Inhibitors of retinyl ester formation also prevent the biosynthesis of 11-cis-retinol

Retinal pigment epithelium 65 kDa protein (RPE65): An update

Vertebrate vision critically depends on an 11-cis-retinoid renewal system known as the visual cycle. At the heart of this metabolic pathway is an enzyme known as retinal pigment epithelium 65 kDa protein (RPE65), which catalyzes an unusual, possibly biochemically unique, reaction consisting of a coupled all-trans-retinyl ester hydrolysis and alkene geometric isomerization to produce 11-cis-retinol. Early work on this isomerohydrolase demonstrated its membership to the carotenoid cleavage dioxygenase superfamily and its essentiality for 11-cis-retinal production in the vertebrate retina. Three independent studies published in 2005 established RPE65 as the actual isomerohydrolase instead of a retinoid-binding protein as previously believed. Since the last devoted review of RPE65 enzymology appeared in this journal, major advances have been made in a number of areas including our understanding of the mechanistic details of RPE65 isomerohydrolase activity, its phylogenetic origins, the relationship of its membrane binding affinity to its catalytic activity, its role in visual chromophore production for rods and cones, its modulation by macromolecules and small molecules, and the involvement of RPE65 mutations in the development of retinal diseases. In this article, I will review these areas of progress with the goal of integrating results from the varied experimental approaches to provide a comprehensive picture of RPE65 biochemistry. Key outstanding questions that may prove to be fruitful future research pursuits will also be highlighted.

Structural biology of 11-cis-retinaldehyde production in the classical visual cycle

The vitamin A derivative 11-cis-retinaldehyde plays a pivotal role in vertebrate vision by serving as the chromophore of rod and cone visual pigments. In the initial step of vision, a photon is absorbed by this chromophore resulting in its isomerization to an all-trans state and consequent activation of the visual pigment and phototransduction cascade. Spent chromophore is released from the pigments through hydrolysis. Subsequent photon detection requires the delivery of regenerated 11-cis-retinaldehyde to the visual pigment. This trans-cis conversion is achieved through a process known as the visual cycle. In this review, we will discuss the enzymes, binding proteins and transporters that enable the visual pigment renewal process with a focus on advances made during the past decade in our understanding of their structural biology.

Non-syndromic retinitis pigmentosa

Retinitis pigmentosa (RP) encompasses a group of inherited retinal dystrophies characterized by the primary degeneration of rod and cone photoreceptors. RP is a leading cause of visual disability, with a worldwide prevalence of 1:4000. Although the majority of RP cases are non-syndromic, 20-30% of patients with RP also have an associated non-ocular condition. RP typically manifests with night blindness in adolescence, followed by concentric visual field loss, reflecting the principal dysfunction of rod photoreceptors; central vision loss occurs later in life due to cone dysfunction. Photoreceptor function measured with an electroretinogram is markedly reduced or even absent. Optical coherence tomography (OCT) and fundus autofluorescence (FAF) imaging show a progressive loss of outer retinal layers and altered lipofuscin distribution in a characteristic pattern. Over the past three decades, a vast number of disease-causing variants in more than 80 genes have been associated with non-syndromic RP. The wide heterogeneity of RP makes it challenging to describe the clinical findings and pathogenesis. In this review, we provide a comprehensive overview of the clinical characteristics of RP specific to genetically defined patient subsets. We supply a unique atlas with color fundus photographs of most RP subtypes, and we discuss the relevant considerations with respect to differential diagnoses. In addition, we discuss the genes involved in the pathogenesis of RP, as well as the retinal processes that are affected by pathogenic mutations in these genes. Finally, we review management strategies for patients with RP, including counseling, visual rehabilitation, and current and emerging therapeutic options.

Cone Health and Retinoids

Cones are photoreceptor cells used for bright light and color vision. Retinoids are vitamin A derivatives, one of which is the 11-cis aldehyde form that serves as the chromophore for both cone and rod visual pigments. In the visual disease, Type 2 Leber congenital amaurosis (LCA2), 11-cis-retinal generation is inhibited or abolished. Work by others has shown that patients with LCA2 have symptoms consistent with degenerating cones. In mouse models for LCA2, early cone degeneration is readily apparent: cone opsins and other proteins associated with the outer segment are delocalized and cell numbers decline rapidly within the first month. Rods would appear normal morphologically and functionally, if not for the absence of chromophore. Supplementation of mouse models of LCA2 with cis-retinoids has been shown to slow loss of cone photoreceptor cells if mice were maintained in darkness. Thus, 11-cis-retinal appears not only to have a role in the light response reaction but also to promote proper trafficking of the cone opsins and maintain viable cones.

Vitamin A derivatives as treatment options for retinal degenerative diseases

The visual cycle is a sequential enzymatic reaction for vitamin A, all-trans-retinol, occurring in the outer layer of the human retina and is essential for the maintenance of vision. The central source of retinol is derived from dietary intake of both retinol and pro-vitamin A carotenoids. A series of enzymatic reactions, located in both the photoreceptor outer segment and the retinal pigment epithelium, transform retinol into the visual chromophore 11-cis-retinal, regenerating visual pigments. Retina specific proteins carry out the majority of the visual cycle, and any significant interruption in this sequence of reactions is capable of causing varying degrees of blindness. Among these important proteins are Lecithin:retinol acyltransferase (LRAT) and retinal pigment epithelium-specific 65-kDa protein (RPE65) known to be responsible for esterification of retinol to all-trans-retinyl esters and isomerization of these esters to 11-cis-retinal, respectively. Deleterious mutations in these genes are identified in human retinal diseases that cause blindness, such as Leber congenital amaurosis (LCA) and retinitis pigmentosa (RP). Herein, we discuss the pathology of 11-cis-retinal deficiency caused by these mutations in both animal disease models and human patients. We also review novel therapeutic strategies employing artificial visual chromophore 9-cis-retinoids which have been employed in clinical trials involving LCA patients.

