Macromolecule transfer in the human trophoblast: transcobalamin II-vitamin B12 uptake.

The accumulation of the large and hydrophilic IgG, TC II-B12 and B12 molecules is demonstrated for the first time in a human placental system which has metabolic and physiological functions. A trypsin-sensitive component is present in the human term placental uptake of TC II-B12, for which a placental membrane receptor has been previously identified; this component is absent for the accumulation of free B12, which has no known receptor. Analyses of the cytosol and incubation media indicate degradation, binding and release of TC II-B12 and B12 as TC II-B12, free B12 and TC I-like complexes. It is suggested that the human placental tissue slice be used for studies involving the binding, uptake and processing of macromolecules as exemplified by TC II-B12.