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Research Article Free access | 10.1172/JCI108621

A saturable high affinity binding site for transcobalamin II-vitamin B12 complexes in human placental membrane preparations.

P A Friedman, M A Shia, and J K Wallace

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Published January 1, 1977 - More info

Published in Volume 59, Issue 1 on January 1, 1977
J Clin Invest. 1977;59(1):51–58. https://doi.org/10.1172/JCI108621.
© 1977 The American Society for Clinical Investigation
Published January 1, 1977 - Version history
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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.

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