Vitamin A metabolism in rod and cone visual cycles

The chromophore of all known visual pigments consists of 11-cis-retinal (derived from either vitamin A1 or A2) or a hydroxylated derivative, bound to a protein (opsin) via a Schiff base. Absorption of a photon results in photoisomerization of the chromophore to all-trans-retinal and conversion of the visual pigment to the signaling form. Regeneration of the 11-cis-retinal occurs in an adjacent tissue and involves several enzymes, several water-soluble retinoid-binding proteins, and intra- and intercellular diffusional processes. Rod photoreceptor cells depend completely on the output of 11-cis-retinal from adjacent retinal pigment epithelial (RPE) cells. Cone photoreceptors cells can use 11-cis-retinal from the RPE and from a second more poorly characterized cycle, which appears to involve adjacent Müller (glial) cells. Recent progress in the characterization of rod and cone visual cycle components and reactions will result in the development of approaches to the amelioration of blinding eye diseases associated with visual cycle defects.

New insights into retinoid metabolism and cycling within the retina

The retinoid cycle is a series of biochemical reactions within the eye that is responsible for synthesizing the chromophore, 11-cis retinal, for visual function. The chromophore is bound to G-protein coupled receptors, opsins, within rod and cone photoreceptor cells forming the photosensitive visual pigments. Integral to the sustained function of photoreceptors is the continuous generation of chromophore by the retinoid cycle through two separate processes, one that supplies both rods and cones and another that exclusively supplies cones. Recent findings such as RPE65 localization within cones and the pattern of distribution of retinoid metabolites within mouse and human retinas have challenged previous proposed schemes. This review will focus on recent findings regarding the transport of retinoids, the mechanisms by which chromophore is supplied to both rods and cones, and the metabolism of retinoids within the posterior segment of the eye.

Retinyl ester homeostasis in the adipose differentiation-related protein-deficient retina

The retinal pigmented epithelium (RPE) plays an essential role in vision, including storing and converting retinyl esters of the visual chromophore, 11-cis-retinal. Retinyl ester storage structures (RESTs), specialized lipid droplets within the RPE, take up retinyl esters synthesized in the endoplasmic reticulum. Here we report studies of mice lacking exons 2 and 3 of the gene encoding adipose differentiation-related protein (Adfp), a structural component of RESTs. We found that dark adaptation was slower in Adfp(Delta2-3/Delta2-3) than in Adfp+/+ mice and that Adfp(Delta2-3/Delta2-3) mice had consistently delayed clearances of all-trans-retinal and all-trans-retinol from rod photoreceptor cells. Two-photon microscopy revealed aberrant trafficking of all-trans-retinyl esters in the RPE of Adfp(Delta2-3/Delta2-3) mice, a problem caused by abnormal maintenance of RESTs in the dark-adapted state. Retinyl ester accumulation was also reduced in Adfp(Delta2-3/Delta2-3) as compared with Adfp+/+ mice. These observations suggest that Adfp plays a unique role in vision by maintaining proper storage and trafficking of retinoids within the eye.

Lecithin:retinol acyltransferase in ARPE-19

The purpose of this study is to investigate if a readily available cell line (APRE-19) may be used to study in vitro function of visual cycle enzymes such as lecithin:retinol acyltransferase (LRAT). Cells incubated with exogenous retinol accumulated intracellular all-trans retinol and all-trans retinyl ester. Membrane proteins from ARPE-19 exhibited LRAT activity, which was inhibited by an LRAT inhibitor, retinyl bromoacetate (RBA). Gene microarray and Western blot results indicated that ARPE-19 cells expressed LRAT transcript and the LRAT protein. Therefore, our data show that ARPE-19 contains an active LRAT enzyme and suggest that it is an appropriate cell system to study visual cycle enzymes.

Acyl CoA:retinol acyltransferase (ARAT) activity is present in bovine retinal pigment epithelium

Visual perception is mediated by a family of G protein-coupled receptors called the opsins. The light-absorbing chromophore in most opsins is 11-cis-retinaldehyde, which is isomerized to all-trans-retinaldehyde upon absorption of a photon. Restoration of light sensitivity to the photobleached opsin requires chemical re-isomerization of the chromophore. This is carried out by an enzymatic pathway called the visual cycle in retinal pigment epithelial cells. The isomerase in this pathway uses fatty-acyl esters of all-trans-retinol as substrate. A retinyl-ester synthase that produces these esters, called lecithin:retinol acyltransferase (LRAT), has been extensively characterized. Based on prior biochemical studies and the phenotype in lrat(-/-) knockout mice, it has been assumed that LRAT is the sole or dominant retinyl-ester synthase in the retinal pigment epithelium. Here we demonstrate the presence of a second ester synthase activity in these cells called acyl CoA:retinol acyltransferase (ARAT). We show that this activity uses palmitoyl coenzyme A as an acyl donor, unlike LRAT which uses phosphatidylcholine. Similar to LRAT, ARAT esterifies both all-trans-retinol and 11-cis-retinol. LRAT and ARAT are both potently inhibited by the retinyl-ester analog, all-trans-retinylbromoacetate, but only ARAT is inhibited by progesterone. Unexpectedly, the maximum turnover rate (V(max)) of ARAT was similar to that of LRAT. However, the Michaelis constant (K(M)) of ARAT was 10-fold higher than the K(M) of LRAT for all-trans-retinol. These observations suggest that ARAT may complement LRAT to provide additional retinyl-ester synthase activity under conditions of high all-trans-retinol. These conditions occur in the retina following exposure to bright light.

The retinal pigment epithelium in visual function

Located between vessels of the choriocapillaris and light-sensitive outer segments of the photoreceptors, the retinal pigment epithelium (RPE) closely interacts with photoreceptors in the maintenance of visual function. Increasing knowledge of the multiple functions performed by the RPE improved the understanding of many diseases leading to blindness. This review summarizes the current knowledge of RPE functions and describes how failure of these functions causes loss of visual function. Mutations in genes that are expressed in the RPE can lead to photoreceptor degeneration. On the other hand, mutations in genes expressed in photoreceptors can lead to degenerations of the RPE. Thus both tissues can be regarded as a functional unit where both interacting partners depend on each other.

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.

