple means of estimating the relative concentration of free 11-cis retinal in the retina/RPE, in disorders exhibiting slowed dark adaptation, from analysis of psychophysical measurements of threshold recovery or from analysis of pigment regeneration kinetics.     

A microspectrophotometric study of normal and artificial visual pigments in the photoreceptors ofXenopus laevis

Absorbance spectra of visual pigments in theXenopus retina were obtained by microspectrophotometry. Principal rods contained a pigment with peak absorbance of 524 ± 2nm. A minority of rods contained a pigment with λ_max at 445 ± 6nm; single cone spectra peaked at 611 ± 4nm.Xenopus tadpoles were maintained on a Vitamin A-free diet for 6–8 weeks, then injected systemically with all-trans retinol or 9-cis retinal. In the principal rod, these resulted in pigments peaking at 511 ± 3nm and 498 ± 2nm, respectively. The normally encountered Vitamin A₂-based pigment of the principal rod had a broader half-width than its Vitamin A₁-based analog. In single cones, all-trans retinol led to the formation of a pigment with λ_max 578 ± 7nm. The substantial shifts seen in the wavelengths of peak absorbance as well as in the shapes of the spectral curves following administration of retinol or 9-cis retinal indicate that at least most of the normally encountered Vitamin A₂-based pigment was replaced by a Vitamin A₁-based pigment.     

Effect of short-term, high-dose retinol on dark adaptation in aging and early age-related maculopathy

PURPOSE: To examine the effect of a short course of high-dose retinol (preformed vitamin A) on dark adaptation in older adults with normal retinal health or early age-related maculopathy (ARM). METHODS: The study design was a randomized, double-masked, placebo-controlled experiment. Adults > or = 50 years of age whose fundus photographs for the eye to be tested psychophysically fell within steps 1 to 9 of the Age-Related Eye Disease Study (AREDS) Grading System were randomly assigned to a 30-day course of 50,000 IU oral retinol or a placebo. At baseline and 30-day follow-up, dark adaptation was tested and the Low Luminance Questionnaire (LLQ), an instrument for assessing difficulty with vision in reduced lighting, was administered. Primary outcomes of interest were rod- and cone-mediated parameters of dark adaptation, with scores on the LLQ's six subscales as secondary outcomes. RESULTS: The sample consisted of 104 participants with 52 each in the intervention and placebo groups. There were no group differences in baseline variables. At 30-days, the dark-adaptation parameters of cone time-constant, cone threshold, rod-cone break, and rod threshold did not differ. The retinol intervention group had significantly larger (i.e., steeper) rod slopes, indicating faster sensitivity recovery, than did the placebo group (P = 0.0419). There were no group differences in scores on the LLQ subscales driving, extreme lighting, emotional distress, general lighting, or peripheral vision. The retinol group had a higher score by five points on the mobility subscale compared with the placebo group (P = 0.0141). Those who had the most self-reported change on the mobility subscale at day 30 were more likely to have greater change in the speed of dark adaptation, as indicated by the rod slope parameter (r = 0.24, P = 0.0141). CONCLUSIONS: A short-term, high-dose course of retinol increased the rate of rod-mediated dark adaptation in older adults who were in the early phases of ARM or were exhibiting normal retinal aging. These results are consistent with the hypothesis that depositions and other structural changes in the retinal pigment epithelium and Bruch's membrane in aging and early ARM cause a localized retinoid deficiency.     

