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PubMed1964

VITAMIN A IN THE VISION OF INSECTS.

Acetone-methanol extracts of honeybees (Apis mellifera) were chromatographed from petroleum ether on columns of aluminum oxide and magnesium oxide:celite. Vitamin A(1) was identified by the Carr-Price (antimony chloride) reaction. These experiments provide the first demonstration of vitamin A in the tissues of an insect. Like retinene, vitamin A is confined to the heads and is not found in either thoraces or abdomens. Dark-adapted bees have very little vitamin A. During light adaptation the vitamin A increases, but at the expense of retinene, which decreases. As much as 0.1 microg of vitamin A/gm of heads has been recovered from light-adapted bees. Two methods are described for demonstrating the enzymic reduction of retinene to vitamin A, using an extract of the heads of honeybees.

Adaptation, Ocular↗
PubMed1978

Visual pigment and photoreceptor sensitivity in the isolated skate retina.

Photoreceptor potentials were recorded extracellularly from the aspartate-treated, isolated retina of the skate (Raja oscellata and R. erinacea), and the effects of externally applied retinal were studied both electrophysiologically and spectrophotometrically. In the absence of applied retinal, strong light adaptation leads to an irreversible depletion of rhodopsin and a sustained elevation of receptor threshold. For example, after the bleaching of 60% of the rhodopsin initially present in dark-adapted receptors, the threshold of the receptor response stabilizes at a level about 3 log units above the dark-adapted value. The application of 11-cis retinal to strongly light-adapted photoreceptors induces both a rapid, substantial lowering of receptor threshold and a shift of the entire intensity-response curve toward greater sensitivity. Exogenous 11-cis retinal also promotes the formation of rhodopsin in bleached photoreceptors with a time-course similar to that of the sensitization measured electrophysiologically. All-trans and 13-cis retinal, when applied to strongly light-adapted receptors, fail to promote either an increase in receptor sensitivity or the formation of significant amounts of light-sensitive pigment within the receptors. However, 9-cis retinal isin. These findings provide strong evidence that the regeneration of visual pigment in the photoreceptors directly regulates the process of photochemical dark adaptation.

Adaptation, Physiological↗
PubMed1978

Formation, conversion, and utilization of isorhodopsin, rhodopsin, and porphyropsin by rod photoreceptors in the Xenopus retina.

The visual pigment content of rod photoreceptors in Xenopus larvae was reduced greater than 90% through a combination of vitamin A-deficient diet and constant light. Thereafter, a dose of either all-trans-retinol or 9-cis-retinal was injected intramuscularly, leading to the formation of a rhodopsin (lambdamax 504 nm) or isorhodopsin (lambdamax 487-493 nm) pigment, respectively. Electrophysiological measurements were made of the threshold and spectral sensitivity of the aspartate-isolated PIII (photoreceptoral) component of the electroretinogram. These measures established that either rhodopsin or isorhodopsin subserved visual transduction with the same efficiency as the 519 nm porphyropsin pigment encountered normally. When animals with rhodopsin or isorhodopsin were kept in darkness or placed on a cyclical lighting regimen for 8 days, retinal densitometry showed that either pigment was being converted to porphyropsin; significantly more porphyropsin was formed as a result of cyclical lighting than after complete darkness.

Animals↗
PubMed1982

Utilization of retinoids in the bullfrog retina.

The capacity to generate 11-cis retinal from retinoids arising naturally in the eye was examined in the retina of the bullfrog, Rana catesbeiana. Retinoids, co-suspended with phosphatidylcholine, were applied topically to the photoreceptor surface of the isolated retina after substantial bleaching of the native visual pigment. The increase in photoreceptor sensitivity associated with the formation of rhodopsin, used as an assay for the appearance of 11-cis retinal in the receptors, was analyzed by extracellular measurement of the photoreceptor potential; in separate experiments using the isolated retina or receptor outer segment preparations, the formation of rhodopsin was measured spectrophotometrically. Treatments with the 11-cis isomers of retinal and retinol induced significant increases in both the rhodopsin content and photic sensitivity of previously bleached receptors. The all-trans isomers of retinyl palmitate, retinol, and retinal, as well as the 11-cis isomer of retinyl palmitate, were inactive by both the electrophysiological and spectrophotometric criteria for the generation of rhodopsin. Treatment with any one of the "inactive" retinoids did not abolish the capacity of subsequently applied 11-cis retinal or 11-cis retinol to promote the formation of rhodopsin. The data are discussed in relation to the interconversions of retinoids ("visual cycle of vitamin A") thought to mediate the regeneration of rhodopsin in vivo after extensive bleaching.