The specific binding of retinoic acid to RPE65 and approaches to the treatment of macular degeneration

RPE65 is essential in the operation of the visual cycle and functions as a chaperone for all-trans-retinyl esters, the substrates for isomerization in the visual cycle. RPE65 stereospecifically binds all-trans-retinyl esters with a K(D) of 47 nM. It is shown here by using a quantitative fluorescence technique, that Accutane (13-cis-retinoic acid), a drug used in the treatment of acne but that causes night blindness, binds to RPE65 with a K(D) of 195 nM. All-trans-retinoic acid binds with a K(D) of 109 nM. The binding of the retinoic acids to RPE65 is competitive with all-trans-retinyl ester binding, and this competition inhibits visual cycle function. A retinoic acid analog that binds weakly to RPE65 is not inhibitory. These data suggest that RPE65 function is rate-limiting in visual cycle function. They also reveal the target through which the retinoic acids induce night blindness. Finally, certain forms of retinal and macular degeneration are caused by the accumulation of vitamin A-based retinotoxic products, called the retinyl pigment epithelium-lipofuscin. These retinotoxic products accumulate during the normal course of rhodopsin bleaching and regeneration after the operation of the visual cycle. Drugs such as Accutane may represent an important approach to reducing the accumulation of the retinotoxic lipofuscin by inhibiting visual cycle function. The identification of RPE65 as the visual cycle target for the retinoic acids makes it feasible to develop useful drugs to treat retinal and macular degeneration while avoiding the substantial side effects of the retinoic acids.

A Palmitoylation Switch Mechanism in the Regulation of the Visual Cycle

RPE65 is essential for the biosynthesis of 11-cis-retinal, the chromophore of rhodopsin. Here, we show that the membrane-associated form (mRPE65) is triply palmitoylated and is a chaperone for all-trans-retinyl esters, allowing their entry into the visual cycle for processing into 11-cis-retinal. The soluble form of RPE65 (sRPE65) is not palmitoylated and is a chaperone for vitamin A, rather than all-trans-retinyl esters. Thus, the palmitoylation of RPE65 controls its ligand binding selectivity. The two chaperones are interconverted by lecithin retinol acyl transferase (LRAT) acting as a molecular switch. Here mRPE65 is a palmitoyl donor, revealing a new acyl carrier protein role for palmitoylated proteins. When chromophore synthesis is not required, mRPE65 is converted into sRPE65 by LRAT, and further chromophore synthesis is blocked. The studies reveal new roles for palmitoylated proteins as molecular switches and LRAT as a palmitoyl transferase whose role is to catalyze the mRPE65 to sRPE65 conversion.

Molecular logic of 11-cis-retinoid biosynthesis in a cone-dominated species

The biochemical pathway to visual chromophore biosynthesis in rod-dominated animals involves minimally a two component system in which all-trans-retinyl esters, generated by the action of lecithin retinol acyltransferase (LRAT) on vitamin A, are processed into 11-cis-retinol by isomerohydrolase. Possible differences in retinoid metabolism in cone-dominated animals have been noted in the literature, so it was of interest to explore whether these differences are tangential or fundamental. Central to this issue is whether cone-dominated animals use an isomerohydrolase (IMH)-based mechanism in the predominant pathway to 11-cis-retinoids. Here, it is shown that all-trans-retinyl esters (tREs) are the direct precursors of 11-cis-retinol formation in chicken retinyl pigment epithelium/retina preparations. This conclusion is based on at least three avenues of evidence. First, reagents that block tRE synthesis from vitamin A also block 11-cis-retinol synthesis. Second, pulse-chase experiments also establish that tREs are the precursors to 11-cis-retinol. Finally, 11-cis-retinyl-bromoacetate, a known affinity-labeling agent of isomerohydrolase, also blocks chromophore biosynthesis in the cone system.

Rpe65 Is a Retinyl Ester Binding Protein That Presents Insoluble Substrate to the Isomerase in Retinal Pigment Epithelial Cells

Photon capture by a rhodopsin pigment molecule induces 11-cis to all-trans isomerization of its retinaldehyde chromophore. To restore light sensitivity, the all-trans-retinaldehyde must be chemically re-isomerized by an enzyme pathway called the visual cycle. Rpe65, an abundant protein in retinal pigment epithelial (RPE) cells and a homolog of beta-carotene dioxygenase, appears to play a role in this pathway. Rpe65-/- knockout mice massively accumulate all-trans-retinyl esters but lack 11-cis-retinoids and rhodopsin visual pigment in their retinas. Mutations in the human RPE65 gene cause a severe recessive blinding disease called Leber's congenital amaurosis. The function of Rpe65, however, is unknown. Here we show that Rpe65 specifically binds all-trans-retinyl palmitate but not 11-cis-retinyl palmitate by a spectral-shift assay, by co-elution during gel filtration, and by co-immunoprecipitation. Using a novel fluorescent resonance energy transfer (FRET) binding assay in liposomes, we demonstrate that Rpe65 extracts all-trans-retinyl esters from phospholipid membranes. Assays of isomerase activity reveal that Rpe65 strongly stimulates the enzymatic conversion of all-trans-retinyl palmitate to 11-cis-retinol in microsomes from bovine RPE cells. Moreover, we show that addition of Rpe65 to membranes from rpe65-/- mice, which possess no detectable isomerase activity, restores isomerase activity to wild-type levels. Rpe65 by itself, however, has no intrinsic isomerase activity. These observations suggest that Rpe65 presents retinyl esters as substrate to the isomerase for synthesis of visual chromophore. This proposed function explains the phenotype in mice and humans lacking Rpe65.

Origin of the vertebrate visual cycle: II. Visual cycle proteins are localized in whole brain including photoreceptor cells of a primitive chordate

The visual cycle system in a primitive chordate, ascidian Ciona intestinalis, was studied by whole-mount in situ hybridization and by whole-mount immunohistochemistry. Three visual cycle proteins, Ciona homologue of RGR (Ci-opsin3), CRALBP (Ci-CRALBP), and BCO/RPE65 (Ci-BCO/RPE65) were widely distributed in the brain vesicle and visceral ganglion. To identify the visual cycle system in a primitive chordate, we compared the localization of photoreceptor-specific proteins (visual pigment and arrestin) and visual cycle proteins (Ci-opsin3 and Ci-CRALBP). The ascidian visual cycle is composed of two cellular compartments, the photoreceptors and the brain vesicle, but some photoreceptor cells also contain visual cycle proteins.

Specific inactivation of isomerohydrolase activity by 11-cis-retinoids

The endergonic trans-->cis isomerization of retinoids is an essential element in rhodopsin regeneration in vertebrates. All-trans-retinyl esters, which are generated by lecithin retinol acyltransferase (LRAT), are on the isomerization pathway. The critical isomerohydrolase activity, which catalyzes the trans-->cis isomerization/hydrolysis reaction of all-trans-retinyl esters, remains to be identified. It is demonstrated here that 11-cis-retinyl bromoacetate (cRBA) is a potent and specific inactivator of the bovine retinyl pigment epithelial (RPE) isomerohydrolase activity, with a measured K(I)=0.19 microM and a pseudo-first-order rate of inactivation k(inh)=1.83 x 10(-3) s(-1). This demonstrates that the isomerization is indeed enzyme-mediated. This inactivator should facilitate the identification and study of isomerohydrolase, or at least an essential component of it. Labeling of crude RPE membranes with 3H-cRBA reveals the presence of several labeled bands that may be isomerohydrolase candidates.

