Abstract
1. 1. Transport of citrate and fumarate across the pig intestinal brush-border membrane (BBM) was investigated using isolated BBM vesicles. 2. 2. Citrate and fumarate uptake was stimulated by an inwardly directed Na + gradient consistent with Na +/citrate (fumarate) co-transport. Cis-inhibition and Irons -stimulation experiments strongly suggest the existence of a common transport site for tri- and dicarboxylates. 3. 3. The protonated forms of citrate (citrate −1, citrate −2) seem to be much better transported than citrate −3, indicated by the strong stimulation of citrate uptake at an extravesicular pH of 5.6 compared to pH 7.8. 4. 4. Uptake of tri- and dicarboxylates seems to be potential-sensitive since citrate and in particular fumarate transport was enhanced by an inside negative potential difference. 5. 5. Kinetics of succinate transport revealed a single carrier-mediated component with apparent kinetic constants of 0.43 nmol/mg protein-3 s ( V max ) and 0.14 mmol/1 ( K m ).
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