Animals↗
PubMed1984

Metaretinochrome in membranes as an effective donor of 11-cis retinal for the synthesis of squid rhodopsin.

Aporetinochrome, which is a protein moiety of retinochrome without chromophore retinal, is found in the membrane containing retinochrome. All of the prosthetic retinal of retinochrome in membranes, which is all-trans retinal, is bound to the chromophoric site on the protein moiety, with protonated Schiff bases showing an absorption band with the maximum at 495 nm. On exposure to light, retinochrome is converted to metaretinochrome at room temperature. The prosthetic retinals of metaretinochrome in membranes, which are 11-cis retinals, are in two states: retinals bound to the chromophoric site with protonated Schiff bases, and the free retinals, which are separated from the protein moiety. These states are suggested from the following observations. (a) The ratio of the absorbance at 470 nm of metaretinochrome to that at 495 nm of the parental retinochrome differs because of differences in samples and is higher in the purer preparations. (b) The difference spectrum of absorption of metaretinochrome caused by alkalinization shows two minimum peaks at approximately 420 and 470 nm. (c) The rate of bleaching of metaretinochrome in membranes with dilute NH2OH is much faster than that of retinochrome, and the absorption band in the near-UV region is more susceptible to NH2OH than the visible absorption band. The state of the prosthetic retinals in metaretinochrome was confirmed directly by the reaction of metaretinochrome in membranes with NaBH4. After treatment with NaBH4, the sodium dodecyl sulfate-polyacrylamide gel electrophoretic pattern shows two fluorescent bands: one at the position that corresponds to the retinochrome protein (mol wt 27,000 +/- 2,000), and another at the front of migration, where no band of protein is observed. Retinoids extracted from the NaBH4-treated metaretinochrome in membranes and analyzed with high-pressure liquid chromatography show a main peak of 11-cis retinol. The results of this and earlier (Seki et al., 1982) papers are summarized, and it is strongly suggested that metaretinochrome in the squid retina may play the role of 11-cis retinal donor for opsin and contribute to the synthesis of the squid rhodopsin.

Absorption↗
PubMed1988

Adaptation of a deep-sea cephalopod to the photic environment. Evidence for three visual pigments.

Watasenia scintillans, a bioluminescent deep-sea squid, has a specially developed eye with a large open pupil and three visual pigments. Photoreceptor cells (outer segment: 476 micron; inner segment: 99 micron) were long in the small area of the ventral retina receiving downwelling light, whereas they were short (outer segment: 207 micron; inner segment: 44 micron) in the other regions of the retina. The short photoreceptor cells contained the visual pigment with retinal (lambda max approximately 484 nm), probably for the purpose of adapting to their environmental light. The outer segment of the long photoreceptor cells consisted of two strata, a pinkish proximal area and a yellow distal area. The visual pigment with 3-dehydroretinal (lambda max approximately 500 nm) was located in the pinkish proximal area, giving high sensitivity at longer wavelengths. A newly found pigment (lambda max approximately 471 nm) was in the yellow distal area. The small area of the ventral retina containing two visual pigments is thought to have a high and broad spectral sensitivity, which is useful for distinguishing the bioluminescence of squids of the same species in their environmental downwelling light. These findings were obtained by partial bleaching of the extracted pigment from various areas of the retina and by high-performance liquid chromatographic analysis of the chromophore, complemented by microscopic observations.

Adaptation, Physiological↗
PubMed1988

Dependency on light and vitamin A derivatives of the biogenesis of 3-hydroxyretinal and visual pigment in the compound eyes of Drosophila melanogaster.

When the fruitfly, Drosophila melanogaster, was reared on media deficient in carotenoids and retinoids, the level of 3-hydroxyretinal (the chromophore of fly rhodopsin) in the retina decreased to less than 1% compared with normal flies. The level of 3-hydroxyretinal increased markedly in flies that were given a diet supplemented with retinoids or carotenoids. The retinas of flies fed on all-trans retinoids and maintained in the dark predominantly contained the all-trans form of 3-hydroxyretinal, and showed no increase in the level of either the 11-cis isomer or the visual pigment. Subsequent illumination of the flies converted substantial amounts of all-trans 3-hydroxyretinal to its 11-cis isomer. The action spectrum of the conversion by illumination showed the optimum wavelength to be approximately 420 nm, which is significantly greater than the absorption maximum of free, all-trans 3-hydroxyretinal. Flies that were fed on carotenoids showed a rapid increase of the levels of 11-cis 3-hydroxyretinal and of visual pigment in the absence of light.

Animals↗
PubMed1988

Fenretinide does not block visual pigment formation in the rat.

