HINDERED CIS ISOMERS OF VITAMIN A AND RETINENE: THE STRUCTURE OF THE NEO-b ISOMER

A short story on how chromophore is hydrolyzed from rhodopsin for recycling

The photocycle of visual opsins is essential to maintain the light sensitivity of the retina. The early physical observations of the rhodopsin photocycle by Böll and Kühne in the 1870s inspired over a century's worth of investigations on rhodopsin biochemistry. A single photon isomerizes the Schiff-base linked 11-cis-retinylidene chromophore of rhodopsin, converting it to the all-trans agonist to elicit phototransduction through photoactivated rhodopsin (Rho*). Schiff base hydrolysis of the agonist is a key step in the photocycle, not only diminishing ongoing phototransduction but also allowing for entry and binding of fresh 11-cis chromophore to regenerate the rhodopsin pigment and maintain light sensitivity. Many challenges have been encountered in measuring the rate of this hydrolysis, but recent advancements have facilitated studies of the hydrolysis within the native membrane environment of rhodopsin. These techniques can now be applied to study hydrolysis of agonist in other opsin proteins that mediate phototransduction or chromophore turnover. In this review, we discuss the progress that has been made in characterizing the rhodopsin photocycle and the journey to characterize the hydrolysis of its all-trans-retinylidene agonist.

Rapid RGR-dependent visual pigment recycling is mediated by the RPE and specialized Müller glia

In daylight, demand for visual chromophore (11-cis-retinal) exceeds supply by the classical visual cycle. This shortfall is compensated, in part, by the retinal G-protein-coupled receptor (RGR) photoisomerase, which is expressed in both the retinal pigment epithelium (RPE) and in Müller cells. The relative contributions of these two cellular pools of RGR to the maintenance of photoreceptor light responses are not known. Here, we use a cell-specific gene reactivation approach to elucidate the kinetics of RGR-mediated recovery of photoreceptor responses following light exposure. Electroretinographic measurements in mice with RGR expression limited to either cell type reveal that the RPE and a specialized subset of Müller glia contribute both to scotopic and photopic function. We demonstrate that 11-cis-retinal formed through photoisomerization is rapidly hydrolyzed, consistent with its role in a rapid visual pigment regeneration process. Our study shows that RGR provides a pan-retinal sink for all-trans-retinal released under sustained light conditions and supports rapid chromophore regeneration through the photic visual cycle.

The Gluopsins: Opsins without the Retinal Binding Lysine

Opsins allow us to see. They are G-protein-coupled receptors and bind as ligand retinal, which is bound covalently to a lysine in the seventh transmembrane domain. This makes opsins light-sensitive. The lysine is so conserved that it is used to define a sequence as an opsin and thus phylogenetic opsin reconstructions discard any sequence without it. However, recently, opsins were found that function not only as photoreceptors but also as chemoreceptors. For chemoreception, the lysine is not needed. Therefore, we wondered: Do opsins exists that have lost this lysine during evolution? To find such opsins, we built an automatic pipeline for reconstructing a large-scale opsin phylogeny. The pipeline compiles and aligns sequences from public sources, reconstructs the phylogeny, prunes rogue sequences, and visualizes the resulting tree. Our final opsin phylogeny is the largest to date with 4956 opsins. Among them is a clade of 33 opsins that have the lysine replaced by glutamic acid. Thus, we call them gluopsins. The gluopsins are mainly dragonfly and butterfly opsins, closely related to the RGR-opsins and the retinochromes. Like those, they have a derived NPxxY motif. However, what their particular function is, remains to be seen.

Ferrocenylimine Palladium (II) Complexes: Synthesis, Characterization and Application in Mizoroki-Heck and Suzuki-Miyaura Cross-Coupling Reactions

