Characteristics of the Microvillus Brush Border of Human Placenta: Insulin Receptor Localization in Brush Border Membranes*

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Abstract
Insulin receptor characteristics were examined in purified brush border membrane from the syncytiotrophoblast of the normal human placenta and quantified during membrane preparation. Insulin receptor concentration was enriched 10- to 15-fold in this preparation, and insulin receptor specific activity followed closely the enrichment values for microvillus plasma membrane markers, alkaline phosphatase, Ca2+- and Mg2+-ATPase, and 5′-nucleotidase during cell fractionation. Insulin receptor concentrations and marker enzyme analyses were compared in whole homogenate, mitochondrial, microsomal, and microvillus fractions, and these fractions were characterized by SDS-gel electrophoresis. Microvillus insulin receptor interactions were dependent on time, [126I]iodoinsulin concentration, protein, and unlabeled hormone concentration's. Competition studies with porcine insulin and [125I]iodoinsulin for this receptor revealed a curvilinear Scatchard plot. Insulinase was demonstrated at 37 C but was minimal at 24 C in the microvillus fraction. Electron microscopy of the microvillus membrane preparation revealed its composition to be mainly spherical closed membrane vesicles and brush border fragments. Sodium dodecyl sulfate-polyacrylamide and isoelectric focusing gels of membrane fractions were compared. Actin was tentatively identified as a major microvillus membrane protein and was further fractionated: βActin and γ-actin were present in approximately equal concentrations. The localization of the insulin receptor in the microvillus brush border of the human placenta suggests that this receptor interacts with maternal, rather than fetal, insulin.