Effects of light and darkness on the visual pigments of amphibian tadpoles

The effects of light and darkness on the visual pigment composition in the eyes of tadpoles ofRana clamitans, Rana catesbeiana, Ambystoma maculatum andXenopus laevis have been investigated. Ranid tadpoles have mainly porphyropsin in the light, but in darkness the proportion of rhodopsin gradually increases over a period of several weeks. The process is reversible over several light-dark cycles. The light effect is extremely rapid: exposure to illumination of 85–150 ft-c. can return the system to nearly pure porphyropsin after 24–48 hr. The reaction appears to be the same at all developmental stages examined, i.e. from Taylor-Kollros stage IV–XV (early prometamorphosis). Illumination retards but does not prevent the switch to rhodopsin at natural or thyroxine-induced metamorphic climax (i.e. forelimb emergence). The dark increase of rhodopsin is not prevented by 0.04% thiourea. If tadpoles which have been permitted to increase their rhodopsin in the dark are illuminated after one eye in each specimen has been covered with black vaseline, the exposed eyes synthesize more porphyropsin. The difference is not observed if normal, clear vaseline is used instead, and it is therefore concluded that the light-stimulated formation of porphyropsin is a local response of the ocular tissues and is not mediated by hormones (such as thyroxine) or extraocular receptors (such as the stirnorgan). The quantities of visual pigment molecules appear to be the same in tadpoles kept in light and darkness, irrespective of the composition of the mixture. Like their adult forms, bullfrog tadpoles at or approaching metamorphic climax always had more porphyropsin in the dorsal retina. The premetamorphic tadpoles kept in the light did not show any significant differences between dorsal and ventral retinal areas, but those kept in darkness sometimes did. No porphyropsin was detectable in any part of the retinas of adultR. clamitans, R. pipiens, R. palustris, R. sylvatica, Hyla crucifer andH. versicolor. No dorsoventral gradient of visual pigment composition was found inR. clamitans tadpoles orXenopus laevis adults. UnlikeRana, Ambystoma tadpoles have pure rhodopsin like the adults.Xenopus tadpoles (stages 58–59) have virtually pure porphyropsin, like the adults. Neither species changes its visual pigments when kept for several weeks in light or darkness. It is suggested that the increase of rhodopsin in the dark occurs because rhodopsin is the major pigment that is incorporated into the outer segment during the renewal process. On the other hand, the rise of porphyropsin in the light is partly a consequence of the interchange of retinol and 3-dehydroretinol between retinal visual pigment and stores in the pigment epithelium, which are composed predominantly of 3-dehydroretinol even in tadpoles kept in the dark. However, ultimately there is total disappearance of retinol and rhodopsin from the eyes of tadpoles kept in continuous light, so it is believed that a further light-driven reaction must be implicated.     

A role for vitamin A in the formation of ocular lipofuscin

BACKGROUND—Lipofuscin granules in the retinal pigment epithelium are lipid protein aggregates which are thought to represent the lifelong accumulation of the non-degradable end products from the phagocytosis of photoreceptor outer segments. Given the increasing evidence for a key role for vitamin A in the formation of ocular lipofuscin, the fluorophores generated by reacting vitamin A with lipid were assessed.
METHODS—Reaction mixtures consisting of vitamin A (retinol) or its aldehyde (retinal) and (a) isolated rod outer segments, (b) the lipid extract of rod outer segments, (c) protein, or (d) liposomes were incubated at either pH 4.5 or 7.0 for up to 42 days. The fluorescence characteristics and mobility of the chloroform soluble fluorophores generated were compared with those extracted from purified human lipofuscin. Finally, the effect of lysosomal degradation on fluorophores generated in the above mixtures was assessed.
RESULTS—Major spectral changes were observed when ROS or liposomes were incubated with retinal. These changes were pH dependent and did not occur if retinal was replaced with retinol. A number of the fluorophores generated exhibited similar fluorescence characteristics and chromatographic mobility to those of lipofuscin. Neither the presence of protein nor exposure to lysosomal enzymes had any effect on the spectral profile or fluorophore mobility of the fluorophores generated.
CONCLUSIONS—These results suggest that some of the chloroform soluble fluorophores of lipofuscin are formed as a direct reaction product of retinal and lipid.

     

Ultrastructural localization of lectin binding sites on the surface of retinal photoreceptors and pigment epithelium.

