The study of placental L-ascorbate (vitamin C) transport mechanism (using microvillous membrane vesicles)

To investigate the placental L-ascorbate (Vitamin C) transport mechanism, the uptake of L-ascorbate into microvillous membrane vesicles prepared from human term placenta was studied using the rapid filtration technique. The uptake of L-ascorbate into microvillous membrane vesicles was osmotically sensitive. This finding indicated that the uptake of L-ascorbate into microvillous membrane vesicles represented transport into the vesicles. The uptake of L-ascorbate into microvillous membrane vesicles was not dependent on sodium electrochemical gradient. The initial rate of uptake was not changed when the intravesicular space was rendered electrically more negative by membrane diffusion potential induced by the use of highly permeant anions. The initial rate of L-ascorbate transport exhibited saturation kinetics with respect to the L-ascorbate concentration; an apparent Km of 1.33 mM and Vmax of 47p mol/mg protein/20 sec was calculated. The uptake of L-ascorbate into microvillous membrane vesicles was competitively inhibited by D-isoascorbate. These results indicated that transport of L-ascorbate across the placental microvillous membrane vesicles was carrier mediated and was passive transport.