5-METHYLTETRAHYDROFOLATE TRANSPORT IN BASOLATERAL MEMBRANE-VESICLES FROM HUMAN LIVER

Transport of 5-methyltetrahydrofolate was studied in vesicles isolated from the basolateral membrane (BLM) of human liver. Uptake was mostly via transport into an osmotically active intravesicular space, with some membrane binding (almost-equal-to 20%). Transport was more rapid with an imposed pH gradient (pH(out) = 5.0, pH(in) = 7.5) as compared with either pH(out) = pH(in) = 5.0 or pH(out) = pH(in) = 7.5. Transport under the influence of a pH gradient showed a transient overshoot; uptake at 60 s was 4.2 times higher than at equilibrium (60 min). Transport in the presence of a pH gradient was saturable; apparent K(m) = 0.55 mumol/L and V(max) = 1.98 nmol.g protein-1.10 s-1. Transport was not saturable at pH(out) = pH(in) = 7.5. Transport was inhibited by the structural analogs 5-formyltetrahydrofolate, folic acid, and methotrexate, and was electroneutral in nature. These results suggest that 5-methyltetrahydrofolate transport in human BLM vesicles is via carrier-mediated cotransport with hydrogen ions.