Taurine transport in human placental microvillous membrane vesicles

Taurine, a βamino acid, is actively transported across the in vitro perfused human placenta. To further characterize the transport mechanism microvillous membrane vesicles (MVV) were prepared from the apical brush border of human term placentae by Mg^{2 +}-aggregation and differential centrifuga-tion. Vesicular uptake of taurine was measured by rapid filtration technique at 25^oC using [U-¹⁴C] taurine. All data are expressed as mean ± SEM. The course of uptake over time in the presence of an inward Na⁺ gradient (n = 4) revealed an "overshoot" phenomena. When Na⁺ was replaced with K⁺ in the incubation medium (n = 4), the overshoot was abolished and the uptake rate was sharply reduced. Uptake velocities at taurine concentrations from 1 to 35 μM were studied (n = 6) showing that taurine uptake was saturable and the K_m was 12.1 μM and the V_max was 36.2 pmol/mg protein/min. Hypotaurine, βalanine, and taurine all inhibited the uptake of taurine by MVV (n = 4). Taurine is actively transported into MVV from human term placentae by a Na⁺-dependent carrier that is inhibited by βamino acids, and therefore conforms to the βcarrier described in other tissues.