Carbon-carbon cross-coupling reactions are essential synthetic tools for synthesizing polymers, natural products, agrochemicals, and pharmaceuticals. Therefore, new catalysts that function with greater efficiency and functional group tolerance are being researched. We have prepared new ferrocenylimine monodentate N and P donor ligands and N^N and N^P bidentate chelating ligands (L1 to L4) employed in stabilizing palladium ions for application in Mizoroki-Heck and Suzuki-Miyaura cross-coupling reactions. The ferrocenylimine ligands were successfully synthesized by Schiff base condensation reactions of acetyl ferrocene with hydrazine monohydrate to afford ferrocenyl hydrazone (L1). Ligand L1 was further treated with aldehydes to give ferrocenyl(2-diphenylphosphino)imine (L3) and ferrocenyl(pyridyl)imine (L3), while phosphination of L1 with chlorodiphenylphosphine afforded L2. The ligands were used to prepare new palladium(II) complexes (C1 to C4) by complexation with [PdCl2(MeCN)2]. All the ligands and complexes were fully characterized using standard spectroscopic and analytical techniques, including 1H NMR and 13C NMR spectroscopy, FT-IR spectroscopy, mass spectrometry and elemental analysis. The complexes (C1 to C4) were tested for efficacies in catalyzing Mizoroki-Heck and Suzuki-Miyaura C-C cross-coupling reactions and proved to be suitable catalyst precursors. Ferrocenyl(2-diphenylphosphine)imino and ferrocenyl-methyl hydrazone palladium(II) complexes C2 and C3 showed the best activities at TONs of up to 201. The ferrocenyl palladium(II) (pre)catalysts demonstrated moderate activity in Mizoroki-Heck reactions involving substrates with substituents on the olefin and aryl halide (including 4-Cl, 4-CH3, -CO2Me and -CO2Et). Density Functional Theory was used to study the mechanism of the Mizoroki-Heck cross-coupling reactions and have led to confirmation of the widely accepted catalytic cycle. Catalyst precursors (C1 to C4) also displayed good activity and selectivity in Suzuki-Miyaura cross-coupling reactions, at 0.5 mol% catalyst loading, with good tolerance to functional groups present on the aryl halide and boronic acid substrates (such as 4-Cl, 4-CHO, 4-COOH, 3-NO2, 3,5-dimethoxy and 4-CH3).

Vitamin A: its many roles-from vision and synaptic plasticity to infant mortality

The recognition that a dietary factor is essential to maintain good and sensitive vision as well as overall health goes back over 3,000 years to the ancient Egyptians. With the discovery of the vitamins at the turn of the twentieth century, fat-soluble vitamin A was soon shown to be the essential factor. In the first half of the twentieth century, the role vitamin A plays in vision, as precursor to the light-sensitive visual pigment molecules in the photoreceptors was elegantly worked out, especially by George Wald and his colleagues. Beginning in the 1960s, with the recognition of the active metabolite of vitamin A, its acid form now called retinoic acid, the roles of vitamin A in maintaining overall health of an organism began to be explored, and this research continues to this day. Receptors activated by retinoic acid, the RARs and RXRs have been shown to regulate gene transcription in a surprisingly wide variety of biological processes from early growth and development to the maintenance of epithelial tissues in many organs, the regulation of the immune system, and even the modulation of synaptic function in the brain involved in mechanisms underlying memory and learning. Therapeutic uses for retinoic acid have been developed, including one for a specific form of leukemia. The story is by no means complete and it is likely more surprises await with regard to this remarkable molecule.

Vitamin A and Vision

Visual systems detect light by monitoring the effect of photoisomerization of a chromophore on the release of a neurotransmitter from sensory neurons, known as rod and cone photoreceptor cells in vertebrate retina. In all known visual systems, the chromophore is 11-cis-retinal complexed with a protein, called opsin, and photoisomerization produces all-trans-retinal. In mammals, regeneration of 11-cis-retinal following photoisomerization occurs by a thermally driven isomerization reaction. Additional reactions are required during regeneration to protect cells from the toxicity of aldehyde forms of vitamin A that are essential to the visual process. Photochemical and phototransduction reactions in rods and cones are identical; however, reactions of the rod and cone visual pigment regeneration cycles differ, and perplexingly, rod and cone regeneration cycles appear to use different mechanisms to overcome the energy barrier involved in converting all-trans- to 11-cis-retinoid. Abnormal processing of all-trans-retinal in the rod regeneration cycle leads to retinal degeneration, suggesting that excessive amounts of the retinoid itself or its derivatives are toxic. This line of reasoning led to the development of various approaches to modifying the activity of the rod visual cycle as a possible therapeutic approach to delay or prevent retinal degeneration in inherited retinal diseases and perhaps in the dry form of macular degeneration (geographic atrophy). In spite of great progress in understanding the functioning of rod and cone regeneration cycles at a molecular level, resolution of a number of remaining puzzling issues will offer insight into the amelioration of several blinding retinal diseases.

