Publisher Summary This chapter discusses separate pools of CNS zinc, which are, vesicular zinc, free zinc, and protein-bound zinc Vesicular zinc is the one which is sequestered in the presynaptic vesicles of a special class of neurons, the zinc-containing neurons, which are found primarily in limbic, cerebrocortical, and corticofugal systems This vesicular zinc can be selectively stained by several histochemical procedures and is, therefore, accessible to histological and histoanalytical studies The second pool, free zinc, is an entirely hypothetical pool of ionic Zn +2 in the cytosol or interstitial fluid that may or may not be present in normal, healthy brain tissue The third pool of zinc is that which is bound firmly into the structure of the many zinc-containing enzymes in the brain The enzymatic zinc is, therefore, a stable pool, involved only in the specific function of the zinc-containing enzymes Zinc metalloenzymes are essential to the normal biology of brain cells, and any experimental treatment that is extreme enough to affect the metalloenzymes, such as prolonged zinc undernutrition or administration of chelators, can disrupt brain function through metalloenzymatic effects

Neurobiology of zinc and zinc-containing neurons.

Zinc, a trace element that influences cell metabolism through a variety of mechanisms, appears to play an integral role in maintaining normal ocular function. This element is present in high concentrations in ocular tissue, particularly in retina and choroid. Zinc deficiency has been shown in a number of species to result in a variety of gross, ultrastructural and electrophysiologic ocular manifestations. The physiological functions for zinc have been studied predominantly in retina and retinal pigment epithelium where zinc is believed to interact with taurine and vitamin A, modify photoreceptor plasma membranes, regulate the light-rhodopsin reaction, modulate synaptic transmission and serve as an antioxidant. Suboptimal zinc status in North America may influence the development and progression of several chronic eye diseases. Zinc supplementation trials and epidemiological studies have produced conflicting results concerning the role of zinc in age-related macular degeneration. Additional well-controlled...

Zinc and the Eye

More than simply cataloging the numerous experimental models in which taurine plays a modulating role, this discussion aims at stimulating further investigation of the potential clinical value of this abundant sulfur amino acid. Both the biomedical investigator and clinician must be struck by the enormous amount of taurine floating freely in the intracellular water of the cells. In cardiac tissue alone, taurine levels of 20 mM or higher may be found.2, 3 Given this abundance of taurine, why is our understanding of its function so elusive? Although it is clear taurine is important in conjugating bile acids to form water-soluble bile salts, only a fraction of available taurine is used for this function, predominantly in young animals and children. While taurine conjugation is the preferred route of bile acid conjugation in the young, changes in adults given 250 mg of taurine daily for two to three weeks are insignificant. Total pool size of bile acid and chenodeoxycholic acid declines. Unchanged are the rate of bile acid synthesis or the secretion rates of biliary cholesterol, bile acid and phospholipids. Biliary cholesterol saturation also stays the same.152 The finding that taurine availability protects against cholestasis induced by monohydroxy bile acids remains confined to guinea pigs.153 The abundance of taurine suggests it may be an osmoregulator of cell volume, and there is convincing evidence that it serves this function in fish. Taurine may play this role in the brain under high osmotic states such as hypernatremia, dehydration and uremia. Evidence is strong that taurine is vital in maintaining retinal function, which may explain why taurine is so abundant in human breast milk. Prolonged TPN feeding of infants demonstrates the importance of taurine in retinal development. We have begun to appreciate the role of the kidney in conserving taurine and how this is perturbed in the neonatal period. Taurine has recently been added to infant formulas (about 50 mg/L). Cataloging what we know of taurine function, however, produces a list of “maybes.” Now is the time for exhaustive, careful taurine research that will produce more definite answers. 

Taurine: Its Biological Role and Clinical Implications

Adult female cats were fed a defined purified diet (taurine-free) either alone or supplemented with 0.05% taurine for at least 6 mo prior to breeding. The reproductive performance by the taurine-depleted females was poor, whereas those receiving dietary taurine had normal pregnancies and deliveries. The taurine-depleted females suffered from severe retinal degeneration, including a large loss of photoreceptor outer segments, and degeneration of the tapetum lucidum, and greatly reduced concentrations of taurine in their body tissues and fluids. Surviving offspring from the taurine-depleted mothers exhibited a number of neurological abnormalities and substantially reduced concentrations of taurine in the body tissues and fluids. Except for greater concentrations of cystathionine in neural tissues, other free amino acids in tissues were unaffected. The specific activities of a number of enzymes involved in the biosynthesis of taurine were unchanged in liver and brain. The composition of maternal milk, total protein, protein amino acids and free amino acids was unchanged except for taurine content, suggesting that the abnormalities in the offspring resulted from the diminished dietary taurine.

Feline maternal taurine deficiency: effect on mother and offspring.

Taurine: retinal function

High resolution electron microscopy of ultrathin sections confirms the presence of a membrane surrounding the tapetal rods in the cat. Cats depleted of taurine exhibit disruption and disorganization of this membrane, probably the first stage of more severe tapetal degeneration. Histochemical localization of zinc shows it to be present on the periphery of the tapetal rods. The amount of zinc present on the periphery of the tapetal rods of taurine depleted cats was greatly reduced. Taurine in feline tapetum, confirmed by autoradiography and direct measurement, was also greatly reduced in taurine-depleted cats. We conclude that both taurine and zinc are localized on the periphery of the tapetal rods and that they contribute to the stability of the membrane. We have also confirmed earlier reports that the cat tapetal rods contain riboflavin and no detectable cysteine.

Histochemical localization of zinc in the feline tapetum. Effect of taurine depletion.

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Histochemical localization of zinc in the feline tapetum. Effect of taurine depletion.