Vitamin A metabolism in the retinal pigment epithelium: genes, mutations, and diseases

Mutations in the genes necessary for the metabolism of vitamin A (all-trans retinol) and cycling of retinoids between the photoreceptors and retinal pigment epithelium (RPE) (the visual cycle) have recently emerged as an important class of genetic defects responsible for retinal dystrophies and dysfunctions. Research into the causes and treatment of diseases resulting from defects in retinal vitamin A metabolism is currently the subject of intense interest, since disorders affecting the RPE are, in principle, more accessible to therapeutic intervention than those affecting the proteins of photoreceptor cells. This chapter presents an overview of the visual cycle, as well as the function of the RPE genes involved in the conversion of vitamin A to 11-cis retinal, the chromophore of the visual pigments. The identification of disease-causing mutations in this group of genes is described as well as the associated phenotypes that range from stationary night blindness to childhood-onset severe visual handicap. Consideration is also given to alternative genetic paradigms potentially relevant to defects in vitamin A metabolism, including a discussion of the relationship of this pathway to age-related macular degeneration, a non-Mendelian disease of late onset. Finally, progress and prospects for targeted therapeutic intervention in vitamin A metabolism are presented, including retinoid and gene replacement therapy. On the basis of early successes in animal models, and plans underway for Phase I/II clinical trials, it is hoped that the near future will bring effective therapies for many retinal dystrophy patients with defects in vitamin A metabolism.

All-trans-retinyl esters are the substrates for isomerization in the vertebrate visual cycle

The identification of the critical enzyme(s) that carries out the trans to cis isomerization producing 11-cis-retinol during the operation of the visual cycle remains elusive. Confusion exists in the literature as to the exact nature of the isomerization substrate. At issue is whether it is an all-trans-retinyl ester or all-trans-retinol (vitamin A). As both putative substrates interconvert rapidly in retinal pigment epithelial membranes, the choice of substrate can be ambiguous. The two enzymes that effect interconversion of all-trans-retinol and all-trans-retinyl esters are lecithin retinol acyl transferase (LRAT) and retinyl ester hydrolase (REH). The retinyl ester or all-trans-retinol pools are radioactively labeled separately in the presence of inhibitors of LRAT and REH, effectively preventing their interconversion. Pulse-chase experiments unambiguously demonstrate that all-trans-retinyl esters, and not all-trans-retinol, are the precursors of 11-cis-retinol. When the all-trans-retinyl ester pool is radioactively labeled, the resulting 11-cis-retinol is labeled with the same specific activity as the precursor ester. The converse is true with vitamin A. These data unambiguously establish all-trans-retinyl esters as the precursors of 11-cis-retinol.

Isomerization and oxidation of vitamin a in cone-dominant retinas: a novel pathway for visual-pigment regeneration in daylight

The first step toward light perception is 11-cis to all-trans photoisomerization of the retinaldehyde chromophore in a rod or cone opsin-pigment molecule. Light sensitivity of the opsin pigment is restored through a multistep pathway called the visual cycle, which effects all-trans to 11-cis re-isomerization of the retinoid chromophore. The maximum throughput of the known visual cycle, however, is too slow to explain sustained photosensitivity in bright light. Here, we demonstrate three novel enzymatic activities in cone-dominant ground-squirrel and chicken retinas: an all-trans-retinol isomerase, an 11-cis-retinyl-ester synthase, and an 11-cis-retinol dehydrogenase. Together these activities comprise a novel pathway that regenerates opsin photopigments at a rate 20-fold faster than the known visual cycle. We suggest that this pathway is responsible for sustained daylight vision in vertebrates.

Lecithin retinol acyltransferase forms functional homodimers

Membrane-bound lecithin retinol acyltransferase (LRAT), an essential enzyme in vitamin A processing, catalyzes the formation of retinyl esters from vitamin A and lecithin. Cloned and expressed LRAT has a molecular mass of 25.3 kDa. The enzyme is not homologous to known enzymes and is, therefore, of substantial interest mechanistically. Along these lines, the functional protomeric state of LRAT is of importance. Gel electrophoretic studies on LRAT in the presence of SDS and disulfide reducing agents show the expected 25 kDa monomer. However, gel electrophoresis in the absence of a reducing agent and/or strong denaturing conditions reveals substantial dimer formation. LRAT monomers can be efficiently and irreversibly cross-linked by thiol reactive bismaleimides in retinal pigment epithelial (RPE) membranes generating LRAT homodimers. Cross-linked LRAT homodimers are fully active catalytically. The experiments suggest that LRAT monomers interact in membranes and form functional homodimers through protein-protein interactions and disulfide bond formation.

Characterization of a dehydrogenase activity responsible for oxidation of 11-cis-retinol in the retinal pigment epithelium of mice with a disrupted RDH5 gene. A model for the human hereditary disease fundus albipunctatus

In the vertebrate retina, the final step of visual chromophore production is the oxidation of 11-cis-retinol to 11-cis-retinal. This reaction is catalyzed by 11-cis-retinol dehydrogenases (11-cis-RDHs), prior to the chromophore rejoining with the visual pigment apo-proteins. The RDH5 gene encodes a dehydrogenase that is responsible for the majority of RDH activity. In humans, mutations in this gene are associated with fundus albipunctatus, a disease expressed by delayed dark adaptation of both cones and rods. In this report, an animal model for this disease, 11-cis-rdh-/- mice, was used to investigate the flow of retinoids after a bleach, and microsomal membranes from the retinal pigment epithelium of these mice were employed to characterize remaining enzymatic activities oxidizing 11-cis-retinol. Lack of 11-cis-RDH leads to an accumulation of cis-retinoids, particularly 13-cis-isomers. The analysis of 11-cis-rdh-/- mice showed that the RDH(s) responsible for the production of 11-cis-retinal displays NADP-dependent specificity toward 9-cis- and 11-cis-retinal but not 13-cis-retinal. The lack of 13-cis-RDH activity could be a reason why 13-cis-isomers accumulate in the retinal pigment epithelium of 11-cis-rdh-/- mice. Furthermore, our results provide detailed characterization of a mouse model for the human disease fundus albipunctatus and emphasize the importance of 11-cis-RDH in keeping the balance between different components of the retinoid cycle.

