Synergism Between Zinc and Taurine in the Visual Sensitivity of the Bullfrog’s Eye

Until now diverse observations on human zinc deficiency syndrome have been observed, such as dermatitis, depressed immunity, stunting, hypogonadism and abnormal dark adaptation and others [1-6]. Zinc has been reported to occur in more than 300 vital metalloenzymes and proteins and play a key role in genetic expression, cell division and growth. Some metalloenzymes (catalytically active metalloproteins containing stoichiometric amounts of zinc firmly bound to active sites) such as alkaline phosphatase, carbonic anhydrase and alcohol dehydrogenase [7], are present in mammalian eye tissue, especially in the retinal pigment epithelium, ciliary body, lens and Muller fibers. Furthermore zinc normally participates in vitamin A metabolism in several ways [8], including the biosynthesis of retinol-binding protein, and the enzymatic conversion of retinol to retinal [9]. Although its exact role in the visual process is not clear and its functional significance remains an enigma, the high level of zinc in ocular tissues may be related to the activity of zinc-dependent enzyme. According to the results of zinc deficiency studies in animal experimentation, zinc may be required for visual sensitivity. Taurine (2-aminoethanesulfonic acid) is a β-amino sulfonic acid that is present in high concentrations in all animal tissues but it is in mammalian systems that the study of taurine generates perhaps its greatest interest due to its wide spectrum of biological activities in various tissues [10-11]. The many possible functions of tarine include the following gamut: (a) neurotransmitter (or neuromodulator) in the central nervous system (CNS); (b) stabilizer of biological membranes in many tissues which affects cardiovascular functions and protects lymphoblastoid cells and spermatozoa against loss of activity; (c) protector of rod outer segments (ROSs) from exposure to toxic levels of light and chemicals; (d) modulator of calcium binding and fluxes; and (e) inhibitor of protein phosphorylation. The retina contains an extremely high amount of taurine and in some animal species the taurine levels in the retina are the highest of any tissues [12]. Greater than half the retinal store of the free amino acid taurine is located in the photoreceptor cell layer [13]. The presence of physiological levels of taurine in the retina seems essential for the normal function of retinal cells as well. Retinal taurine depletion results in a marked decrease in the amplitude of the aand b-waves of the electroretinogram (ERG), which are eventually abolished [1415]. Severe damage of photoreceptor structure is also observed, characterized by disruption and twisting of the otherwise highly ordered structure of the discs membranes [16]. Vesiculation and swelling of both outer and inner segments of photoreceptors also characterizes the pathology of Journal of Photoscience (2000), Vol. 7(3), pp. 115 121