Visual purple

Vitamin A is necessary for growth and reproduction, resistance to infection, maintenance and differentiation of epithelial tissues, stability and integrity of membrane structures, and the process of vision. In terms of the last function, vitamin A is a component of rhodopsin or visual purple, a photosensitive pigment in the eye that is needed for vision in dim light. An early mild clinical symptom of vitamin A deficiency is night blindness a severe deficiency of this fat-soluble vitamin results in xerophthalmia, an eye condition leading to blindness. 

The membranes of the rod discs are -60% protein and 40% lipid (Table 8-3). About 80% of the protein is rhodopsin (visual purple), a lipoprotein that is insoluble in water but soluble in detergent solutions. Digito-nin is widely used to disperse rhodopsin molecules because it causes no change in optical properties. In addition to rhodopsin, in the outer segment discs of frog retinal rods, there are -65 molecules of phospholipid and smaller amounts of other materials for each molecule of rhodopsin (Table 8-3). The cone cells have a similar architecture but have a different shape and contain different light receptors. The receptors in the cones are present in deep indentations of the plasma membrane rather than in discs within the cytoplasm. 

The principal physiological functions of tins vitamin include growth, production of visual purple, maintenance of skin and epithelial cells, resistance to infection, gluconeogenesis. mucopolysaccharide synthesis, bone development, maintenance of myelin and membranes, maintenance of color and peripheral vision, maintenance of adrenal cortex and steroid hormone synthesis, Specific vitamin A deficiency diseases include xerophthalmia, nyctalopia, hemeralopia, keratomalacia, and hyperkeratosis. 

In 1912, Hopkins reported a factor m milk needed for the growth of rats. In 1913, Osborne and Mendel demonstrated that milk factor is fat soluble, and present in other fats also, McCollum and Davis, in 1913-1915, identified milk factor (fat-soluble A) in butter and egg yolk. In 1917, McCollum and Simmonds found xerophthalmia in rats due to lack of fat-soluble A. In 1920, Drummond renamed fat-soluble A, vitamin A. In 1930, Moore determined that carotene is a precursor of vitamin A. See also . During 1930-1937, Karrer et al, isolated and synthesized vitamin A. In 1935, Wald reported visual purple in retina to be a complex of protein and vitamin A. 

The photosensitive compound of most mammalian eyes is a protein called opsin to which is covalently coupled an aldehyde of vitamin A, and is called rhodopsin or visual purple. 

Visual purple occurs in all vertebrates. It is not distributed evenly over the retina. It is messing in the fovea, and in the regions outside the fovea, its concentration undoubtedly increases to a maximum in the region about 20° off center, corre-sponding to the high density of rods in this region. Therefore, to see an object best in the dark, one should not look directly at it. 

Pigment from sweet cherries Prunus avium), elder (Sambucus), plums (Prunus domestica), tulips (Tu-lipa), and blackthorn (Prunus spinosa). K. is purported to have, among others, a regenerative effect on visual purple (rhodopsin). 

In mammals, (1IZ)-Retinal, generated from retinol in the retina, is the photo-reactive chromophore, which forms a Schiff-base to a lysine residue of opsin, a G-protein-coupled receptor (GPCR) protein, to give rhodopsin. This visual purple is concentrated in the outer parts of the rod and cone photoreceptors. Upon light absorption, the chromophore converts photons into a chemical signal by isomerisation to (all )-retinal, which causes a conformational change of... 

To give but one example of the importance of imines in biological systems, the active form of vitamin A aldehyde (retinal) is bound to the protein opsin in the human retina in the form of an imine called rhodopsin or visual purple (see Chemical Connections 4B). The amino acid lysine (see Table 18.1) provides the primary amino group for this reaction ... 

Ditarpenes terpenes derived from four isoprene units (C20H32). Phytol, an aliphatic D., is important as the ester component of chlorophyll and as a part of vitamins K and E. Other examples of naturally occurring D. are Stevioside (see), vitamin A, retinol (chromophore of visual purple), certain alkaloids (cassaine, aconitine), and hormones such as gibberellins, trisporic acids and antheridiogens. Many cyclic D. are acids (e.g. the resinic acid, abie-tic acid Fig.). 

Synthesis of visual purple during the visual process. 

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