[Placental transport of taurine in brush border microvilli].

Placental transport of taurine was studied in isolated brush border microvillous plasma membrane vesicles by a rapid filtration technique. Brush border microvillous plasma membrane vesicles were prepared from syncytio trophoblast of human term placenta by a method of differential centrifugation and calcium precipitation. The specific activities of alkaline phosphatase, 5' nucleotidase and gamma-GTP in the membrane preparation were enriched to 13-14 times, 12-13 times, and 5-6 times respectively, as high as those in the homogenate. The membrane vesicles exhibit uptake of 3H-labeled taurine into an osmotically reactive intravesicular space. Taurine uptake by vesicles was stimulated specifically by an inward sodium gradient, and replacement of NaCl in the transport medium by KCl, LiCl, and choline chloride had no effect on the transport activity of the vesicles. Taurine transport is inhibited competitively by the presence of beta alanine and GABA. The initial rate of transport followed saturation kinetics with respect to the taurine concentration: An apparent Km of 0.22mM and Vmax of 67 pmol/mg protein were calculated in 20 seconds. These results indicate that transport of taurine across the placental brush border membrane is sodium dependent and carrier mediated.