Effect of phytate added to a casein-based diet on endogenous zinc and study of pancreatic/biliary fluid fractions in rats
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The purpose of the current research was to study the effect of phytate and Ca level on the endogenously secreted zinc. Sprague-Dawley rats (48) were fed a casein-based diet with added Na phytate containing either high (1.6 %) or low (0.8 %) Ca for 4 weeks to reduce the body Zn pool. After the depletion period, 65Zn was given IP to label the endogenous pool. Feces were collected for 3 wks (2 wks of the initial collection and 1 wk after dietary crossover). The ratios of excreted radioactivity phytate/non-phytate were determined as a measure of the phytate effect on endogenous zinc. Mean fecal 65Zn radioactivity was higher in the phytate, low Ca group (4582 ± 345 cpm/d) than in nonphytate, low Ca group (3990 ±215 cpm/d) during the initial collection period as well as during the crossover collection period, phytate group (3077 ±146 cpm/d) vs. non-phytate group (2376 ± 96 cpm/d) (p<0.0001). For the high Ca diets, the fecal 65Zn was higher for the phytate than the non-phytate during the 14 day collection (p<0.0001) but was not different after dietary crossover (p>0.05). Ratios of phytate/non-phytate ranged from 0.66-1.77 for low Ca diet and 0.64-1.99 for the high Ca diet. Since ratios above 1 represent a phytate effect on complexing endogenous Zn, this study showed phytate effect at both dietary Ca levels. At the elevated dietary Ca level, the phytate effect was synergized by dietary Ca (p<0.05). At the termination of the collection period, the common bile duct was cannulated with small bore tubing and pancreatic/biliary fluid collected with protein stimulation. Pancreatic/bniary fluid was applied to a Sephadex G-75 column and eluted with 0.01 mol/L Tris buffer, pH 8.1. Subfractions (2.5 mL) were collected for analysis of protein, total Zn concentration and 65Zn radioactivity. Distribution of protein in subfractions showed a peak corresponding to the high molecular weight protein standard (>66 kDa) in the pancreatic/biliary fluid. A more remarkable small molecular weight fraction was eluted near the 6.5 kDa protein standard. This demonstrates the presence of small molecular weight compounds in pancreatic/biliary fluid associated with zinc which may serve as ligands for the secretion of endogenous Zn into the duodenum. These ligands may dissociate Zn in the duodenum thus making it vulnerable to phytate complexation. It was estimated that the zinc concentration in the pancreatic/biliary fluid for one day (311.40 |ig/ day) was about 2.9 times greater than the zinc amount of dietary zinc consumed for one day (107.89 p-g/ day). Also, the dietary phytate group showed 72% of the zinc reabsorption which was enough to replace 63% of zinc which was secreted from pancreas for the zinc homeostasis.