The ferritin-lectin complexes of Ricinus communis agglutinin (RCA-120), wheat germ agglutinin (WGA), concanavalin A (con A), soybean agglutinin (SBA) and fucose binding protein (FBP) have been used to study the distribution of sugars along the plasma membranes of frog photoreceptor and pigment epithelial cells. The tissues were fixed prior to labeling in order to prevent the lectin-induced rearrangement of binding sites. The plasma membrane of different regions of the rod and cone photoreceptor cells bound each lectin to a different extent. In all cases, the outer segment plasma membrane bound more of the ferritin-lectin complex than did the inner segment plasma membrane. This might reflect the specific differentiation of the outer segment plasma membranes, related to their role in the visual process. The intensity of binding of each lectin to the pigment epithelium plasma membrane was similar to its binding to the rod outer segment plasma membrane. In cone outer segments, where one side of the disk membrane is accessible to the incubation medium, direct binding of lectin to the disk edges, as well as to the plasma membrane, was seen. Cone outer segments bound more of the α-D-N-acetylgalactosamine and D-galactose specific lectin, SBA, than did rod outer segments. No differences were observed in the binding of ferritin-lectin complexes to rod outer segments from dark or light adapted frogs.     

Effects of Monocular Light Deprivation on the Diurnal Rhythms in Retinal and Choroidal Thickness

PurposeTo determine the effects of monocular light deprivation on diurnal rhythms in retinal and choroidal thickness.MethodsTwenty participants, ages 22 to 45 years, underwent spectral domain optical coherence tomography imaging every three hours, from 8 AM to 8 PM, on two consecutive days. Participants wore an eye patch over the left eye starting at bedtime of day 1 until the end of the last measurement on day 2. Choroidal, total retinal, photoreceptor outer segment + retinal pigment epithelium (RPE), and photoreceptor inner segment thicknesses were determined.ResultsFor both eyes, significant diurnal variations were observed in choroidal, total retinal, outer segment + RPE, and inner segment thickness (P < 0.001). For light-deprived eyes, choroid diurnal variation persisted, although the choroid was significantly thinner at 8 AM and 11 AM (P < 0.01) on day 2 compared to day 1. On the other hand, diurnal variations in retinal thickness were eliminated in the light-deprived eye on day 2 when the eye was patched (P > 0.05). Total retinal and inner segment thicknesses significantly decreased (P < 0.001) and outer segment + RPE thickness significantly increased (P < 0.05) on day 2 compared to day 1.ConclusionsBlocking light exposure in one eye abolished the rhythms in retinal thickness, but not in choroidal thickness, of the deprived eye. Findings suggest that the rhythms in retinal thickness are, at least in part, driven by light exposure, whereas the rhythm in choroidal thickness is not impacted by short-term light deprivation.     

Parallel visual cycles in the zebrafish retina

Vertebrate vision necessitates continuous recycling of the chromophore 11-cis retinal (RAL). The classical (or canonical) visual cycle employs a number of enzymes located in the photoreceptor outer segment and RPE (retinal pigment epithelium) of the retina to regenerate 11-cis RAL from all-trans RAL. Cone-dominant species are believed to utilize a second, intra-retinal, pathway for 11-cis RAL generation, involving retinal Müller glia cells. This review summarizes the efforts made in zebrafish to gain a better understanding of the role of these two visual cycles for rod and cone photoreceptor chromophore recycling.     

