Phosphorylation of peptides derived from isolated soybean protein: Effects on calcium binding, solubility and influx into Caco-2 cells

Calcium homeostasis in the human body is maintained primarily via the absorption of calcium through the intestine. In order to maintain an efficient absorption of calcium with minimal calcium loss due to the formation of calcium phosphate precipitates in the small intestinal lumen, we developed a calcium-binding mediator using peptides derived from isolated soybean protein (ISP). ISP was modified via tryptic digestion and chemical phosphorylation using sodium trimetaphosphate, thereby generating soybean phosphopeptides (SPP), and this was followed by conducting a binding reaction with calcium chloride. We have established an optimized procedure and reaction conditions for maximal phosphorylation and calcium binding. Consequently, the phosphorylation of soybean peptides resulted in considerable improvement in their calcium binding activities. Next, we demonstrated that SPP was able to render calcium ions resistant to precipitate formation with inorganic phosphates, which suggested the enhancement of calcium bioavailability. Finally, we noted that the addition of calcium-bound SPP induced an increase in cytosolic calcium concentration in the intestinal Caco-2 cells, due to an influx of calcium. These findings provide a new basis by which we may assess the possibility that SPP, as a potent calcium carrier, can be utilized in the prevention of poor absorption of dietary calcium in animals.