Preferential uptake of long chain polyunsaturated fatty acids by isolated human placental membranes

Fatty acid uptake by the placenta is thought to be a carrier-mediated process, however the mechanism by which long chain polyunsaturated fatty acids (LCPUFA) are preferentially accumulated from the maternal circulation to the fetal tissues is still unclear. To examine the role of the placenta in this process, binding of four different radiolabelled fatty acids (-14C-oleate, -14C-linoleate, [14C]a-linolenate and [14C]arachidonate) to human placental membranes was studied. Binding of fatty acid was found to be time- and temperature dependent. At equilibrium, the total binding of oleate was highest (5.1 +/- 0.1 nmoles/mg protein) followed by linoleate (2.8 +/- 0.31 nmoles/mg protein) and arachidonate (2.06 +/- 0.4 nmoles/mg protein) and alpha-linolenate binding was lowest (0.5 +/- 0.1 nmoles/mg protein). However, oleate had the lowest specific binding (37% of the total binding) whereas arachidonate had the highest specific binding (approximately 86% of the total binding) followed by linoleate and a-linolenate (62%, and 69% of the total binding, respectively). Binding of each [14C] fatty acid was also assessed in the presence of 20-fold excess of other unlabelled ligands. Binding sites seem to have preference for the binding of [14C] fatty acids in the following order: arachidonic acid >>> linoleic acid >> a-linolenic acid >>>>> oleic acid, whereas BSP and a-tocopherol did not show any competition with any of the [14C] fatty acids. These data suggest that the fatty acid binding sites in placental membranes are specific for the fatty acids but that they have heterogeneous affinities. Trans fatty acids (elaidic and linoelaidic acids) also competed very strongly for the [14C] fatty acid binding. Polyclonal antiserum raised against placental FABPpm inhibited binding of these [14C] fatty acids but with variable degrees of inhibition; EFA/LCPUFA binding was much more than that of oleate. Our data suggest that EFA/LCPUFA bound to albumin are preferentially transported by human placental membranes and that the placental FABPpm may be involved in the sequestration of EFA/LCPUFA by the placenta.