Retina, taurine

There are wide differences amongst species in the capacity to synthesize taurine. The chick embryo and the chick can convert inorganic sulphate to taurine (41-43). The adult rat is able to convert inorganic sulphate to taurine whereas this conversion cannot be demonstrated to occur in the cat (44, 45). However until recently It was a matter of discussion whether in retina taurine is synthesized In situ either it Is taken up from the blood stream through the pigment epithelium (32). 

Taurine Is the predominant free amino acid in ox retina(48) where it is present at a concentration of 20-22 umoles/g w. w, tissue and of 16-18 /jmoles/100 mg protein.Sucn concentrations are higher in comparison to those reported in chick and rat retina by Pasantes-Morales et al.(6) and lower than that found in rat retina (4), In addition in the pigment epithelium of ox retina taurine is present at a concentration of 1.8-2.2 )jmoles/100 mg protein thus the retina/pigment epithelium ratio is about 8.5 1. 

The high concentration of taurine in cells is an indication that it is an important molecule but not all functions or their importance are known. Cell membranes are impermeable to taurine so that it must be formed in the cell, within which its concentration is very high. The concentration ratio across the membrane is also very high in cells in the retina (400), neurones in the brain (500) and in some tumour cells (7000). Unfortunately, the precise role of taurine in these particular cells is not known. Of the many functions that are known (Huxtable, 1992 Schuller-Levis Park, 2003) the following is a summary ... 

The maintenance of the structure and function of photoreceptors in the eye depends on taurine. As an antioxidant, it may protect the cell membranes in the retina. 

Militante JD, Lombardini JB (1999) Taurine uptake activity in the rat retina protein kinase C-independent inhibition by chelerythrine. Brain Res 818 368-374... 

Taurine Osmotic agent in retina and used for conjugation of bile acids dietary essential for cats... 

Taurine in the Central Nervous System There is a relatively high concentration of taurine in the central nervous system - higher than would be expected for a neurotransmitter and without a specific anatomical localization. As in the retina, the main function of taurine in the central nervous system seems to be as an osmolyte (Hussy et al., 2000 Saransaari and Oja, 2000). 

They become blind- A taurine deficiency results in degeneration of the retinas the tapetum Juddum, a layer of cells behind the retina, degenerptes,... 

The regulatory amino acid taurine in the retina requires an efficient uptake system to maintain the high physiological concentration of taurine in the retina. Stimulation and inhibition of PKC activity with TP A and with staurosporine, respectively, produced no significant effect on taurine uptake. On the other hand, chelerythrine significantly inhibited the taurine uptake systems, presumably through a PKC-independent mechanism [104]. 

Pow DV, Sullivan R, Reye P, Hermanussen S. 2002. Localization of taurine transporters, taurine, and 3H taurine accumulation in the rat retina, pituitary, and brain. Glia 37 153-168. 

Salceda R and Pasantes-Morales H (1975) Calcium coupled release of ( S) taurine from retina Bram Res 96, 206-211 Schlaepfer W W. (1977) Structural alterations of peripheral nerve induced by the calcium lonophore A 23187 Bram Res 136, 1-9 Schon F and Kelly J S (1974) The characterization of [ H]-GABA uptake into the satellite glial cells of rat sensory ganglia Brain Res 66, 289-300... 

We focused our attention on L-Cys (Scheme 19.2) because of its importance as a sulfur-containing semiessential amino acid, which can be biosynthesized in humans and yet, due to its relatively low content in food, it is also used as a food additive (denoted as E920). L-Cys is an important building block of proteins that are used throughout the body, and it can physiologically be transformed to glutathione (a powerful antioxidant [22]), or taurine (essential for cardiovascular function, development and function of skeletal muscles, the retina, and the central nervous system [23]). Thus, the main aim of this study [14] was to employ TLC-densitometry and TLC-ESI-MS in order to demonstrate an ability of L-Cys to spontaneously undergo chiral conversion and condensation, when dissolved in 70% aqueous acetonitrile. 

Other retin d degenerations that may be confused with toxic reactions in the retina of laboratory species include nutritional retinal degeneration due to taurine deficiency in cats and vitamin E deficiency in dogs. 

The role of taurine in the retina has been thoroughly reviewed elsewheref( and is the subject of several papers at this meeting It is of interest that taurine uptake into platelets from patients with retinitis pigmentosa has recently been shown to be decreased-. Platelets are considered to be useful models for synaptosomes in the central nervous system and have been much used in tudies of the uptake mechanisms of various biogenic amines. Since retinitis pigmentosa may also be found in patients with Friedreich s ataxia, it would be of interest to examine the transport and content of taurine in platelets from patients. 

Decreased availability of taurine in cerebellum, retina, lens, disposing to... 

Gaull, Taurine deficiency in the kitten Exchange and turnover of ( S) taurine in brain, retina and other tissues, J. Nutr. lo8 l -62 (1978). 

B. Ehinger, Glial uptake of taurine in the rabbit retina, Brain Res. 60 512 (1973) ... 

Although various physiological roles were suggested for taurine in different tissues like brain, heart, retina, the origin of this substance in these tissues is still unclear. Taurine could be provided by the diet and could also be synthesized. Until now, three main pathways were described or postulated in mammals for this biosynthesis ... 

According to a recent report hypotaurine is oxidized into taurine by ultraviolet irradiation, which suggests that a non enzymatic oxidation could occur on the other hand, an enzymic oxidation of hypotaurine into taurine, catalysed by hypotaurine oxidase, was described in various tissues of the rat 7 and in the retina of different species 8, in view of the poor knowledge which still exists about the metabolism of hypotaurine and especially about its enzymic oxidation into taurine, we decided to initiate experiments in order to contribute to a better understanding of this last step. 

Those human infants fed the currently available synthetic formulas often have taurine-containing foods added to the diet within a few weeks or months of birth, and the time scale for induction of retinal degeneration in the cat is of the order of months. Furthermore, the concentration of taurine in retina of most species, although high, is not as great as in the retina of the cat. This high concentration of taurine may render the cat retina especially vulnerable. 

Mandel, P, Pasantes-Morales, H, and Urban, P. F, 1976, Taurine, a putative transmitter in retina, "Transmitters in the Visual Process, S. L Bonting, ed., Pergamon Press, New York. 

Pasantes-Morales, H., Klethi, J., Urban, P. F., and Mandel, P., 1972, The physiological role of taurine in retina Uptake and effect on electroretinogram (ERG), Physiol. Chem. Phys. U 339. 

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