High-affinity binding is essential for enhancement of intestinal Fe2+ and Ca2+ uptake by bile salts

Both calcium and iron are bound with high affinity by premicellar bile salts having cholanic ring 7-OH and/or 12-OH groups, forming soluble cation-bile salt complexes. The authors of the current study recently showed that premicellar taurocholate markedly enhances intestinal iron and calcium uptake. However, the relationship of high-affinity binding to the observed uptake enhancement was unknown. In the current study, this relationship was examined by studying taurodehydrocholate (TDHC) binding and intestinal uptake of both cations. Ca2+ binding was measured by noting depression of [Ca2+] activity in solutions containing constant total Ca concentrations (1 mmol/L) and varying [TDHC] (0.5-50 mmol/L). Fe2+ binding was assessed by equilibrium dialysis studies of 59FeSO4 (0.179-1.79 mmol/L) and TDHC (0.5-50 mmol/L). Effects of TDHC on intestinal Fe2+ and Ca2+ uptake were measured in isolated perfused intestinal segments in vivo in seven and eight Sprague-Dawley rats, respectively. TDHC, lacking ring OH groups, did not bind either cation with high affinity and had no effect on their intestinal uptake. These results suggest that high-affinity binding is essential for bile salt-induced enhancement of intestinal Fe2+ and Ca2+ uptake.