J. NUTRITION, 82: '64 256
Pran Vohra and F.H. Kratzer
One mechanism by which a chelating agent might improve mineral availability depends upon the chelating agent having a stronger stability constant for the metal than the metal binding substance in the feed so that the metal is complexed with the chelating agent in the gastrointestinal tract. The metal chelate can then be absorbed, if it is a relatively small molecule. After absorption the metal might be available for specific body functions if it can be removed from the chelating agent. This means that the various systems in which the metal is required for proper functioning (e.g., enzymes) should have higher stability constants for the metal than the chelating with which it is absorbed. To satisfy the requirements of this mechanism, a chelating agent, to be effective in improving metal availability, would need to have a stronger stability constant than he binding agent in the feed and a weaker stability constant than the tissue system.
CDTA and DTPA with stability constants of more than 18 for zinc also were not very suitable for improving the availability of zinc. Our data lead us to speculate that chelating agents with stability constants below 13 may not be able to release zinc effectively from its "bound" form from the diets containing isolated soybean protein. The stability constant of the chelating factor in this protein must be higher than 13 but lower than 14.5. Once this zinc has been released from the dietary factor, another chelating substance may be involved in the living organism with a stability constant lower than 16.5 but higher than 14.5 which transports zinc to the various tissues where it is needed. EDTA may be interfering slightly in releasing it complexed zinc to the "transporting" chelating system of the poult. This appears to be definitely the case for CDTA and DTPA, which complex zinc so strongly that its availability to the tissue transport system is of a low order.
LITERATURE CITED
1. Kratzer FH, Allred JB, Davis PN, Marshall BJ, and Vohra P: 1959; The effect of autoclaving soybean protein in the addition of ethylenediaminetetraacetic acid on the biological availability of dietary zinc for turkey poults. J. Nutrition, 68: 313.
2. Davis PN, Norris LC, and Kratzer FH 1962. Interference of soybean proteins with the utilization of trace minerals. J. Nutrition, 77:217.
3. Scott ML and Zeigler, TR 1963. Evidence for natural chelates which aid in the utilization of zinc by chicks. Agr. Food Chem, 11:123.
4. Kratzer FH and Starcher B 1963. Quantitative relation of EDTA to availability of zinc for turkey poults. Proc. Soc. Exp. Biol. Med, 113:424.
5. Schwarzenbach G and Ackermann, H. 1948. Complexon. XII. Die homologen der Athylen-diamin-tetraessigsaure und ihre erdalkali-komplexe. Helv. Chim. Acta, 31:1029.
6. Martell AE and Chaberek, Jr., S. 1950. The preparation and properties of some N1N-disubstituted ethylenediaminedipropionic acids. J. Am. Chem. Soc., 72:5357.
Tracer studies have indicated significant absorption of EDTA, Zn-EDTA, and Ca-EDTA complexes from the gastrointestinal tract of chickens and hens (unpublished data of Koike, Vohra, Darwish and Kraizer; abstracts in Poultry Sci., 42:1263, 1281, 1963.
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