[Study on human placental beta-alanine and taurine transport mechanism (using microvillous membrane vesicles].

Using microvillous membrane vesicles prepared from human full term placenta, we studied the placental beta-amino acid transport mechanism. The transport of amino acids into microvillous membrane vesicles was studied by a filtration technique using a millipore filter. The uptake of beta-alanine into microvillous membrane vesicles was dependent on Na+ electrochemical gradient (extravesicular greater than intravesicular). The initial rate of this Na+ gradient dependent beta-alanine transport exhibited saturation kinetics with respect to the beta-alanine concentration: an apparent Km of 0.24 mM and Vmax of 46 pmol/mg protein/20 sec were calculated. Taurine inhibited beta-alanine uptake into microvillous membrane vesicles, but on the other hand L-alanine didn't inhibit this beta-alanine uptake. The L-alanine uptake into microvillous membrane vesicles was Na+ electrochemical gradient dependent and the initial rate of this Na+ dependent L-alanine uptake into vesicles was faster than the uptake of Na+ itself into vesicles. On the other hand, the initial rate of Na+ dependent beta-alanine and taurine uptake into vesicles was slower than the uptake of Na+ itself into vesicles. These results indicated that there existed a beta-amino acid specific transport system in human placental microvillous membrane, and placental taurine transport was carried out by this system. And it was also indicated that this placental beta-amino acid transport mechanism is quite different from that of L-alanine.