Sodium-dependent transport of riboflavin in brush border membrane vesicles of rat small intestine is an electrogenic process.

Transport of riboflavin across the intestinal brush border membrane in rats was studied using the brush border membrane vesicle technique. Uptake of riboflavin into the membrane vesicles was predominantly the result of transport into an osmotically reactive space. Binding of the substrate to the membrane surface increased as a function of time and with lowering of the incubation buffer pH. The transport of riboflavin at low substrate concentrations was carrier mediated and Na+, but not K+, dependent with a distinct overshoot phenomenon. Uptake as a function of substrate concentration revealed dual transport characteristics in the presence of a Na+ gradient but linearity in the presence of a K+ gradient. The apparent Km value of the saturable, carrier-mediated and Na(+)-dependent component was calculated by two independent methods to be 0.25 or 0.38 mumol/L. Because uptake of riboflavin into brush border membrane vesicles in the presence of a Na+ gradient was increased by a valinomycin-induced K(+)-diffusion potential and by sodium thiocyanate vs. NaCl (intravesicular negative membrane potential), riboflavin transport is most likely an electrogenic process. The Na(+)-dependent riboflavin uptake into the vesicles was inhibited by structural analogues at low substrate concentrations only and increased by a low buffer pH.