Chapter 29: historical aspects of the major neurological vitamin deficiency disorders: overview and fat-soluble vitamin A

The vitamine doctrine: Although diseases resulting from vitamin deficiencies have been known for millennia, such disorders were generally attributed to toxic or infectious causes until the "vitamin doctrine" was developed in the early 20th century. In the late-19th century, a physiologically complete diet was believed to require only sufficient proteins, carbohydrates, fats, inorganic salts, and water. From 1880-1912, Lunin, Pekelharing, and Hopkins found that animals fed purified mixtures of known food components failed to grow or even lost weight and died, unless the diet was supplemented with small amounts of milk, suggesting that "accessory food factors" are required in trace amounts for normal growth. By this time, Funk suggested that deficiencies of trace dietary factors, which he labeled "vitamines" on the mistaken notion that they were "vital amines," were responsible for such diseases as beriberi, scurvy, rickets, and pellagra. Vitamin A deficiency eye disease: Night blindness was recognized by the ancient Egyptians and Greeks, and many authorities from Galen onward advocated liver as a curative. Outbreaks of night blindness were linked to nutritional causes in the 18th and 19th centuries by von Bergen, Schwarz, and others. Corneal ulceration was reported in 1817 by Magendie among vitamin A-deficient dogs fed for several weeks on a diet limited to sugar and water, although he erroneously attributed this to a deficiency of dietary nitrogen (i.e. protein). Subsequently, corneal epithelial defects, often in association with night blindness, were recognized in malnourished individuals subsisting on diets now recognizable as deficient in vitamin A by Budd, Livingstone, von Hubbenet, Bitot, Mori, Ishihari, and others. During World War I, Bloch conducted a controlled clinical trial of different diets among malnourished Danish children with night blindness and keratomalacia and concluded that whole milk, butter, and cod-liver oil contain a fat-soluble substance that protects against xerophthalmia. Early retinal photochemistry: In the 1870s, Boll found that light causes bleaching of the retinal pigment, and suggested that the outer segments of the rods contain a substance that conveys an impression of light to the brain by a photochemical process. Shortly thereafter, Kühne demonstrated that the bleaching process depends upon light, and was reversible if the retinal pigment epithelium was intact. Kühne proposed an "optochemical hypothesis," a prescient concept of photochemical transduction, attributing vision to a photochemical change in visual purple (rhodopsin) with resulting chemical products stimulating the visual cells and thereby conveying a visual image. Vitamin A: In 1913, Ishihara proposed that a "fatty substance" in blood is necessary for synthesis of both rhodopsin and the surface layer of the cornea, and that night blindness and keratomalacia develop when this substance is deficient. That year McCollum and Davis (and almost simultaneously Mendel and Osborne) discovered a fat-soluble accessory food factor (later called "fat-soluble A") distinct from the water-soluble anti-beriberi factor (later called "fat-soluble B"). By 1922 McCollum and colleagues distinguished two vitamins within the fat-soluble fraction, later named vitamins A and D. In 1925 Fridericia and Holm directly linked vitamin A to night blindness in animal experiments using rats, and in 1929 Holm demonstrated the presence of vitamin A in retinal tissue. In the 1930s, Moore, Karrer, Wald, and others established the provitamin role of beta-carotene. Karrer and colleagues isolated beta-carotene (the main dietary precursor of vitamin A) and retinol (vitamin A), and determined their chemical structures. In 1947, Isler and colleagues completed the full chemical synthesis of vitamin A. Modern retinal photochemistry: Beginning in the 1930s, Wald and colleagues greatly elaborated the photochemistry of vision, with the discovery of the visual cycle of vitamin A, demonstration that rhodopsin is decomposed by light into retinal (the aldehyde form of vitamin A) and a protein (opsin), elaboration of the enzymatic conversions of various elements in the rhodopsin system, and discovery that the rhodopsin system is dependent on a photoisomerization of retinal. In 1942, Hecht and colleagues demonstrated that a single photon could trigger excitation in a rod. In 1965, Wald suggested that a large chemical amplification was necessary for this degree of light sensitivity, likely by a cascade of enzymatic reactions. Later studies elaborated this cascade and found that an intermediary in the photoisomerization of retinal interacts with transducin, a G-protein, to activate phosphodiesterases that control cyclic GMP levels, which in turn modulate the release of neurotransmitter from the rod cell. Public health: Although the availability of vitamin A through food fortification and medicinal supplements virtually eliminated ocular vitamin A deficiency from developed countries by the second half of the 20th century, vitamin A deficiency remains a serious problem in developing countries as indicated by global surveys beginning in the 1960s. Millions of children were shown to be vitamin A deficient, with resultant blindness, increased susceptibility to infection, and increased childhood mortality. Beginning in the 1960s, intervention trials showed that vitamin A deficiency disorders could be prevented in developing countries with periodic vitamin A dosing, and in the 1980s and 1990s, large randomized, double-blind, placebo-controlled clinical trials demonstrated the marked efficacy of vitamin A supplementation in reducing childhood mortality.