An adaptive ERG technique to measure normal and altered dark adaptation in the mouse

The time-course of dark adaptation provides valuable insights into the function and interactions between the rod and cone pathways in the retina. Here we describe a technique that uses the flash electroretinogram (ERG) response to probe the functional integrity of the cone and rod pathways during the dynamic process of dark adaptation in the mouse. Retinal sensitivity was estimated from the stimulus intensity required to maintain a 30 μV criterion b-wave response during a 40 min period of dark adaptation. When tracked in this manner, dark adaptation functions in WT mice depended upon the bleaching effects of initial background adaptation conditions. Altered dark adaptation functions, commensurate with the functional deficit were recorded in pigmented mice that lacked cone function (Gnat2 ^cplf3 ) and in WT mice injected with a toxin, sodium iodate (NaIO₃), which targets the retinal pigment epithelium and also has downstream effects on photoreceptors. These data demonstrate that this adaptive tracking procedure measures retinal sensitivity and the contributions of the rod and/or cone pathways during dark adaptation in both WT control and mutant mice.     

Long-lived photoproducts of the green-rod pigment of the frog, Rana temporaria.

The slow thermally decaying photoproducts formed from the P437₁ of the frog green rods have been studied by microspectrophotometry. The first detectable photoproduct is metarhodopsin II (MR II,λ_max 390 nm) which is followed by a MR III withλ_max at about 445 nm, “retinal” (λ_max 380–390 nm) and retinol. The sequence of photoproducts can be fitted by the model proposed by Baumann (1972) for frog rhodopsin P505₁. However, in the green rod, in contrast to the photoproducts of P505₁, MR II is randomly orientated in the outer segment membranes whereas MR III. “retinal” and retinol are orientated parallel to the long axis of the outer segment. It is suggested that this orientation of photoproducts is consistent with other cone-like properties of the green rod.     

Visual pigment loss after light-induced shedding of rod outer segments.

The amount of visual pigment in the eyes of adult frogs kept in darkness for about 5 weeks increases by approximately one-third. There is a comparable fall in the amount of vitamin A stored in the pigment epithelium (RPE), due to a steep deeline in the all-trans isomer. The proportion in the 11-cis configuration, however, increases from 15% to nearly 50%. Similarly, tadpoles kept in complete darkness for a week have more visual pigment than those exposed to a diurnal 12 hr light/dark cycle. The difference is correlated with rod outer segment (ROS) length which also increases in the dark. When dark animals are exposed to light for 2 hr, massive shedding of fragments from the tips of the ROS results in a decrease in ROS length. The engulfment of these shed ROS fragments results in an increase in the phagosome content of the RPE. After 2·5 hr in darkness the bleached visual pigment in these phagosomes does not regenerate, and any unbleached material present appears to be degraded. Both effects may be attributed to the action of lysosomal enzymes following phagocytosis by the RPE. During the 24 hr after shedding, the ROS elongate at a rate suggesting that renewal has been accelerated by a factor of 3–5 fold. The amount of visual pigment in the dark-adapted eye recovers in parallel with this increase in length.     

The effect of light on the quantity of phagosomes in the pigment epithelium.

The effect of light on the number of phagosomes within the pigment epithelium was studied using Rana pipiens tadpoles dark-adapted for various intervals before light stimulation. Two populations of phagosomes were observed. One consists of large packets of rod outer segment discs 4–10 μm long and 5 μm wide. Light stimulation following various periods of darkness results in a steady increase in the density of these large phagosomes reaching a peak 2–3·5 times above control levels after 2 hr of light. The maximum number of large phagosomes was obtained in animals dark adapted for 3 days before light stimulation. The second population of phagosomes consists of small packets of outer segment discs 1–2 μm in diameter. The number of these phagosomes does not change with different conditions of illumination. Electron microscopic analysis indicates that the small phagosomes are lost from the rod outer segment tips as small whorls of 5–30 discs.     

