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Zinc is relatively abundant in biological materials. Among the other well-characterized zinc proteins are the Cu-Zn superoxide dismutases other forms have Fe or Mn , carbonic anhydrase an abundant protein in red blood cells responsible for maintaining the pH of the blood , alcohol dehydrogenase, and a variety of hydrolases involved in the metabolism of sugars, proteins, and nucleic acids. Zinc is a common element in nucleic-acid polymerases and transcription factors, where its role is considered to be structural rather than catalytic.
Interestingly, zinc enhances the stereoselectivity of the polymerization of nucleotides under reaction conditions designed to simulate the environment for prebiotic reactions. Recently a group of nucleic-acid binding proteins, with a repeated sequence containing the amino acids cysteine and histidine, were shown to bind as many as eleven zinc atoms necessary for protein function transcribing DNA to RNA.
A survey of the sequences of many nucleic-acid binding proteins shows that many of them have the common motif required to form zinc fingers. Other zinc-finger proteins called steroid receptors bind both steroids such as progesterone and the progesterone gene DNA Chapter 8.
Much of the zinc in animals and plants has no known function, but it may be maintaining the structures of proteins that activate and deactivate genes. Copper and iron proteins participate in many of the same biological reactions:.
The two metal ions also function in concert in proteins such as cytochrome oxidase, which catalyzes the transfer of four electrons to dioxygen to form water during respiration. Whether any types of biological reactions are unique to copper proteins is not clear. However, use of stored iron is reduced by copper deficiency, which suggests that iron metabolism may depend on copper proteins, such as the serum protein ceruloplasmin, which can function as a ferroxidase, and the cellular protein ascorbic acid oxidase, which also is a ferrireductase.
Cobalt is found in vitamin B 12 , its only apparent biological site. Vitamin-B 12 deficiency causes the severe disease of pernicious anemia in humans, which indicates the critical role of cobalt. The most common type of reaction in which cobalamin enzymes participate results in the reciprocal exchange of hydrogen atoms if they are on adjacent carbon atoms, yet not with hydrogen in solvent water:.
An important exception is the ribonucleotide reductase from some bacteria and lower plants, which converts ribonucleotides to the DNA precursors, deoxyribonucleotides, a reaction in which a sugar -OH is replaced by -H.
Cellular extracts were prepared from Caco-2 cells grown as described above. The cell suspensions were transferred to a 1. The supernatant was transferred to a 0. The cells were washed and a cellular extract was prepared. To determine 55 Fe and 64 Cu uptake on Caco-2 cells incubated at equilibrium with different concentrations of metal, Caco-2 cells were incubated as described and supplemented with 5, 10, 20 and 50 uM of Fe, Cu or Zn for 7 days.
Cells were washed and intracellular metal radioisotope and total concentration was measured in a Beta counter and by AAS. For competition studies, Caco-2 cells were grown for 7 days as above and then incubated in transport buffer with a 55 Fe: Zn, 55 Fe: Cu, 64 Cu: Fe or 64 Cu: Zn in varying ratios from 1: 0. After the incubation, cells were washed and intercellular radioisotope was measured. Intracellular content of Fe, Cu and Zn in Caco-2 cells. As expected, when extracellular Fe increased in the media 0.
However, in the same conditions, both Cu and Zn decreased from 0. When Caco-2 cells were incubated with increasing Cu concentrations, intracellular Cu content increased from 0. In this situation, Fe decreased from 0. Cells incubated in basal conditions, had a ratio of 1: 2: 4 of Fe: Cu: Zn, respectively. To study the effect of the intracellular metal content on the uptake of Fe or Cu, Caco-2 cells were incubated with four concentrations 0.
Independently of the intracellular Cu or Zn concentrations Figs. However, Fe uptake in cells grown in different Cu concentrations was lower than in cells grown in different Zn concentrations. In the same way, intracellular Fe or Zn concentrations Figs. Cu uptake in cells grown in different Zn concentrations was lower than in cells grown in different Fe concentrations.
Studies of competition between metals Fe vs. Cu or Zn and Cu vs. Fe or Zn. Fe at However, increased extracellular Zn concentrations did not affect Cu uptake Fig.
Effect of different molar ratios over 55 Fe or 64 Cu uptake. Caco-2 cells were incubated with different molar ratios of 64 Cu: Fe: Zn or 55 Fe: Cu: Zn 1: 1: 1 a 1: 5; from 10 uM to 50 uM , and the uptake of Fe and Cu was studied.
Under these conditions, a ratio of 1: 1: 1 of 64 Cu: Fe: Zn Fig. Zn, Cu and Fe are essential mineral elements that exhibit important interactions and possible competitive inhibition of transport and bioavailability Reinstein et al. In this work, we studied the effects of iron, copper and zinc, alone or in combination in different metals ratios, on the absorption of each other.
However, Zn did not inhibit Cu uptake. When two metals were given together there was an evident inhibition in the uptake, especially over 1: 10 molar ratio. Based on these results, a concern arises about the concentrations of minerals currently used in the programs of food fortification with multinutrients.
These findings are especially relevant for the infantile milk formula field, since most of these preparations are fortified with copper, iron and zinc in molar ratios of 1: 5.
Future research is required to find the optimal molar ratios among these microminerals. Zn-Cu competition has been exploited beneficially for the treatment of Wilson's disease to avoid Cu accumulation Barone, Recent progress in the field of metal ion transport has advanced significantly our understanding of the mechanisms of intestinal metal ion absorption under normal and pathological conditions.
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