Identification of specific binding sites for acetylated low density lipoprotein in microvillous membranes from human placenta.

The ability of microvillous membranes isolated from human placenta to specifically bind human low density lipoprotein (LDL) modified by acetic anhydride has been investigated. The presence of saturable high affinity binding sites specific for [125I]acetyl-LDL was demonstrated. Scatchard analysis of the binding data, obtained at 4 degrees C, revealed a single class of sites with a mean KD value of 3.63 +/- 1.16 micrograms acetyl-LDL protein/ml, and a maximal binding capacity of 335.1 +/- 148.8 ng acetyl-LDL protein/mg of membrane protein. At 37 degrees C, the binding capacity was increased, while the KD value was not modified. The specificity of these binding sites was assessed by competition studies: unlabelled acetyl-LDL were effective competitors, whereas native LDL, VLDL and HDL3 were ineffective. Conversely, unlabelled acetyl-LDL failed to prevent the binding of native [125I]LDL to placental microvilli. The [125I]acetyl-LDL binding was partially inhibited (about 35%) by dextran sulfate and fucoidin, and was abolished by a pretreatment of the microvillous membranes with pronase. The binding sites specific for acetyl-LDL are present during all the gestation and are distinctly different from the binding sites for native LDL, previously characterized in placental microvilli. These 2 types of binding sites may be related to the high amount of cholesterol required by the human placenta for progesterone synthesis and trophoblastic growth.