Confronting complexity: the interlink of phototransduction and retinoid metabolism in the vertebrate retina

Absorption of light by rhodopsin or cone pigments in photoreceptors triggers photoisomerization of their universal chromophore, 11-cis-retinal, to all-trans-retinal. This photoreaction is the initial step in phototransduction that ultimately leads to the sensation of vision. Currently, a great deal of effort is directed toward elucidating mechanisms that return photoreceptors to the dark-adapted state, and processes that restore rhodopsin and counterbalance the bleaching of rhodopsin. Most notably, enzymatic isomerization of all-trans-retinal to 11-cis-retinal, called the visual cycle (or more properly the retinoid cycle), is required for regeneration of these visual pigments. Regeneration begins in rods and cones when all-trans-retinal is reduced to all-trans-retinol. The process continues in adjacent retinal pigment epithelial cells (RPE), where a complex set of reactions converts all-trans-retinol to 11-cis-retinal. Although remarkable progress has been made over the past decade in understanding the phototransduction cascade, our understanding of the retinoid cycle remains rudimentary. The aim of this review is to summarize recent developments in our current understanding of the retinoid cycle at the molecular level, and to examine the relevance of these reactions to phototransduction.

Cloning and characterization of a human beta,beta-carotene-15,15'-dioxygenase that is highly expressed in the retinal pigment epithelium

Retinoids play a critical role in vision, as well as in development and cellular differentiation. beta,beta-Carotene-15,15'-dioxygenase (Bcdo), the enzyme that catalyzes the oxidative cleavage of beta,beta-carotene into two retinal molecules, plays an important role in retinoid synthesis. We report here the first cloning of a mammalian Bcdo. Human BCDO encodes a protein of 547 amino acid residues that demonstrates 68% identity with chicken Bcdo. It is expressed highly in the retinal pigment epithelium (RPE) and also in kidney, intestine, liver, brain, stomach, and testis. The gene spans approximately 20 kb, is composed of 11 exons and 10 introns, and maps to chromosome 16q21-q23. A mouse orthologue was also identified, and its predicted amino acid sequence is 83% identical with human BCDO. Biochemical analysis of baculovirus expressed human BCDO demonstrates the predicted beta,beta-carotene-15,15'-dioxygenase activity. The expression pattern of BCDO suggests that it may provide a local supplement to the retinoids available to photoreceptors, as well as a supplement to the retinoid pools utilized elsewhere in the body. In addition, the finding that many of the enzymes involved in retinoid metabolism are mutated in retinal degenerations suggests that BCDO may also be a candidate gene for retinal degenerative disease.

Two histidine residues are essential for catalysis by lecithin retinol acyl transferase

Lecithin retinol acyl transferase (LRAT) is a novel membrane bound enzyme that catalyzes the formation of retinyl esters from vitamin A and lecithin. The enzyme is both essential for vision and for the general mobilization of vitamin A. The sequence of LRAT defines it as a novel enzyme unrelated to any other protein of known function. LRAT possesses a catalytically essential active site cysteine residue. The enzyme also contains six histidine residues. It is shown here that two of these residues (H57 and H163) are essential for catalysis. A mechanistic hypothesis is presented to account for these observations.

IRBP enhances removal of 11- cis -retinaldehyde from isolated RPE membranes

Interphotoreceptor retinoid-binding protein (IRBP) greatly enhances the conversion of all- trans -retinol to 11- cis -retinal by the retinal pigment epithelium (RPE) and facilitates 11- cis -retinal release from the RPE. However, the mechanisms by which IRBP exerts these effects are not clear. Using a model system of purified bovine IRBP and isolated bovine RPE membranes, we investigated the possibility that IRBP may favor the delivery of all- trans -retinol to, or the release of 11- cis -retinal from, RPE membranes. As the interphotoreceptor space contains serum retinol-binding protein (RBP) and serum albumin in addition to IRBP, we similarly examined the exchange of retinoids between RPE membranes and human RBP or bovine serum albumin (BSA). Isolated RPE membranes were loaded with radioactive 11- cis -retinal and incubated with solutions of IRBP, RBP, BSA or with buffer alone. Membranes (pellet) and retinoid-binding protein or buffer (supernatant) were separated by centrifugation and analysed for radioactive 11- cis -retinal. Membranes incubated with buffer alone released only 4-5% of their 11- cis -retinal, while 25 microm IRBP removed 18-35%. More retinal was released as the membrane concentration was reduced. In contrast, RBP and BSA removed little retinal, even though both proteins are capable of binding this retinoid. Similar results were obtained with bovine liver membranes, consistent with the idea that the effects of IRBP do not depend on an RPE surface receptor for IRBP. IRBP was also markedly superior to RBP and BSA in removing all- trans -retinol from RPE membranes. In addition, IRBP efficiently delivered bound all- trans -retinol to membranes; however, in contrast to their differential removal of retinoids, all three binding proteins delivered comparable amounts of retinol to membranes. (This result supports the practice of using BSA as a retinoid carrier in in vitro experimental systems). We conclude that, whereas IRBP shares with other retinoid-binding proteins the ability to deliver retinol to membranes, IRBP is unique in its capacity to remove 11- cis -retinal from membranes. This may be the feature of IRBP that drives the vitamin A cycle to efficiently produce 11- cis -retinal.

