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Effects of Phytate and Calcium on the Reabsorption of Endogenous Zinc in Zinc-Depleted Rats
Kwun, In-Sook,Oberleas, Donald The Korean Nutrition Society 1997 Journal of Nutrition and Health Vol.31 No.7
Endogenous zinc is important for maintaining zinc homeostasis because the size of endogenous zinc pool is almost 3-4 times bigger than that of dietary zinc. The purpose of this study was to examine the phytate effect on the reabsorption of endogenous zinc and the additional Ca effect on the phytate effect. Rats were fed a casein-based diet with added sodium phytate containing either high(1.6%) or low(0.8%) Ca concentrations for 4 weeks to reduce the body zinc pool. After the depletion period, $^{65}ZN$ was given by intraperitoneal injection to label the endogenous zinc pool. Rats were then assigned into phytate or non-phytate group within the same Ca group. Feces were collected for 2 weeks of the initial collection period and 1 week after dietary crossover. The ratios of excreted fecal $^{65}ZN$ radioactivity of phytate group : non-phytate group were determined as a measure of the phytate effect on the endogenous zinc. Mean fecal $^{65}ZN$ radioactivity was higher in the phytate group than in the non-phytate group during the entire 3 weeks of the collection period in the low Ca group, and during the initial collection period in the high Ca group(p<0.0001). This study showed an adverse phytate effect on endogenous zinc at both high and low dietary Ca levels. Elevated dietary Ca levels showed a synergistic effect on the phytate effect on endogenous zinc(p<0.05). These results imply greater phytate effect on zinc homeostasis rather than on zinc bioavailability through complexing with the endogenous zinc which is larger portion than the dietary zinc on zinc homeostasis.
Identification and Molecular Size of Zinc - Binding Ligands in Pancreatic / Biliary Fluid of Rats
In-Sook Kwun,Donald Oberleas 한국식품영양과학회 1997 Preventive Nutrition and Food Science Vol.2 No.1
The exocrine pancreatic secretion is an important factor in the maintenance of zinc homeostasis. The daily pancreatic secretion of zinc into the gastrointestinal tract may be two or more times the daily dietary zinc intake. The objective of this study was to examine the distribution of proteins and zinc in pancreatic/biliary fluid following intraperitoneal (65)^Zn injection into dietary prepared Sprague-Dawley rats. Distribution of zinc-binding protein in Sephadex G-75 subfractions showed a peak corresponding to the high molecular weight protein standard(<66kDa) in the pancreatic/biliary fluid. Zinc also was associated with the 29~35kDa molecular weight proteins. These are similar in size with zinc-containing enzymes, carboxypeptidase A and carboxypeptidase B. A more remarkable small molecular weight fraction eluted beyond the 6.5kDa standard protein peak. These results show the presence of small molecular weight compounds in pancreatic/biliary fluid associated with zinc. These small molecular weight compounds may serve as zinc-binding ligands for the secretion of enogenous zinc into the duodenum. These findings suggest that these lignads may dissociate zinc in the duodenum thus making it vulnerable to complexation with phytate in the upper gastrointestinal tract rendering the zinc unavailable for reabsorption.
In-Sook Kwun,Donald Oberleas 한국식품영양과학회 1997 Preventive Nutrition and Food Science Vol.2 No.1
The pancreas is an important organ in the maintenance of zinc homeostasis. Endogenous zinc is continuously secreted via pancreatic exocrine fluid or to a lesser extent in bile. Much of the endogenous secretion must be reabsorbed to sustain zinc homeostasis. The objective of this study was to estimate the relative size of the pancreatic/biliary zinc pool in comparision to the dietary zinc intake, and to study the effect of the phytate and calcium on the zinc homeostasis using a rat model. At the termination of the experiment, pancreatic/biliary fluid was collected from the rats. Both radioactivity and total zinc were measured and the relative size of the pancreatic/biliary zinc pool was estimated. To determine the effect of phytate and calcium on zinc homeostasis, dietary zinc intake, the amount of zinc in pancreatic/biliary fluid and fecal zinc excretion were measured. The flow rate of pancreatic/biliary fluid. as corrected for tubing constriction, gives the corrected zinc concentration in the pancreatic/biliary fluid was 2.2 times higher than dietary zinc intake. To maintain zinc homeostasis, zinc absorption/reabsorption was very efficient in the current model; 76%, 88% of absorption/reabsorption for low calcium group and high calcium group, and 81%, 85% for phytate group and non-phytate group, respectively.
Molecular Size and Distribution of Zinc - binding Ligands in Rat Pancreatic Tissue
In-Sook Kwun,Donald Oberleas 한국식품영양과학회 1997 Preventive Nutrition and Food Science Vol.2 No.3
The pancreas is an important organ in the maintenance of zinc homeostasis. The pancreatic tissue used in this study was obtained from rats fed varying levels of dietary Ca and phytate followed by intraperitoneal (65)^Zn injection. The objective of this study was to determine the molecular size and distribution of compounds that may represent zinc-binding complexes in pancreatic tissue homogenates. The supernatant of the homogenized pancreatic tissue was separated using a Sephadex G-75 column with Tris buffer at pH 8.1. All subfractions were assayed for zinc, protein and (65)^Zn activity. The elution of subfractions from pancreatic tissue homogenates showed a prominent peak corresponding to the high molecular weight protein standard (>66kd). A small molecular weigth protein (<6.5kd), that was absorbed at 280㎚, was also present; prominently in low Ca group, however not much as in high Ca group. These small compounds may combine weakly with zinc in pancreatic tissue and serve as zinc-binding ligands in pancreatic/biliary fluid. In the duodenum, these ligands dissociate zinc into an ionic form which becomes vulnerable to phytate complexation.