George Wald memorial talk

George Wald was born in 1906 in New York City to immigrant parents. An early and voracious reader, he soon developed a wide range of interests and entered New York University as a pre-law student, the first in his family to attend college. Shortly shifting to pre-medicine, he graduated college in biology. For graduate work, he joined the laboratory of Selig Hecht, a pioneer in vision research, at Columbia University. In 1932, four months before Hitler came to power, George went to Berlin to do postdoctoral work in the laboratory of Otto Warburg and there found vitamin A in the retina. This launched his life-long explorations of the molecular basis of vision for which he received the Nobel Prize in Physiology or Medicine in 1967. During the 1960s, George became increasingly involved in anti-war and anti-nuclear activities, writing and travelling widely, including multiple trips to commemorations of the bombings of Hiroshima and Nagasaki sponsored by Japanese colleagues. He considered these activities part of being a biologist, someone concerned with life. In his final years, he turned to questions about consciousness, writing and speaking about 'Life and Mind in the Universe'.

Low-temperature trapping of early photointermediates of alpha-isorhodopsin

Alpha-Isorhodopsin, an artificial visual pigment with a 9-cis-4,5-dehydro-5,6-dihydro(alpha)retinal chromophore, was photolyzed at low temperatures and absorption difference spectra were collected as the sample was warmed. A bathorhodopsin (Batho)-like intermediate absorbing at ca 495 nm was detected below 55 K,a blue-shifted intermediate (BSI)-like intermediate absorbing at ca 453 nm was observed when the temperature was raised to 60 K and a lumirhodopsin (Lumi)-like intermediate absorbing at ca 470 nm was found when the sample was warmed to 115 K. Photointermediates from this pigment were compared to those of native rhodopsin and 5,6-dihydroisorhodopsin. As in native rhodopsin, Batho is the first intermediate detected in alpha-isorhodopsin, though unlike native rhodopsin at low temperatures BSI is observed prior to Lumi formation. Alpha-Isohodopsin behaves similarly to 5,6-dihydroisorhodopsin, with the same early intermediates observed in both artificial visual pigments lacking the C5-C6 double bond. The transition temperature for BSI formation is higher in alpha-isorhodopsin, suggesting an interaction involving the chromophore ring in BSI formation. The transition temperature for Lumi formation is similar for these two pigments as well as for native rhodopsin, suggesting comparable changes in the protein environment in that transition.

Study of the shape of the binding site of bovine opsin using 10-substituted retinal isomers

The 9-cis, 11-cis, 13-cis, and all-trans isomers of 10-fluoro-, 10-chloro-, 10-methyl-, and 10-ethylretinals have been prepared and characterized. Results of their interaction with bovine opsin are reported. The data have been analyzed in terms of the conformational properties of the retinal isomers and their steric compatibility with the binding site as defined by the two-dimensional map disclosed earlier. The need to expand the active zone and previously undetected restrictions in the third dimension are noted.

