title: A saturable high affinity binding site for transcobalamin II-vitamin B12 complexes in human placental membrane preparations.
abstract: Studies were designed to evaluate the binding of binding of vitamin B12 to cell membrane preparations from human placenta. The transcobalamin II-vitamin B12 complex (TCII-B12), which has a much greater affinity for the membranes than vitamin B12 alone, binds to a single saturable binding site with an approximate Ka = 7.2 mM-1. The binding requires a divalent cation and is temperature-dependent. Free TCII can compete with TCII-B12 for the binding site but has somewhat less affinity than does TCII-B12. Rat TCII-B12 has an affinity constant that is less than one-fifth that of human TCII-B12; human TCI-B12, bovine TCII-B12, hog intrinsic factor-B12 (IF-B12), and human IF-B12 do not bind to the membranes. Pretreating the membranes with trypsin causes a marked decrease in subsequent binding; this suggests the binding site includes a relatively exposed membrane protein. These data suggest that a specific cell surface receptor for the TCII-B12 complex exists in placenta. This TCII-B12 receptor can be solubilized with Triton X-100.
mesh_terms: D001665:Binding Sites; D001798:Blood Proteins / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism*; D005260:Female; D006801:Humans; D010920:Placenta / Q000166:cytology*; D011247:Pregnancy; D011955:Receptors, Drug; D014155:Transcobalamins / Q000378:metabolism*; D014805:Vitamin B 12 / Q000378:metabolism*
chemical_list: D001798:Blood Proteins; D011955:Receptors, Drug; D014155:Transcobalamins; D014805:Vitamin B 12