Transfer Of Retinol Research Articles

Myeloid bodies in the pigment epithelium of a teleost embryo, the viviparous Poecilia reticulata

Appearance of myeloid bodies (MB) in the retinal pigment epithelium (RPE) precedes photoreceptor outer segment development in Poecilia reticulata embryos reared under a 12 hrs LD cycle, in constant darkness (DD) and constant light (LL). When first formed, MB are predominantly continuous with rough endoplasmic reticulum (RER). The same is observed in the peripheral growth zone of the developed eye, whereas in differentiated parts, MB are continuous with the smooth endoplasmic reticulum (SER). At onset of photomechanical movements, wavy MB predominate in light-adapted LD embryos, are exclusively present in LL and are located in the RPE processes. SER abounds. Straight MB predominate in dark-adapted LD embryos, are exclusively present in DD and contain electrondense material between lamellae. Diurnal appearance of electrondense material may be coupled with transfer of retinol, mediated by various transport proteins.

Characterization, Localization, and Biosynthesis of an Interstitial Retinol‐Binding Glycoprotein in the Human Eye

Human eyes contain an Mr 135K retinol-binding protein that is analogous to interstitial retinol-binding protein ( IRBP ) in the subretinal space of bovine eyes. It is a glycoprotein, because it binds 125I-concanavalin A, 125I-wheat germ agglutinin and 125I-Lens culinaris hemagglutinin. It does not bind Ricinus communis agglutinin I. After desialation, it binds Ricinus communis agglutinin I, but loses its capacity to bind wheat germ agglutinin. These observations, coupled with the known specificities of these lectins, suggest that at least one of the oligosaccharide chains is a sialated , biantennary complex type containing fucose. Both by direct analysis of dissected ocular tissues and by immunocytochemistry it was shown that human interstitial retinol binding protein is an extracellular protein that is confined predominantly to the subretinal space. Monkey retinas incubated in vitro in medium containing [3H]leucine were shown to synthesize and secrete this protein into the medium, a conclusion that was confirmed by immunoprecipitation with an immunoglobulin fraction prepared from rabbit antibovine IRBP serum. Virtually no other labeled proteins were detectable in the medium. It is concluded that interstitial retinol-binding protein meets many of the requirements for a putative transport protein implicated in the transfer of retinol between the pigment epithelium and retina during the visual cycle, and that the neural retina may play an important role in regulating its amount in the subretinal space.

In Vitro transfer of chylomicron retinol and retinyl esters

The redistribution of rat chylomicron retinoids following incubation with fasting- or postheparin human plasma was investigated. With fasting plasma, chylomicron retinol appeared among higher density lipoprotein acceptors and density greater than 1.21 gm/ml plasma proteins; only small amounts of retinyl ester were found therein. With postheparin plasma, retinyl ester-containing chylomicron remnants with densities spanning the low- and high density lipoprotein distributions were generated; appreciable quantities of retinyl esters appeared among rho greater than 1.019 lipoproteins only in the presence of postheparin plasma. These observations are consistent with the conservation of retinyl esters, but not retinol, among chylomicrons and their catabolic products.

In vivo uptake of chylomicron [3H]retinyl ester by rat liver: evidence for retinol transfer from parenchymal to nonparenchymal cells.

We have studied hepatic uptake of chylomicron retinyl ester. Chylomicrons were obtained from intestinal lymph of rats that were given retinol in groundnut oil by intraduodenal injection. When lymph was injected intravenously into normal rats, the radioactivity was cleared from blood with t1/2 approximately equal to 10 min. Retinyl ester was taken up initially by the liver, which, after 30 min, contained 80-90% of the radioactivity injected. Initially, most of the radioactivity was in hepatocytes, but after 30 min it disappeared from these cells and reappeared in nonparenchymal liver cells. After 2 hr these cells contained more radioactivity than the hepatocytes. When lymph was injected into vitamin A-deficient rats or rats given vitamin A in the form of retinoic acid, the plasma clearance and initial hepatic uptake of radioactivity were similar to that found in control animals. However, the nonparenchymal cells in these animals did not accumulate radioactivity. The current data suggest that vitamin A (in chylomicron remnants) is taken up initially by hepatocytes and then is released from these cells and delivered mainly to nonparenchymal liver cells in normal animals. In vitamin A-deficient rats, the vitamin is transferred from the hepatocytes to extrahepatic tissues.

