LINOLEIC-ACID TRANSPORT BY HUMAN PLACENTAL SYNCYTIOTROPHOBLAST MEMBRANES

The placenta syncytiotrophoblast is the site of exchange of nutrients, lipids and minerals between the mother and the fetus. In order to cha racterize the transport of fatty acids by the placenta, we purified bi polar syncytiotrophoblast brush border and basal plasma membranes from human placenta. These purified brush border and basal plasma membrane s enriched 3-fold and 22-fold, respectively, in sodium/potassium-ATPas e and 27-fold and 6-fold in alkaline phosphatase activity, compared wi th the placental homogenates. Fatty acid transport was performed at di fferent fatty acid/albumin ratios to evaluate the optimal uptake condi tions. The maximal transport efficiency, for linoleic acid bound to al bumin by sonication, was obtained with a 6:1 fatty acid/albumin ratio in brush border and basal plasma membranes. The linoleic acid transpor t observed with brush border membranes followed Michaelis-Menten kinet ics, with a Michaelis constant of 7.89 +/- 0.01 mu M and a maximal inc orporation rate of 30.80 +/- 6.39 pmol . mg(-1). min(-1). Linoleic aci d transport was very low in basal plasma membranes and we obtained a M ichaelis constant of 0.95 +/- 0.01 mu M and a maximal incorporation ra te of 1.62 +/- 5.06 pmol . mg(-1). min(-1). In order to show that lino leic acid accumulated within brush border and plasma membrane vesicles , and to eliminate the possibility of a non-specific binding of fatty acid to these membranes, we demonstrated by an osmolarity experiment, the decrease of the linoleic acid transport in brush border and basal plasma membranes obtained in the presence of 455 mu M essential fatty acid at 23 degrees C for 180 min. The results presented in this study suggest that linoleic acid is transported significantly by syncytiotro phoblast brush border membranes and basal plasma membranes. Thus, it m ay represent a unidirectional transport from mother to fetus through t he brush border membranes facing the mother, followed by transport at a slower rate through basal plasma membranes facing the fetus.