Maintenance of retinoid metabolism in human retinal pigment epithelium cell culture

If transplantation of cultured retinal pigment epithelium (RPE) or iris pigment epithelium (IPE) is to be successful in the treatment of ocular disease, it is imperative to demonstrate that these cells can perform all of their necessary metabolic functions. Unfortunately, a critical function of the RPE, retinoid metabolism, is often lost rapidly in culture. We have examined whether or not nonspecific proteolytic enzymes commonly used in cell isolation and serial passaging may be responsible for this loss of function, and we have investigated novel isolation and passaging techniques which can alleviate this loss of retinoid metabolism.RPE cells were obtained from human donor eyes by enzymatic and nonenzymatic methods. Cells were cultured either on control tissue culture inserts or on inserts coated with a layer of thermally responsive poly(N -isopropylacrylamide-co-cinnamoylcarbamidemethylstyrene). Upon confluence, cells were detached either by trypsinization or by lowering dish temperature. Retinoid metabolism of cells was assessed after isolation and culture by incubating membrane fractions with3H-all- trans -retinol. Retinoid metabolism was also measured in freshly isolated IPE, corneal endothelium (CE), an RPE cell line (D407), and two hepatocyte cell lines (Hepa 6 and HepG2). Membrane fractions from cells isolated nonenzymatically or using collagenase/hyaluronidase formed 11- cis -retinol, retinal isomers and retinyl esters. Retinoid metabolism of RPE cells freshly isolated by trypsinization showed no 11- cis -retinal and little 11- cis -retinol formation. Nondamaged cells cultured on thermally responsive surfaces detached in sheets upon temperature change. They showed metabolism similar to that of cells freshly isolated by nonenzymatic means. After trypsinization, confluent cultures dissociated into individual cells, but these cells showed poor retinoid metabolism, including no detectable retinyl esters or 11- cis -retinoid isomers. IPE, CE and Hepa 6 did not show any retinoid metabolism. D407 and HepG2 produced retinals, but not the 11- cis isomer.RPE cells isolated using trypsin lose the ability to form critical intermediates in the visual cycle. Collagenase/hyaluronidase or nonenzymatic cell isolation techniques enable these functions to be maintained. After cell culture, thermally responsive surfaces allow nonenzymatic cell detachment and excellent maintenance of retinoid metabolism.

Preferential release of 11-cis-retinol from retinal pigment epithelial cells in the presence of cellular retinaldehyde-binding protein

In photoreceptor cells of the retina, photoisomerization of 11-cis-retinal to all-trans-retinal triggers phototransduction. Regeneration of 11-cis-retinal proceeds via a complex set of reactions in photoreceptors and in adjacent retinal pigment epithelial cells where all-trans-retinol is isomerized to 11-cis-retinol. Our results show that isomerization in vitro only occurs in the presence of apo-cellular retinaldehyde-binding protein. This retinoid-binding protein may drive the reaction by mass action, overcoming the thermodynamically unfavorable isomerization. Furthermore, this 11-cis-retinol/11-cis-retinal-specific binding protein potently stimulates hydrolysis of endogenous 11-cis-retinyl esters but has no effect on hydrolysis of all-trans-retinyl esters. Apo-cellular retinaldehyde-binding protein probably exerts its effect by trapping the 11-cis-retinol product. When retinoid-depleted retinal pigment epithelial microsomes were preincubated with different amounts of all-trans-retinol to form all-trans-retinyl esters and then [3H]all-trans-retinol was added, as predicted, the specific radioactivity of [3H]all-trans-retinyl esters increased during subsequent reaction. However, the specific radioactivity of newly formed 11-cis-retinol stayed constant during the course of the reaction, and it was largely unaffected by expansion of the all-trans-retinyl ester pool during the preincubation. The absence of dilution establishes that most of the ester pool does not participate in isomerization, which in turn suggests that a retinoid intermediate other than all-trans-retinyl ester is on the isomerization reaction pathway.

The endogenous chromophore of retinal G protein-coupled receptor opsin from the pigment epithelium

The recent identification of nonvisual opsins has revealed an expanding family of vertebrate opsin genes. The retinal pigment epithelium (RPE) and Müller cells contain a blue and UV light-absorbing opsin, the RPE retinal G protein-coupled receptor (RGR, or RGR opsin). The spectral properties of RGR purified from bovine RPE suggest that RGR is conjugated in vivo to a retinal chromophore through a covalent Schiff base bond. In this study, the isomeric structure of the endogenous chromophore of RGR was identified by the hydroxylamine derivatization method. The retinaloximes derived from RGR in the dark consisted predominantly of the all-trans isomer. Irradiation of RGR with 470-nm monochromatic or near-UV light resulted in stereospecific isomerization of the bound all-trans-retinal to an 11-cis configuration. The stereospecificity of photoisomerization of the all-trans-retinal chromophore of RGR was lost by denaturation of the protein in SDS. Under the in vitro conditions, the photosensitivity of RGR is at least 34% that of bovine rhodopsin. These results provide evidence that RGR is bound in vivo primarily to all-trans-retinal and is capable of operating as a stereospecific photoisomerase that generates 11-cis-retinal in the pigment epithelium.

Molecular and biochemical characterization of lecithin retinol acyltransferase

The enzyme responsible for conversion of all-trans-retinol into retinyl esters, the lecithin retinol acyltransferase (LRAT) has been characterized at the molecular level. The cDNA coding for this protein was cloned and its amino acid sequence deduced. LRAT is composed of a polypeptide of 230 amino acid residues with a calculated mass of 25.3 kDa. Tissue distribution analysis by Northern blot showed expression of a 5.0-kilobase transcript in the human retinal pigment epithelium as well as in other tissues that are known for their high LRAT activity and vitamin A processing. Affinity labeling experiments using specific compounds with high affinity for LRAT and monospecific polyclonal antibodies raised in rabbits against two peptide sequences for LRAT confirmed the molecular mass of LRAT as a 25-kDa protein. High performance liquid chromatography analysis of the reaction product formed by HEK-293 cells transfected with LRAT cDNA confirmed the ability of the transfected cells to convert [3H]all-trans-retinol into authentic [3H]all-trans-retinyl palmitate as chemically determined.

Lack of effect of RPE65 removal on the enzymatic processing of all-trans-retinol into 11-cis-retinol in vitro

RPE65 is a major membrane associated protein found in the vertebrate retinal pigment epithelium (RPE). Various studies have shown this protein to be essential for visual function, possibly at the level of the processing of retinoids. The pigment epithelium is the anatomical site in which the visual chromophore 11-cis retinal is generated. The two critical RPE enzymes in the isomerization pathway are lecithin retinol acyl transferase (LRAT) and isomerohydrolase, which processes all-trans-retinyl esters into 11-cis-retinol. Both enzymes are membrane bound. It is shown here that RPE65 can be largely extracted (90-95%) from RPE membranes by 1 M KCl by itself, or with added detergent CHAPS. The almost quantitative extraction of RPE65 from RPE membranes has little or no effect on in vitro LRAT and isomerohydrolase activities in quantitative enzymatic assays using RPE membranes, suggesting that RPE65 may not have an important role to play in the enzymatic processing of all-trans-retinol into 11-cis-retinol in vitro.

