Retinal Ischemia Journals

 Retinal, also known as retinaldehyde, is a form of vitamin A. It was originally called retinene, and renamed after it was discovered to be vitamin A aldehyde. Retinal is one of the many forms of vitamin A (the number of which varies from species to species). Retinal is a polyene chromophore, bound to proteins called opsins, and is the chemical basis of animal vision. Retinal allows certain microorganisms to convert light into metabolic energy.   Vertebrate animals ingest retinal directly from meat, or they produce retinal from carotenoids, either from one of two carotenes (α-carotene, β-carotene) or from β-cryptoxanthin, a type of xanthophyll. These carotenoids must be obtained from plants or other photosynthetic organisms. No other carotenoids can be converted by animals to retinal, and some carnivores cannot convert any carotenoids at all. The other main forms of vitamin A, retinol, and a partially active form, retinoic acid, may both be produced from retinal.   Invertebrates such as insects and squid use hydroxylated forms of retinal in their visual systems, which derive from conversion from other xanthophylls. Opsins are proteins and the retinal-binding visual pigments found in the photoreceptor cells in the retinas of eyes. An opsin is arranged into a bundle of seven transmembrane alpha-helices connected by six loops. In rod cells the opsin molecules are embedded in the membranes of the disks which are entirely inside of the cell. The N-terminus head of the molecule extends into the interior of the disk, and the C-terminus tail extends into the cytoplasm of the cell. In cone cells the disks are defined by the cell's plasma membrane so that the N-terminus head extends outside of the cell. Retinal binds covalently to a lysine on the transmembrane helix nearest the C-terminus of the protein through a Schiff base linkage. Formation of the Schiff base linkage involves removing the oxygen atom from retinal and two hydrogen atoms from the free amino group of lysine, giving H2O. Retinylidene is the divalent group formed by removing the oxygen atom from retinal, and so opsins have been called retinylidene proteins.   Opsins are prototypical G protein-coupled receptors (GPCRs).[13] Bovine rhodopsin, the opsin of the rod cells of cattle, was the first GPCR to have its X-ray structure determined.[14] Bovine rhodopsin contains 348 amino acid residues. The retinal chromophore binds at Lys296.   Although mammals use retinal exclusively as the opsin chromophore, other groups of animals additionally use four chromophores closely related to retinal. These are 3,4-didehydroretinal (vitamin A2), (3R)-3-hydroxyretinal, (3S)-3-hydroxyretinal (both vitamin A3), and (4R)-4-hydroxyretinal (vitamin A4). Many fish and amphibians use 3,4-didehydroretinal, also called dehydroretinal. With the exception of the dipteran suborder Cyclorrhapha, the so-called higher flies, all insects examined use the (R)-enantiomer of 3-hydroxyretinal. The (R)-enantiomer is to be expected if 3-hydroxyretinal is produced directly from xanthophyll carotenoids. Cyclorrhaphans, including Drosophila, use (3S)-3-hydroxyretinal. Firefly squid have been found to use (4R)-4-hydroxyretinal.