Low-density lipoprotein binding sites in the microvillous membranes of human placenta at different stages of gestation

Abstract

Microvillous membranes isolated from early gestation placentas (8–12 weeks of amenorrhoea) and from mid-term placentas (20–22 weeks of amenorrhoea) were used to study the specific binding of low-density lipoprotein (LDL) to the trophoblast. The purity of the microvillous preparations has been assessed by electron microscopy and by their enrichment in two membrane markers, 5'-nucleotidase and alkaline phosphatase. Evidence was presented demonstrating the existence of saturable binding sites for [ 125I]LDL in placental microvilli as early as 6 weeks of pregnancy. The apparent K D values for these binding sites have been determined by Scatchard analyses to be 6.98 ± 0.83 and 6.57 ± 0.81 μg protein LDL/ml, for early gestation and mid-term preparations, respectively. This apparent K D value was unaffected by a pretreatment of the membranes by heparin, as indicated by the mean values of 7.13 ± 0.89 and 6.97 ± 0.75 μg protein LDL/ml obtained for immature microvilli preincubated with or without heparin, respectively. Large variations of binding capacity were observed in each gestational age group and no significant difference was found between them. These results indicate that the LDL binding sites of the human placenta, located on the microvillous membranes, (i) are present as early as the 6th week of pregnancy, and (ii) display the same high affinity and specificity for LDL as those of the term trophoblast.