title: Human placental microvilli contain high-affinity binding sites for folate.
abstract: Uptake of [3H]pteroylglutamic acid [( 3H]PteGlu) was studied in microvilli isolated from the syncytiotrophoblast of the human term placenta. The effect of changes in medium osmolality on the equilibrium uptake of [3H]PteGlu was negligible, which suggested that the observed uptake represented binding to proteins on or within the microvilli rather than translocation of the vitamin from the incubation medium to a free state in the intravesicular fluid. Equilibrium uptake experiments performed over a wide range of [3H]PteGlu concentrations disclosed a class of binding sites with an association constant of 0.3 nM-1 as well as a second class of sites with high capacity and low affinity. Binding of [3H]PteGlu at the high-affinity sites was inhibited by tetrahydrofolate and N5-methyltetrahydrofolate, but not by several other structural analogues. It is likely that the high-affinity binding sites are receptors for maternal plasma folate; however, their role in placental transport or storage of the vitamin was not delineated in these studies.
mesh_terms: D000621:Aminoisobutyric Acids / Q000378:metabolism; D001665:Binding Sites; D003902:Detergents / Q000494:pharmacology; D005260:Female; D005492:Folic Acid / Q000031:analogs & derivatives / Q000378:metabolism* / Q000494:pharmacology; D006801:Humans; D066298:In Vitro Techniques; D008871:Microvilli / Q000187:drug effects / Q000378:metabolism; D009994:Osmolar Concentration; D010920:Placenta / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D011247:Pregnancy; D012964:Sodium / Q000378:metabolism
chemical_list: D000621:Aminoisobutyric Acids; D003902:Detergents; C100049:2-aminoisobutyric acid; D005492:Folic Acid; D012964:Sodium