PARTIAL CHARACTERIZATION OF FOLATE UPTAKE IN MICROVILLOUS MEMBRANE-VESICLES ISOLATED FROM HUMAN PLACENTA

The purpose of the present study was to determine biochemical paramete rs of folate uptake, and the putative contribution of the membrane-anc hored folate receptor in microvillous membrane vesicles obtained from the syncytiotrophoblast of human term placenta. Uptake of [H-3]-pteroy lglutamic acid (PGA) by microvillous membrane vesicles was pH dependen t with a maximum at pH 6.0, and attained equilibrium at 60 min of incu bation. Uptake was higher in the presence of an inward pH gradient (pH (out) = 6.0; pH(in) = 7.5) than in the absence of the gradient (pH(out ) = pH(in) = 6.0). The effect of changes in medium osmolality showed t hat both binding to the vesicular membrane and internalization contrib uted to the measured [H-3]-PGA uptake. Equilibrium uptake experiments using [H-3]-PGA concentrations within the physiological range of folat e in blood serum showed that saturation was achieved at 30 nM and reve aled a single class of binding sites with a Kd of 1.8 nM for [H-3]-PGA . Cleavage of the glycosyl-phosphatidylinositol moiety of the folate r eceptor, which anchors the receptor to the membrane, with phosphatidyl inositol-specific phospholipase C resulted in a reduction of about 80% in [H-3]PGA uptake. In conclusion, our results showed that the folate uptake in the maternally facing membrane of the human placenta presen ts a saturable component and is mediated by the folate receptor to ens ure an adequate maternal-fetal folate transfer.