[Study on placental L-glutamine transport mechanism using microvilli vesicles]

Using microvillous (brush border) membrane vesicles prepared from human term placenta, the uptake of L-glutamine was studied using a rapid filtration technique. The uptake of L-glutamine into the vesicles was osmotically sensitive. A Na+ electrochemical gradient (extravesicular greater than intravesicular) stimulated the initial rate of L-glutamine uptake and the Na+ dependent uptake of L-glutamine into vesicles showed a typical overshoot phenomenon. This overshoot and the initial rate of uptake were markedly increased when the intravesicular space was rendered electrically more negative by membrane diffusion potentials, induced by the use of highly permeant anions. A similar stimulation of L-glutamine uptake was observed when membrane potential (inside negative) was imposed by K+ diffusion potentials via valinomycin. These results indicated that a sodium dependent uptake of L-glutamine into the microvillous membrane vesicles was dependent on the electrical potential difference of membrane. The initial rate of L-glutamine transport exhibited saturation kinetics with respect to L-glutamine concentration; the apparent Km of 0.42 mM and Vmax of 1.54 nmol/mg protein/20 sec were calculated. The uptake of L-glutamine into the vesicles was competitively inhibited by L-alanine.