Retinol transfer from rat liver cytosol retinyl ester lipoprotein complex to serum retinol-binding protein

Cytosol retinyl ester lipoprotein complex from rat liver was capable of transferring its unesterified retinol component to serum aporetinol-binding protein. In the presence of serum albumin and aporetinol-binding protein, 68% of retinyl ester was hydrolyzed and up to 30% of unesterified retinol was transferred from cytosol retinyl ester lipoprotein complex to serum aporetinol-binding protein in 24 h at 30 °C. The reconstituted retinol-retinol-binding protein complex showed biochemical and biophysical properties similar to native retinol-retinol-binding protein. Both native and reconstituted retinol-retinol-binding proteins had identical uv, CD, and fluorescence spectra as well as binding affinity to prealbumin. Treatment of cytosol retinyl ester lipoprotein with sulfhydryl reagent, with 1 n NaCl, or with diisopropyl fluorophosphate (0.14 m m) abolished the hydrolysis of retinyl ester; however, the activity of retinol transfer from cytosol retinyl ester lipoprotein complex to serum retinol-binding protein was still unaffected. The activity of retinol transfer was proportional to the amount of retinol content in the complex and the amount of aporetinol-binding protein. These experiments suggest that the cytosol retinyl ester lipoprotein complex serves three major functions: (i) as a storage form of retinyl ester and retinol; (ii) as an enzyme for hydrolyzing its own retinyl ester ligand; and (iii) as a medium for transfer of unesterified retinol to serum retinol-binding protein.

Retinol uptake by isolated retinal pigment epithelium plasma membranes

The uptake of retinol has been investigated by incubating suspensions of isolated bovine retinal pigment epithelium plasma membranes in the presence of [ 3 H]retinol-retinol-binding protein complex. After SDS gel electrophoresis of the labelled membranes, [ 3 H]retinol is found to be associated with a protein band of molecular weight 16000. The binding process is saturable, specific and shows a linear dependence on the membrane concentration in the incubation mixture. A dissociation constant of the order of 10 −9 has been calculated for the retinol-receptor interaction. These observations confirm previous data obtained with isolated pigment epithelium cells and suggest the existence of an intermediate step, at the membrane level, in the transfer of retinol from its plasma carrier to the cell cytoplasm.

Hereditary retinal dystrophy in the rat: Rhodopsin, retinol, vitamin A deficiency

Measurements were performed on young hooded (B) and albino (Ba) rats with inherited visual cell degeneration of the Bourne-Campbell-Tansley type, in comparison to heterozygotes. The animals were reared and maintained in a dark environment. Rhodopsin of the dark-maintained B and Ba increased between 16 and 35 days to levels twice those of controls. These high levels were maintained up to the age of 100 days. Estimates indicate that no more than 50% of the rhodopsin is contained in surviving visual cells at the age of 25 days, only 25% at 35 days and virtually none at 80 days. The transfer of retinol from bleached rhodopsin to the pigment epithelium (Pe) is increasingly slowed from 17 days on as a function of the accumulation of debris. The conversion of retinal from bleached rhodopsin to retinol also occurs at a lower rate; about 45% of the retinal is converted to retinol at the end of a 10-min period of light exposure in 37-day-old Ba compared to 80% in the controls. The rate of rhodopsin regeneration after 1 hr exposure to strongly bleaching light is decreased after the age of 17 days; it is only about 30% normal for 6 hr in darkness at 37 days. Maintenance on a vitamin A deficient diet accelerates the rate of visual cell death and ERG decline as early as 12 days after withholding vitamin A from weaned B rats.