Formation and storage of 11-cis retinol in the eyes of lobster (Homarus) and crayfish (Procambarus)

Modes of storage and mechanisms of formation of 11-cis retinoids in the eyes of animals vary widely among the major phyla. We here describe evidence from two species of macruran decapod crustacea that point to different processes from those known in insects, the other group of arthropods for which there is extensive data. The eyes of the lobster (Homarus) contain about 300 pmol of retinal, somewhat less free retinol, and variable amounts (up to 1000+ pmol) of two retinyl esters, over 90% of which contain retinol in the 11-cis configuration. The major ester contains the long chain, polyunsaturated fatty acid docosahexaenoate (C22:6), but retinyl oleate (C18:1) is also present. Crayfish (Procambarus) contain the same retinyl esters, although in much smaller amounts. Homogenates of the eyes of both species are capable of isomerizing all-trans retinyl docosahexaenoate to the 11-cis configuration without using the energy of light. Crude fractionation of homogenates shows isomerase activity associated with membranes. The reaction mechanism has not been explored in detail, but on the basis of present evidence it may be similar to that found in vertebrate pigment epithelium. It is clearly different from the light-dependent processes known in insects (Hymenoptera and Diptera) and cephalopod mollusks, where isomerization takes place at the level of the aldehyde and 11-cis retinyl esters are not present as major storage reserves.

The retinoids of seven species of mantis shrimp

Eyes of stomatopod crustaceans, or mantis shrimps, contain the greatest diversity of visual pigments yet described in any species, with as many as ten or more spectral classes present in a single retina. In this study, the eyes of seven species of mantis shrimp from three superfamilies of stomatopods were examined for their content of retinoids. Only retinal and retinol were found; neither hydroxyretinoids nor dehydroretinoids were detected. The principal isomers were 11-cis and all-trans. The eyes of most of these species contain stores of 11-cis retinol, principally as retinyl esters, and in amounts in excess of retinal. Squilla empusa is particularly noteworthy, with over 5000 pmoles of retinol per eye.

Retinol esterification in bovine retinal pigment epithelium: reversibility of lecithin:retinol acyltransferase

Esterification of all-trans-retinol is a key reaction of the vertebrate visual cycle, since it produces an insoluble, relatively non-toxic, form of the vitamin for storage and supplies substrate for the isomerization reaction. CoA-dependent and -independent pathways have been described for retinol esterification in retinal pigment epithelium (RPE). The CoA-independent reaction, catalysed by lecithin:retinol acyltransferase (LRAT) was examined in more detail in this study. Addition of retinol to RPE microsomes results in a burst of retinyl ester synthesis, followed by a rapid apparent cessation of the reaction. However, [3H]retinol, added when retinyl ester synthesis has apparently ceased, is rapidly incorporated into retinyl ester without a net increase in the amount of ester. The specific radioactivities of [3H]retinol and [3H]retinyl ester reach the same value. [14C]Palmitate from palmitoyl-CoA is incorporated into preexisting retinyl ester in the absence of net ester synthesis, too. These exchange reactions suggest that the reaction has reached equilibrium at the plateau of the progress curve and that only the accumulation of retinyl ester, and not its synthesis, has stopped during this phase of the reaction. Studies with geometrical isomers of retinol revealed that the rate of exchange of all-trans-retinol with all-trans-retinyl esters was about 6 times more rapid than exchange of 11-cis-retinol with 11-cis-retinyl ester. This is the first demonstration of the reversibility of LRAT and the first example of stereospecificity of retinyl ester synthesis in the visual system. Reversal of the LRAT reaction could contribute to the mobilization of 11-cis-retinol from 11-cis-retinyl ester pools.

Affinity labeling of lecithin retinol acyltransferase

Lecithin retinol acyltransferase (LRAT) transfers acyl groups regiospecifically from the sn-1 position of lecithins to all-trans-retinol (vitamin A) and similar retinoids. LRAT is essential for the biosynthesis of 11-cis-retinal, the visual pigment chromophore. LRAT is also required for the general dietary mobilization of vitamin A. The enzyme is membrane-bound and has been solubilized and partially, but not completely, purified. It is demonstrated here that all-trans-retinyl alpha-bromoacetate (RBA) is a potent irreversible affinity labeling agent of LRAT. The measured KI = 12.1 microM and the pseudo-first-order rate constant for inhibition is kinh = 8.2 x 10(-4) s-1. The specificity of the inhibition process is further evidenced by the observation that alpha-bromoacetate derivatives of hydrophobic alcohols which are not substrates for LRAT, such as cholesterol and beta-ionol, are not inhibitors of the enzyme. Labeling of the partially purified enzyme with 3H-RBA showed a single radiolabeled band of molecular weight approximately 25,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis.

Kinetic mechanism of lecithin retinol acyl transferase

Lecithin retinol acyl transferase transfers acyl groups regiospecifically from the 1-position of lecithins to all-trans-retinol (vitamin A) and similar retinoids. LRAT is essential for the biosynthesis of 11-cis-retinal, the visual pigment chromophore, and is also required for the general dietary mobilization of vitamin A. The kinetic mechanism of this enzyme is described here, KM and Vmax values were determined for the substrates dipalmitoylphosphatidylcholine (DPPC) [1.38 microM and 0.17 microM/(min-mg), respectively] and for all-trans-retinol [0.243 microM and 0.199 microM/(min-mg), respectively]. In order to distinguish between a ping-pong bi-bi mechanism and a rapid equilibrium random or ordered bi-bi mechanism, the velocity of product formation as a function of one of the substrates at different fixed concentrations of the other substrate was measured. The parallel lines generated are entirely consistent with a ping-pong bi-bi mechanism in which DPPC first binds to LRAT and acylates it and rule out both simple random binding and ordered kinetic mechanisms. Further evidence for a ping-pong bi-bi mechanism comes from partial exchange reaction studies which show that LRAT can catalyze acyl group interchange between two different lecithin derivatives. Finally, the ping-pong reaction was established as being ordered, using the potent and reversible dead-end inhibitor 13-desmethyl-13,14-dihydro-all-trans-retinyl trifluoroacetate. This compound proved to be competitive with respect to DPPC, with a KI = 11.4 microM, and uncompetitive with respect to all-trans-retinol.

