An endogenous carrier-mediated uptake system for folate in oocytes of Xenopus laevis

We investigated the existence of an endogenous uptake system for folate in Xenopus laevis oocytes. This was done by performing uptake measurements using [3H]folic acid. Uptake of folic acid was linear with time for 4 h of incubation, and was similar in collagenase-treated and non-treated oocytes. The uptake process was carrier-mediated, as suggested by the saturation of folic acid uptake with concentration, and by the ability of unlabelled folic acid and its related compounds to significantly inhibit the uptake of [3H]folic acid. The apparent Km and Vmax of the uptake process were 42 +/- 7 nM and 10.56 +/- 0.46 fmol per oocyte per 2 h, respectively. The uptake of folic acid was independent of the presence of Na+ in the incubation medium, but was highly pH dependent with severe inhibition occurring at pH lower than 6.5. Folic acid uptake was energy- and temperature-dependent, and was significantly inhibited by the anion transport inhibitors DIDS and SITS. These results demonstrate the existence of an endogenous carrier-mediated system for folic acid uptake in Xenopus oocytes. Further characterization of the molecular mechanism of folic acid uptake and its regulation in this non mammalian in vitro unicellular system may prove useful in furthering our understanding of folate movement across biological membranes.