The subcellular distribution of vitamin B12 was studied during its absorption in the guinea-pig ileum. Animals were fed either (57Co) or (58Co) cyanocobalamin and killed two or four hours later. At two hours labelled cyanocobalamin was concentrated in brush border and lysosomal fractions of ileal homogenates, with some remaining in the sample layer. In contrast, at four hours labelled cyanocobalamin was concentrated predominantly in the cytosol with much smaller peaks in the brush border and lysosomal fractions. These findings are consistent with vitamin B12 absorption by receptor-mediated endocytosis. It is suggested that, after binding at the brush border, vitamin B12 is first sequestrated within lysosomes, and then released into the cytosol, from where it leaves the cell to enter the portal blood.
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