Retinoids bound to interstitial retinol-binding protein during light and dark-adaptation

High-performance liquid chromatography was used to determine the types and amounts of retinoids bound to interstitial retinol-binding protein (IRBP) during light- and dark-adaptation in frogs. IRBP was separated from CRBP and CRA1BP by ion-exchange chromatography and quantitated by determining the amount of Serva Blue R dye bound to it in stained sodium dodecyl sulfate polyacrylamide gels. The amount of IRBP was not significantly different in light- and dark-adapted eyes (0.15 +/- 0.05 nmol/eye compared with 0.18 +/- 0.08 nmol/eye). In the dark-adapted state, IRBP bound mainly 11-cis retinol and 11-cis retinal in quantities that summed to about 1 mol/mol IRBP. After the onset of light-adaptation, all-trans retinol increased from its very low dark-adapted level, peaked at 0.2 mol/mol IRBP and then declined to the dark-adapted level again. Concomitantly, the total retinoid bound to IRBP fell, mainly because there was a drop in the amount of 11-cis retinal. During dark-adaptation, the amount of 11-cis retinal increased. No significant changes were seen in the amount of 11-cis retinol in light and darkness. These findings support the hypothesis that when rhodopsin is bleached IRBP transports all-trans retinol from the retina to the pigment epithelium and that it delivers 11-cis retinal to the rod outer segments for rhodopsin regeneration.     

Rhodopsin content and rod outer segment length in albino rat eyes: Modification by dark adaptation

Whole eye rhodopsin content and rod outer segment length have been determined in albino rat eyes after different periods of dark adaptation, dark-rearing or cyclic light maintenance. The rhodopsin content in the eyes of dark-reared albino rats is approximately 50% higher than that in the eyes of littermates reared in cyclic light (in-cage illumination less than 15 ft-cd). The same increase in rhodopsin can be obtained in albino rats after only a 10-day dark adaptation period. Furthermore, the increased level of rhodopsin is the same as that in the eyes of pigmented rats reared in cyclic light. The increase in rhodopsin content in the albino rat eyes is due at least in part to an increase in rod outer segment length. The increase in length with dark adaptation and dark-rearing was somewhat variable, with an average increase of approximately 25%. No apparent increase was found in rod outer segment diameter, rod outer segment disc packing density or eye size. Rod outer segment lengths were consistently longer in the superior hemisphere of the eye than in the inferior hemisphere in both dark-adapted albino rats and pigmented rats maintained in cyclic light, but not in albino rats maintained in cyclic light.     

The decay of long-lived photoproducts in the isolated bullfrog rod outer segment: relationship to other dark reactions.

Isolated bullfrogrodouter segments have been used to investigate whetherrhodopsinregeneration, light activated rhodopsinphosphorylationor aspects of “dark adaptation” in vitro are correlated with rhodopsin photoproduct decay. We find that the presence ofphotoproductsdoes limit rhodopsin regeneration initiated by addition of excess 11-cis retinal. This regeneration proceeds with a biphasic time course only as metarhodopsin II and metarhodopsin III decay into opsin and all-trans retinal. Regeneration initiated after decay of these photoproducts is about 20 times faster and is proportional to the concentration of visual pigment molecules still in the bleached state. The presence of long-lived photoproducts does not alter the light activated phosphorylation of outer segment membranes, suggesting that this reaction must arise at an earlier step in the photolytic sequence. Further, photoproduct decay and rhodopsin regeneration have no effect on the recovery of the sodium permeability of the plasma membrane in dark adapting outer segments.     

Alcohol dehydrogenase and retinol dehydrogenase in bovine retinal pigment epithelium.

An NAD dependent alcohol dehydrogenase isolated from cytosol of bovine retinal pigment epithelium and catalysing ethanol oxidation, plays no role in the retinol-retinene interconversion.High retinol-retinene oxidoreductase activity was found in subcellular fractions of retinal pigment epithelium. This NADP-NADPH linked oxidoreductase, was extracted by sonication from mitochondria and microsomes and some catalytic properties were examined.The possible physiological significance of this enzyme in the metabolism of retinol and retinene in the pigment epithelium may be deduced by considering the participation of the pigment epithelium with regard to degradation of rod outer segments (ROS); the enzyme could be so located as to facilitate the reduction of the retinene, released by rhodopsin of phagocytosed ROS.Other physiological roles are discussed.