Vision photoreceptors

Porphyropsin. Pigment of vision (photoreceptor protein) in the retina of fresh water fish and some amphibians. The absorption maximum of P. is 520-530 nm. P. consist of the chromophore 3,4-didehydro-11-cii- retinal and the protein scotopsin. The biological activity is similar to that of rhodopsin. 

Vision is vital for human activities, and eyes are very sensitive to a number of toxic insults induced by chemical compounds. The most serious outcome is permanent eye damage which may be so severe as to cause loss of vision. The eye consists of the cornea and conjunctiva, the choroid, the iris, and the ciliary body. It also contains the retina, which is of neural origin, and the optic nerve. The retina contains photoreceptors, a highly specific light-sensitive type of neural tissue. The eye also contains the lens and a small cerebrospinal fluid system, the aqueous humor system, that is important for the maintenance of the steady state of hydration of the lens and thus the transparency of the eye. 

Vitamin A is essential throughout life, including foetal development, but perhaps its most well researched role is that in vision where 11 -cis retinaldehyde is the initial part of the photoreceptor complex in rods and cones. Retinoic acid induces differentiation in epithelial cells and deficiency leads to... 

Wolken, J.J. (1966). Biophysics and biochemistry of the retinal photoreceptors. In Vision pp. 52-61. Thomas, Spring-field, IL. 

AMD is a deterioration of the central portion of the retina (the macula). The macula facilitates central vision and high-resolution visual acuity because it has the highest concentration of photoreceptors in the retina. The loss of central... 

Rhodopsin is a seven ot-helix trans-membrane protein and visual pigment of the vertebrate rod photoreceptor cells that mediate dim light vision. In this photoreceptor, retinal is the chromophore bound by opsin protein, covalently linked to Lys296 by a Schiff base linkage. Kpega et al.64 have studied NMR spectra of Schiff bases being derivatives of all-frans retinal and amino-p-cyclodextrins as a model of rhodopsin, where p-cyclodextrin plays a role of a binding pocket. On the basis of analysis of the chemical shift differences for the model compound in the presence and in the absence of adamantane carboxylate, it has been shown that the derivative of 3-amino-p-cyclodextrin forms dimer in water and retinoid is inserted into p-cyclodextrin cavity [31]. 

Mutations in rhodopsin and other photoreceptor proteins are linked to retinitis pigmentosa. Retinitis pigmentosa (RP) is a group of inherited retinopathies that affects about 1 in 4,000 humans [26], RP maybe classified into four types autosomal dominant (19%), autosomal recessive (19%),X-linked (8%) and allied diseases (54%). RP is characterized by loss of night vision in the early stage, followed by loss of peripheral vision. Chromosomal loci for numerous RP genes have been mapped and mutations characterized [27]. 

Vision results from signals transmitted to the brain by about 125 million sensors located in the retina. These photoreceptors are of two types, called cones and rods. Cones work under intense light - that is, during daylight hours - and this mode of vision is called photopic vision. Rods work under dim lighting conditions, and this is called scotopic vision. 

For vision to continue, c/s-retinal must be regenerated. The /rans-retinal is reduced to /rans-retinol (i.e. the aldehyde is converted to alcohol) and is isomerised to c/s-retinal this is oxidised to c/s-retinol (see Figure 15.9(b)). Two different cells are involved the oxidation of retinal to retinol occurs in the photoreceptor cell. The retinol is then released and is taken up by the adjacent epithelial cell where it is isomerised to c/s-retinol and then reduced to... 

Two types of photoreceptor cell are found in the human retina—rods and cones. Rods are sensitive to low levels of light, while the cones are responsible for color vision at higher light intensities. 

Vitamin A is essential for proper functioning of the retina, for the integrity of epithelial tissue, for growth and bone development and for reproduction. For vision the active vitamin appears to be retinal as the chromophore of both rods and cones is 11-cis-retinal which, in combination with the protein opsin, forms the photoreceptor rhodopsin. Retinoic acid is the active form associated with growth, differentiation, and transformation. Both all-trans and 9-cis retinoic acid act as a steroid hormone to affect cellular differentiation, especially for morphogenesis, reproduction and for immune responses. At... 

Shand J, Foster RG 1999 The extraretinal photoreceptors of non-mammaKan vertebrates. In Archer SN, Djamgoz MBA, Loew ER, Partridge JC, Vallerga S (eds) Adaptive mechanisms in the ecology of vision. Kluwer Academic Publishers, Dordrecht, Netherlands, p 197—222 Smyth RD, Saranak J, Foster KW 1988 Algal visual systems and their photoreceptor pigments. Prog Phycol Res 6 255-286... 

The Gt- and Gg-proteins are also classed as Gi-proteins, based on sequence homologies. The Gt- and Gg-proteins are involved in transmitting sensory signals. Signal transmission in the vision process is mediated via G-proteins known as transducins (Gt). The Gt-proteins are activated by the photoreceptor rhodopsin and are located in the rods and cones of the retina. The sequential effector molecules of the Gt-proteins are cGMP-specific phosphdiesterases (see Fig. 17.9). 

In connection with the problem of oscillations discussed by previous speakers and other types of dynamical behavior of membranes, it would probably be timely to mention here in some more detail the experiments with vision rhodopsin that were performed in our institute by using the Mossbauer spectroscopy method [G. R. Kalamkarov et al., Doklady Biophys., 219, 126 (1974)]. These experiments manifested the existence of reversible photo-induced conformational changes in the photoreceptor membrane even at such low temperature as 77°K. We have labeled various samples of solubilized rhodopsin and of photoreceptor membranes by iron ascorbate enriched with Fe57 and looked for the change of Mossbauer spectra caused by the illumination of our samples. 

Vision is a process in which light is absorbed by a pigment in a photoreceptor cell (by a dye in the eye) and the photochemistry that ensues ultimately produces a transient electrical signal that is transmitted to the brain and interpreted as a visual image. There is much that is not fully understood about this process, but we shall discuss briefly the chemistry involved. 

The chromophores are aggregated into a liquid crystalline stmcture wherein they are able to conduct the excited electrons resulting from photoexcitation at a given location to a second location where they are de-excited in the process (developed in Chapter 11) of generating a signal within the dendrites of the photoreceptor cells. The chromophores of vision are produced in the RPE cells of the retina and not in the photoreceptor cells as conventionally assumed. This complex procedure is developed in Chapter 7. 

There are a large number of reasons to revert to an alternate baseline. The most important is the fact that the chromophores of vision are not formed within the photoreceptor cell. There is an extensive database on how they are formed and stored within the RPE cells and subsequently transported to the Outer Segments for coating of the protein substrate known as opsin. 

M-R A unique situation occurs in vision. The dendrites of the photoreceptor cell are in direct quantum-mechanical contact with the disks coated with the chromophores. The probability of the energy being transferred to the suitable structure in the dendrites is much higherthan the probability of direct de-excitation (probability 0.0). 

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