Friedman P A, Shia M A, Wallace J K
J Clin Invest. 1977 Jan;59(1):51-8. doi: 10.1172/JCI108621.
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.
开展了多项研究以评估维生素B12与人胎盘细胞膜制剂的结合情况。转钴胺素II - 维生素B12复合物（TCII - B12）对细胞膜的亲和力比单独的维生素B12大得多，它与一个单一的可饱和结合位点结合，其近似解离常数Ka = 7.2 mM-1。这种结合需要二价阳离子，且依赖于温度。游离的TCII可以与TCII - B12竞争结合位点，但亲和力比TCII - B12略低。大鼠的TCII - B12的亲和常数不到人类TCII - B12的五分之一；人类的TCI - B12、牛的TCII - B12、猪的内因子 - B12（IF - B12）以及人类的IF - B12均不与这些细胞膜结合。用胰蛋白酶预处理细胞膜会导致随后的结合显著减少；这表明结合位点包含一种相对暴露的膜蛋白。这些数据表明胎盘中存在TCII - B12复合物的特异性细胞表面受体。这种TCII - B12受体可用曲拉通X - 100溶解。
