Abstract
Transcobalamin II is the blood cobalamin binding protein that delivers cobalamin to target cells via a receptor-mediated endocytosis. We have studied the receptor binding of human transcobalamin II in differentiated HT 29 cells. The on-rate constant and the off-rate constant were estimated at 5.8 nM −1 min −1 and 0.015 min −1, respectively. Scatchard analysis of the transcobalamin II-[ 57Co] cobalamin binding to HT 29 cells showed a Ka at 0.14 pM −1 and 31,000 receptor sites per 15 days-aged cell. The binding was inhibited by EDTA and nearly abolished at pH 5.0. The transcobalamin II-[ 57Co] cobalamin bound to plasma membrane was eluted as a high molecular cross-linked complex in Superose 6 gel filtration. Electron microscope radioautography showed the endocytosis of iodinated rabbit transcobalamin II in HT 29 cells. In conclusion, transcobalamin II receptor is present in HT 29 cells and may be involved in a receptor-mediated endocytosis of transcobalamin-cobalamin complex.
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