Human ileal brush border membrane vesicles were prepared from intestines obtained from cadaveric renal allograft donors. The energetics and kinetics of taurocholate transport were studied. Fifty-five percent of equilibrium uptake (picomoles per mg protein) resulted from binding to the vesicle surface or incorporation into an osmotically insensitive compartment. The initial rate of transport was stimulated fourfold by an inwardly directed Na+ gradient when compared with a K+ gradient, and cation gradient-dependent differences persisted throughout the initial 5 min of incubation (p < 0.05). Taurocholate uptake was half-maximally stimulated by a Na+ concentration of 23 ± 4 mM. A Hill transformation of this plot gave a slope (n) of 0.97, indicating a 1:1 (mol/mol) Na+-taurocholate coupling ratio. Generation of a negative inside diffusion potential by anion substitution or valinomycin-induced K+ diffusion potential failed to alter bile salt uptake, suggesting an electroneutral transport mechanism. When Na+-dependent uptake velocity (10 s) was examined over a range of taurocholate concentration (0.036-0.9 mM), the plot described a rectangular hyperbola. The mean apparent Michaelis constant was 0.037 ± 0.007 mM and maximum velocity was 1093 ± 329 pmol taurocholate per milligram protein per 10 s. These data confirm and extend animal studies of ileal bile salt transport. Taurocholate uptake by the human ileal brush border occurs by a Na+-dependent, carrier-mediated electroneutral mechanism. According to this model, a single Na ion is coupled with a single taurocholate anion and transported across the brush border by a carrier mechanism that is driven by a transmembrane Na+ gradient.
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