[Study on changes in placental L-alanine transport activity during gestation (using microvillous membrane vesicles]

Using microvillous membrane vesicles prepared from human normal early and full term placenta, we investigated the transport mechanism of L-alanine and the change in its transport activity during gestation. We estimated the purity of microvillous membrane vesicles prepared from human placenta from the relative specific activities (microvilli versus homogenate) of the membrane's maker enzymes, alkaline phosphatase (ALP), 5'-nucleotidase, and gamma-glutamyltranspeptidase ( gamma-GTP). In early pregnancy (12-15 weeks gestational age), the relative specific activities (microvilli versus homogenate) were calculated to be: ALP: 15.3, 5'-nucleotidase: 14.0, gamma-GTP: 8.3, and in full term pregnancy (37-40 weeks gestational age) the relative specific activities (microvilli versus homogenate) were calculated to be: ALP: 16.0, 5'-nucleotidase: 14.8, gamma-GTP: 7.5. The uptake of L-alanine into microvillous membrane vesicles was Na+ electrochemical gradient (extravesicular greater than intravesicular) dependent and this Na+ dependent uptake was membrane-potentially sensitive both in early pregnancy and in full term pregnancy. The kinetics parameter of the initial L-alanine uptake into microvillous membrane vesicles were calculated to be: Km: 0.78 +/- 0.20 mM, Vmax: 0.62 +/- 0.21 nmol/mg protein/20 sec in early pregnancy, Km: 0.80 +/- 0.24 mM, Vmax: 3.53 +/- 0.70 nmol/mg protein/20 sec in full term pregnancy. In conclusion, the placental transport mechanisms of L-alanine in both early and full term pregnancy were the same, and the L-alanine transport activity of full term pregnancy was much greater than that of early pregnancy.