Photoreceptor processes: some problems and perspectives

Visual photoreceptors from both vertebrates and invertebrates are characterized by extensive elaboration of membrane which contains visual pigment (rhodopsin). Visual pigments in all phyla examined are chemically similar: the chromophore is 11-cis retinaldehyde attached by an aldimine linkage (Schiff base) to a membrane protein, opsin. The effect of light is to isomerize the chromophore to the all-trans configuration. Beyond these fundamental similarities, several specific areas are discussed in which variations and differences appear. (1) Light causes vertebrate visual pigments to bleach, liberating the chromophore. Most invertebrate visual pigments do not bleach in the light, but instead form a thermally stable metarhodopsin, with the chromophore in the all-trans configuration still attached to the opsin. (2) In the disk membranes of vertebrate rod and cone outer segments, the rhodopsin molecules are oriented with their chromophores nearly coplanar with the disks. Within this plane, however, both rotational and translational diffusion are possible. In the microvillar membranes of arthropod and cephalopod rhabdoms, on the other hand, the situation is less clear. There is evidence for some preferential orientation of chromophores that implies restrictions on Brownian rotation. (3) In the outer segments of vertebrate receptors, absorption of light by rhodopsin causes the plasma membrane to hyperpolarize due to a decrease in sodium conductance, possibly mediated by calcium ions. In most invertebrate photoreceptors, light causes a depolarization due to an increase in conductance, principally to sodium ions. A subsequent entry of calcium causes a partial repolarization of the membrane, due to a decrease in sodium conductance. (4) For vertebrate receptors, log threshold is directly proportional to the fraction of rhodopsin bleached (Dowling-Rushton relationship). The proportionality constant varies in different preparations from less than four to more than 30, and the physical basis for the relationship is unknown. For invertebrates, by contrast, the dependence of sensitivity on rhodopsin concentration is much less dramatic and may well depend simply on the probability of quantum catch. (5) In most species, vertebrate and invertebrate, the accumulation of photoproduct probably has no effect on membrane conductance, but several possible exceptions exist. (6) Photoregeneration of rhodopsin from metarhodopsin is likely an important mechanism of recovery in certain arthropods such as diurnal insects, but dark mechanisms of recovery also exist in all phyla. In no single case are they adequately understood.

The vitamin A of a euphausiid crustacean

The vitamin A of the euphausiid crustacean, Meganyctiphanes norvegica, consists almost wholly of the hindered cis isomer, neo-b (11-cis). In this animal vitamin A is concentrated almost entirely in the eyes; and its properties so closely resemble those of pure neo-b vitamin A as not in themselves to indicate that any other isomer is present. However, Fisher et al. (1955 b) have isolated a small fraction from this material which may be neo-c vitamin A (11, 13-dicis). The neo-b isomer was identified by its absolute absorption spectrum, the changes of absorption spectrum on isomerization, oxidation to neo-b retinene, and synthesis from the latter of rhodopsin. This identification is also in good accord with new, revised bioassays of Meganyctiphanes vitamin A by Plack et al. (1956).

The vitamin A of the lobster

In many crustacea, including the lobster, the bulk of the vitamin A of the whole animal is concentrated in the eyes. Recently Fisher, Kon, and Thompson found that vitamin A extracted from the eyes of euphausiid crustacea has only about one half the biological potency of the same amount of the all-trans acetate or fish liver vitamin A. In the present experiments the vitamin A of the lobster eye is found to consist almost entirely of the hindered cis isomer, neo-b, the precursor in the vertebrate retina of the visual pigments rhodopsin and iodopsin. This isomer is known to have a low biological potency in the rat, only about one quarter that of all-trans vitamin A. In the lobster eye it is virtually all extractable with petroleum ether, about 30 per cent in the form of free alcohol, about 70 per cent in the form of esters. It was identified by its absorption spectrum, as derived from measurements on crude extracts, and measured directly in purified preparations; the changes in absorption which accompany isomerization; oxidation to the corresponding retinene; and synthesis from the latter of rhodopsin. The examination of an extract of euphausiid eyes, provided by Dr. Kon, also revealed the presence of neo-b vitamin A virtually alone. This may be the characteristic condition in the eyes of Eucarid crustacea. It is peculiar in that the neo-b isomer, being a sterically hindered form, is ordinarily expected to be represented in any equilibrium mixture of geometric isomers in very small amount. Apparently certain crustacea have ways of circumventing the difficulties implicit in producing and retaining this isomer, and store it in the eye virtually alone.