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Zinc is essential for normal growth and development in all organisms. This ion serves both as a structural element and a dynamic signaling ion, released within the cell or to the extracellular milieu. The majority of cellular zinc is tightly bound to intracellular proteins, where it serves as a functional and/or structural component of an estimated 3000 enzymes and proteins involved in virtually all cellular processes. However, some zinc exists in its ionic form (Zn2+) and constitutes a pool that is identifiable with chelating agents or by autometallography. This pool of zinc, which can be present in high concentrations, e.g., in vesicles or in low concentrations, e.g., in soluble cellular fractions, has been implicated in cellular signaling. The clinical symptoms of severe zinc deficiency in humans include dermatitis, diarrhea, alopecia, re-current infections and neurological disturbances. Yet, increases of cellular ionic zinc are toxic, as shown in many experimental paradigms, in both the central nervous system, related, for example, to ischemia, and in peripheral tissues, e.g. pancreatic cell loss related to diabetes.
The last few years have witnessed dramatic progress on all frontiers of Zn2+ biology aided by powerful new tools, such as highly-specific fluorescent zinc probes, and transgenic animal models. The sensitivity and specificity of these techniques has resulted in increased understanding of many novel aspects of the physiological and patho-physiological functions of zinc. Progress has been especially pronounced with regard to the activity and functions of zinc in the central nervous system, where Zn2+ interacts with the receptors of major neurotransmitters (GABA, NMDA and glycine) and influences important physiological and patho-physiological processes, including LTP, memory consolidation and neurodegeneration. Intriguing links have also been discovered between zinc transporters and diabetes as well as acrodermatitis entheropathica, a severe genetic disorder. Studies on the function of zinc in other peripheral tissues and organs (e.g., heart, intestine, liver, immune, eye, etc.) are actively being pursued.
Given the rapid pace of recent advances, it may not be imprudent to suggest that zinc ions will soon rival calcium ions in their perceived importance for cell biology. Think zinc!