Abnormal retinal function has been reported in patients receiving the synthetic retinoid fenretinide. In this study, the vitamin A deprived rat was used as a model to test the possibility that fenretinide is interfering with the vitamin A transport system and/or blocking the normal uptake of 11-cis retinal into the visual pigment. Assessing retinal function by electroretinogram measurements and visual pigment formation by competition studies, our data demonstrate normal uptake of vitamin A and normal visual pigment formation in the rat after treatment with the drug.

Animals↗
PubMed1975

Function of the fat-soluble vitamins.

Much has been learned about the function of the fat-soluble vitamins in recent years. Their mechanisms are divergent ranging from a role in transcription of DNA or differentiation to a direct enzymatic activation of the gamma-carboxylation of glutamate residues of a preexisting protein. Thus the only resemblance so far among these physiologically active agents is their lipid solubility. However, in general the fat-soluble vitamins play important roles in specialized functions carried out by highly differentiated organisms.

Animals↗
PubMed1988

Uptake and metabolism of beta-carotene and retinal by C3H/10T1/2 cells.

Both beta-carotene (beta-C), a vitamin A precursor, and vitamin A itself have been shown to reversibly inhibit neoplastic transformation in 10T1/2 cells during the progression phase of carcinogenesis. In order to determine whether the activity of beta-C in these cells may be attributed to conversion to vitamin A or is intrinsic to the carotenoid molecule, the uptake and metabolism of beta-C, and of retinal, the immediate product of dioxygenase-cleavage of beta-C, was studied in 10T1/2 cells. Cellular uptake of 2.6 nmol/10(6) cells occurred 24 h after treatment with 10(-5) M beta-C. Thereafter, cell levels remained relatively stable between 1 and 2 nmol/10(6) cells over the 1-week treatment period. Upon removal of beta-C from the medium, cellular levels decreased by approximately 80% in 2 weeks, then stabilized. Retinal was rapidly and quantitatively converted to retinol by 10T1/2 cells, suggesting that the inhibitory action of retinal on neoplastic transformation in these cells is due to its conversion to retinol, and that any enzymatic conversion of beta-C to retinal by these cells would be expected to result in retinol as the end product. Using [14C]beta-C, we found no evidence for formation of [14C]retinol, [14C]retinal or [14C]retinoic acid using sensitive HPLC. We therefore conclude that beta-C has intrinsic chemopreventive activity in 10T1/2 cells, perhaps due to its anti-oxidant properties.

Animals↗
PubMed2006

Beyond retinotopic mapping: the spatial representation of objects in the human lateral occipital complex.

The spatial representation in the human ventral object-related areas (i.e., the lateral occipital complex [LOC]) is currently unknown. It seems plausible, however, that it would diverge from the strict retinotopic mapping (characteristic of V1) to a more invariant coordinate frame, thereby allowing for reliable object recognition in the face of eye, head, or body movement. To study this, we compared the fMRI activation in LOC when object displacement was limited to either the retina or the screen by manipulating eye position and object locations. We found clear adaptation in LOC when the object's screen position was fixed, regardless of the object's retinal position. Furthermore, we found significantly greater activation in LOC in the hemisphere contralateral to the object's screen position, although the visual task was constructed in a way that the objects were present equally often on each of the 2 retinal hemifields. Together, these results indicate that a sizeable fraction of the neurons in LOC may have head-based receptive fields. Such an extraretinal representation may be useful for maintenance of object coherence across saccadic eye movements, which are an integral part of natural vision.

Adult↗
PubMed1994

A new locus for autosomal dominant retinitis pigmentosa on the short arm of chromosome 17.

Retinitis pigmentosa (RP) is a group of genetically and clinically heterogeneous retinopathies, some of which have been shown to result from mutations in two different known retinal genes, rhodopsin (3q) and peripherin-rds (6p). Three additional anonymous loci at 7p, 7q and pericentric 8 have been implicated by linkage studies. There are still, however, a few families in which all known loci have been excluded. In this report we present data indicating a location, on the short arm of chromosome 17, for the autosomal dominant RP (ADRP) locus in a large South African (SA) family of British ancestry. Positive two-point lod scores have been obtained for nine markers (D17S938, Z = 5.43; D17S796, Z = 4.82; D17S849, Z = 3.6; D17S786, Z = 3.55; TP53, Z = 3.55; D17S578, Z = 3.29; D17S960, Z = 3.16; D17S926, Z = 1.51; D17S804, Z = 0.47 all at theta = 0.10 except D17S804 and D17S926, theta = 0.20). These data provide definitive evidence for the localization of an ADRP gene on chromosome 17p. The human recoverin gene has been localized to 17p13.1 and was consequently a prime candidate for ADRP in the family studied. However, mutation screening of the three exons of this gene failed to produce any evidence of recoverin being the gene involved in the pathogenesis of ADRP in this SA family.

