Abstract
The specific cobalamin transport proteins chemically resemble their receptors in cell membranes. They are therefore assumed to have developed as gene duplications of their receptors. Since the transport proteins have a great tendency to form oligomers, their attachment to their receptors seems to be analogous to the formation of an oligomer. However, the theoretical possibility exists that the receptor is the transport protein itself, internalized in the cell membrane. The ileum from gastrectomized rats is here shown to contain the intrinsic factor receptor, which disproves this hypothesis about the ileal receptor. Free intrinsic factor was found in extracts from control rat intestinal mucosa but not in that of gastrectomized animals.
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