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 (approximately 20%). Transport was more rapid with an imposed pH gradient (pHout = 5.0, pHin = 7.5) as compared with either pHout = pHin = 5.0 or pHout = pHin = 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 Km = 0.55 mumol/L and Vmax = 1.98 nmol.g protein-1.10 s-1. Transport was not saturable at pHout = pHin = 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.