Molecular mechanisms in visual pigment regeneration

The photochemical bleaching of vertebrate rhodopsin results in the cis to trans isomerization of the 11-cis-retinal protonated Schiff base. Hydrolysis of the Schiff base leads to the formation of opsin and all-trans-retinal. In order for vision to proceed, the enzymatic trans to cis isomerization of a retinoid must occur. Since retinoids exist as alcohols, aldehydes, or esters in the eye, there are potentially nine different routes for isomerization. Moreover, 11-cis-retinoids are approximately 4 kcal/mol higher in energy than their all-trans isomers. Thus, not only must the isomerization route be defined, but an energy source must be identified to power this process. It was discovered that the energy is provided for in a minimally two-step process involving membrane phospholipids as the energy source. First, all-trans-retinol (vitamin A) is esterified in the retinal pigment epithelium by lecithin retinol acyl transferase to produce an all-trans-retinyl ester. Second, this ester is directly transformed into 11-cis-retinol by an isomerohydrolase enzyme, in a process that couples the negative free energy of hydrolysis of the acyl ester to the formation of the strained 11-cis-retinoid.

Localization of retinal photoisomerase in the compound eye of the honeybee

The distribution of honeybee retinal photoisomerase, a soluble light-requiring enzyme that stereospecifically forms 11-cis retinal, was investigated by immunoelectron microscopy and by HPLC. Immunolocalization with polyclonal antibodies shows that the highest concentration of retinal photoisomerase is located in the proximal portion of the primary pigment cells in large aggregates (approximately 2 microns diameter). Photoisomerase is also located in the peripheral portion of the photoreceptor cells, laterally displaced from the rhabdom, but in much lower concentration. Because of the larger volume of the photoreceptor cells, about half of the total immunoreactivity is associated with the primary pigment cells. Dissection of the eye with the subsequent use of HPLC to assay for photoisomerase activity showed that most of the photoisomerase activity is associated with tissues near the cornea. The same tissue also supports the reduction of 11-cis retinal to 11-cis retinol. These biochemical findings are consistent with the immunolocalization of retinal photoisomerase to the high-concentration aggregates in the primary pigment cells that surround the crystalline cones. The major synthesis of 11-cis retinol therefore takes place in the primary pigment cells, and the retinoid must be moved into the photoreceptor cells to be available to newly synthesized opsin. The immunoreactivity of the photoreceptor cells appears to reflect the presence of some isomerase without an attached retinoid chromophore.

All-trans to 11-cis retinol isomerization in nuclear membrane fraction from bovine retinal pigment epithelium

Isomerization of all-trans to 11-cis retinol has been studied in a membrane preparation from the nuclear fraction of bovine retinal pigment epithelium. When the nuclear membrane preparation deprived of endogenous retinoids is incubated with 4.5 microM all-trans-retinol, the mean value calculated for the isomerase activity is 1.32 nmol 11-cis retinol formed hr-1 mg protein-1. Simultaneous formation of all-trans and 11-cis retinyl esters is also observed in the nuclear preparation. When assayed under the same experimental condition, RPE 150,000 g post-nuclear sediment shows about 70% of the isomerase activity found in the nuclear membrane fraction. Treatment of the nuclear membrane fraction with 0.5% (w/v) CHAPS produces a 200,000-g supernatant retaining 80% of the total isomerase activity and leads to a modest purification of the enzyme activity. Apparent values for Km and Vmax of the solubilized enzyme are 1.6 microM and 2.5 nmol 11-cis retinol formed h-1 mg protein-1, respectively. Bovine serum albumin and beta-lactoglobulin effectively stimulate the isomerization reaction. The mechanism underlying this activating effect remains unclarified at present. Some hypotheses are discussed.

Membrane phospholipids and the dark side of vision

The key step in the visual pigment regeneration process is an enzyme-catalyzed trans to cis retinoid isomerization reaction. This reaction is of substantial general interest, because it requires the input of metabolic energy. The energy is needed because the 11-cis-retinoid reaction products are approximately 4 kcal/mol higher in energy than their all-trans congeners. In the retinal pigment epithelium a novel enzymatic system has been discovered which is capable of converting all-trans-retinol into all-trans retinyl esters, by means of a lecithin retinol acyl transferase (LRAT), followed by the direct processing of the ester into 11-cis-retinol. In this process the free energy of hydrolysis of a retinyl ester, estimated to be approximately -5 kcal/mol, is coupled to the endothermic (+4 kcal/mol) isomerization reaction, resulting in an overall exothermic process. The overall process is analogous to ATP-dependent group transfer reactions, but here the energy is provided by the membrane phospholipids. This process illustrates a new role for membranes: they can serve as an energy source.

Retinoids and carotenoids in bovine pineal gland

The retinoids and carotenoids in bovine pineal gland were analyzed by high-performance liquid chromatographic methods. The major retinoid observed was all-trans retinol, with an average level of 579 pmol/g wet tissue. In agreement with a previous report, retinal was not detected; however, we could not detect any retinyl ester in bovine pineal gland. Methods similar to those used in studying metabolism of vitamin A in the bovine eye were used to study the esterification and isomerization reactions of retinoids in the pineal gland. Neither esterification nor isomerization reactions occurred. These results suggest that the key retinoids and enzymes involved in visual function are absent in the bovine pineal gland. The major carotenoid, which has not previously been reported in pineal gland, was beta-carotene at an average level of 1830 pmol/g wet tissue.

Uptake and isomerization of all-trans retinol by isolated bovine retinal pigment epithelial cells: further clues to the visual cycle

The site and substrate for all-trans to 11-cis isomerization in the visual cycle have remained obscure for several decades. Only recently studies on a subcellular level have begun to shed some light on these phenomena. We have addressed this system on a cellular level by utilizing intact isolated bovine retinal pigment epithelial cells, maintained during short-term incubation in vitro. Supplementation with labeled all-trans retinol incorporated in a lipid vesicle carrier, in a range of 1-6 nmol per 10(6) cells, resulted in a rapid uptake of retinol. The majority of the internalized retinol was processed prior to mixing with endogenous retinoid pools and most of it was converted into all-trans retinylester. Up to 10% of the incorporated label was isomerized yielding 11-cis retinol, 11-cis retinaldehyde and 11-cis retinylester. The kinetics of the 11-cis retinoid formation indicated that 11-cis retinol is the first isomerization product. The level of 11-cis retinol apparently 'triggered' further processing into other 11-cis retinoids. An updated model with discussion topics is presented for the retinoid pathway relevant to the visual cycle.