The binding of high and low density lipoproteins to human placental membrane fractions

Abstract

It was shown previously that human placental trophoblastic cells use principally lipoprotein cholesterol for progesterone biosynthesis and that the rate of de novo synthesis of cholesterol is low. In addition, it was demonstrated that cholesterol derived from maternal plasma low density lipoprotein (LDL) rather than high density lipoprotein (HDL), is the principal source of placental cholesterol. In the present investigation, membrane fractions derived from human placenta were used to identify and characterize specific binding sites for both HDL and LDL. Pretreatment of membrane fractions with heparin resulted in an increase in the specific binding capacity for [125I]iodo-LDL 1.5 times that in membrane fractions not pretreated with heparin. Heparin pretreatment did not affect significantly the specific binding capacity of placental membranes for [125I]iodo-HDL. The specific binding capacity for [125I]iodo-LDL was 107 ng LDL protein mg-1 membrane protein, with an approximate Kd of 77 microgram LDL protein ml-1 in membranes pretreated with heparin. The specific binding capacity for [125I]iodo-HDL was much greater, equal to 323 ng HDL protein mg-1 membrane protein, with an approximate Kd of 152 microgram HDL protein ml-1. Each [125I]iodolipoprotein was specifically displaced by the corresponding respective nonradiolabeled lipoprotein. Preincubation of membranes with trypsin and pronase caused reductions in the specific binding capacity for [125I]iodo-LDL of 88% and 100%, respectively. Incubation of membranes with heparin caused displacement of [125I]iodo-LDL. However none of these treatments affected [125I]iodo-LDL. However none of these treatments affected [125I]iodo-HDL binding capacity. Similar binding sites for "125I]iodo-LDL and [125I]iodo-HDL were demonstrated in cells prepared from human placenta by trypsin digestion and maintained in monolayer culture.