Zinc: A Promising Agent in Dietary Chemoprevention of Cancer (Report)
Numerous enzymes and proteins utilize first transition and Group IIB elements to carry out their biological functions and zinc is the most widely used of these elements in biology. Among the zinc enzymes are oxidoreductases, tranferases, hydrolases, lyases, isomerases and ligases. Indeed, zinc is the only metal encountered in each enzyme class. The notable selection and frequent utilization of zinc as the predominant functional element of so many biological molecules have been understood in terms of its chemical properties and its use in biochemical systems. Two properties of zinc need to be highlighted. First, unlike other metals, including those of IIB series, zinc is virtually nontoxic even at higher doses (1). The homeostatic mechanisms that regulate its entry into, distribution in and excretion from cells and tissues are so efficient that no disorders are known to be associated with its excessive accumulation in contrast to iron, copper, mercury and other metals (2). Second, its physical and chemical properties, including its generally stable association with macromolecules and its co-ordination flexibility, makes it highly adaptable to meet the needs of proteins and enzymes that carry out diverse biological functions (3,4). These chemical properties form the basis for the extensive participation of zinc in protein, nucleic acid, carbohydrate and lipid metabolism as well as in the control of gene transcription and other fundamental biological processes. Zinc is required in the diet of human beings in trace quantities, which is approximately 15 mg Zn/day (5). It is found in all body tissues and fluids in relatively high concentrations, with 85 per cent of the whole body zinc in muscle and bone, 11 per cent in the skin and the liver and the remaining in all the other tissues (6). The average amount of Zn in the adult body is about 1.4-2.3 g (6). Zinc is present at higher concentrations in liver followed by pancreas, kidney, heart, pituitary, adrenal, and prostrate. There are many reports that clearly emphasize that zinc is a principal limiting factor in the nutrition of children and adolescents and that its deficiency probably accounted for the growth retardation so commonly seen in such age groups. Zinc is virtually nontoxic to living organisms. It is the only pre-, post-, and transitional element that is neither cytotoxic nor systematically toxic, nor is it carcinogenic, mutagenic, or teratogenic. Zinc is not stored in the body and excess intakes result in reduced absorption and increased excretion. However, there are reports on a few cases of acute Zn poisoning (7). Although zinc is an essential element and is nontoxic at lower doses (1), yet its metabolic role is not clearly known (8). It has been considered a trace metal of prime concern as it is essential for carrying out function of various DNA and RNA synthesizing enzymes (9). It is a part of most cellular aspects of body and participates in all major metabolic pathways and is involved in the development and maintenance of competent immune system.