![]() ![]() Taurine enters RPE cells by means of a high-affinity Na + and Cl − dependent taurine transporter (TauT SLC6a6), , ]. Retinal pigment epithelial (RPE) cells take up considerable amounts of taurine only the uptake of proline is higher. The structure of taurine does not include the carboxyl group typical of amino acids instead, it bears a sulfonate group. Although taurine can be synthesized endogenously from cysteine, it is primarily acquired through the diet. Taurine (2-aminoethanesulfonic acid) is an endogenous sulfur-containing β-amino acid that is not incorporated into protein but is the most abundant constituent of the amino acid pool of photoreceptor cells. Taken together these findings support the postulate that A1T serves as a reservoir of vitamin A aldehyde, with diminished A1T explaining reduced photoreceptor light-sensitivity, accentuated ONL thinning in Abca4 −/− mice and attenuated bisretinoid formation. Consistent with a reduction in carbonyl scavenging activity by taurine, methylglyoxal-adducts were also increased in the presence of β-alanine. Treatment of agouti and albino Abca4 −/− mice with β-alanine and GES was associated with reduced bisretinoid measured chromatographically. The thinning of ONL that is indicative of reduced photoreceptor cell viability in albino Abca4 −/− mice was more pronounced in β-alanine treated mice. As a secondary effect of retinal degeneration, A1T was not detected and taurine was significantly reduced in mice carrying a P23H opsin mutation. Oral treatment of BALB/cJ mice with β-alanine reduced ocular A1T and the mice exhibited significantly lower scotopic and photopic a-wave amplitudes. Here we have undertaken to examine the effects of taurine depletion using the transport inhibitors guanidinoethyl sulfonate (GES) and β-alanine. Mechanisms for sequestration of retinoid include the formation of a reversible Schiff base between retinaldehyde and taurine (A1-taurine, A1T), the most abundant amino acid in photoreceptor cells. Various carrier proteins confer water solubility and maintain the 11- cis-retinoid configuration. These bisretinoid fluorophores initiate photooxidative processes having adverse consequences for retina. Otherwise inefficient handling of retinaldehyde can lead to the formation of fluorescent di-retinal compounds within the outer segments of photoreceptor cells. Vitamin A aldehyde is sequestered by reversible Schiff base formation with phosphatidylethanolamine (PE) and subsequently undergoes NADPH-dependent reduction. ![]() To facilitate the movement of retinoids through the visual cycle and to limit nonspecific chemical reaction, multiple mechanisms are utilized to handle these molecules when not contained within the binding pocket of opsin. ![]()
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