Calcium-Binding Proteins↗
PubMed2005

Retinal degeneration associated with RDH12 mutations results from decreased 11-cis retinal synthesis due to disruption of the visual cycle.

Retinoid dehydrogenases/reductases catalyze key oxidation-reduction reactions in the visual cycle that converts vitamin A to 11-cis retinal, the chromophore of the rod and cone photoreceptors. It has recently been shown that mutations in RDH12, encoding a retinol dehydrogenase, result in severe and early-onset autosomal recessive retinal dystrophy (arRD). In a cohort of 1011 individuals diagnosed with arRD, we have now identified 20 different disease-associated RDH12 mutations, of which 16 are novel, in a total of 22 individuals (2.2%). Haplotype analysis suggested a founder mutation for each of the three common mutations: p.L99I, p.T155I and c.806_810delCCCTG. Patients typically presented with early disease that affected the function of both rods and cones and progressed to legal blindness in early adulthood. Eleven of the missense variants identified in our study exhibited profound loss of catalytic activity when expressed in transiently transfected COS-7 cells and assayed for ability to convert all-trans retinal to all-trans retinol. Loss-of-function appeared to result from decreased protein stability, as expression levels were significantly reduced. For the p.T49M variant, differing activity profiles were associated with each of the alleles of the common p.R161Q RDH12 polymorphism, suggesting that genetic background may act as a modifier of mutation effect. A locus (LCA3) for Leber congenital amaurosis, a severe, early-onset form of arRD, maps close to RDH12 on chromosome 14q24. Haplotype analysis in the family in which LCA3 was mapped excluded RDH12 as the LCA3 gene and thus suggests the presence of a novel arRD gene in this region.

Alcohol Oxidoreductases↗
PubMed1975

Effects of zinc deficiency on the oxidation of retinol and ethanol in rats.

The zinc metalloenzyme alcohol dehydrogenase was assayed in subcellular fractions of liver and retina from zinc-deficient and control rats using retinol and ethanol as substrates. When the zinc concentrations of these tissues were reduced because of feeding inadequate dietary zinc, alcohol dehydrogenase was significantly lowered, and the retinol-retinal as well as the ethanol-ethyladehyde conversion was significantly reduced. While in older rats no changes occurred in liver zinc and alcohol dehydrogenase, the retina was more sensitive to the lack of dietary zinc than the liver. Young rats nursed by zinc-deficient dams showed the most severe changes in both tissues. The data suggest that zinc deficiency affects the utilization of vitamin A as well as the catabolism of ethanol.

Alcohol Oxidoreductases↗
PubMed1977

Influence of zinc deficiency on retinal reductase and oxidase activities in rat liver and testes.

The influence of zinc deficiency on vitamin A metabolism in rats was investigated by assessing two specific enzymes involved in its metabolism viz retinal reductase and retinal oxidase in the liver as well as in the testes. The activity of retinal reductase in the liver was not altered in zinc deficiency. Retinal oxidase activity on the other hand, was increased approximately 1.5 fold over the pair-fed controls. In contrast, both retinal reductase and oxidase in the testes were decreased during zinc deficiency. The effects of zinic deficiency on vitamin A metabolism in the liver could be partly attributed to the secondary effect of reduced food intake and growth. However, the effects seen in the testes on the metabolic enzymes of vitamin A appear to be due to zinc deficiency per se. The liver concentration of vitamin A (microgram/g) as well as total vitamin A (microgram/liver) were higher in the zinc-deficient rats when compared to the zinc-sufficient rats although not significantly different from the pair-fed controls. In aggreement with previous reports, the plasma vitamin A in zinc-deficient rats were found to be markedly lowered compared to zinc sufficient ad libitum controls.

Alcohol Oxidoreductases↗
PubMed1978

Retinol-binding proteins in human testis cytosol.

In the cytosol fraction of human testes a specific binding protein for retinol has been isolated by sephadex column chromatography and by gradient centrifugation. The protein has a sedimentation coefficient of 2S, and its molecular weight is estimated by gel filtration to be approximately 16,000 daltons. Competitive binding studies with labeled retinol and 200-fold molar excess of unlabeled retinal, retinoic acid, retinyl palmitate and retinyl acetate indicate that the isolated protein is specific for retinol. Retinyl esters bind to the protein as effectively as retinol; however retinal binds to a lesser degree than retinol esters. In addition, a lower molecular weight protein of approximately 4,500 daltons was found in the cytosol that has comparatively lower affinity for retinol binding than the 16,000-dalton protein.

Binding, Competitive↗