Intrinsic factor-mediated modulation of cyanocobalamin–N-sulfonyl-acridinium-9-carboxamide chemiluminescence

Two cyanocobalamin-N-sulfonyl-acridinium-9-carboxamides with linkage through the N(10) or 9-position were prepared from B(12)-e-carboxylic acid. The noncovalent association of intrinsic factor with these ligands resulted in specific modulation of the associated chemiluminescence signal either by quenching or changing the emission profile. Either effect was sufficient to formulate a homogeneous assay to detect vitamin B(12) in buffer.

The transport of vitamin B12 through polarized monolayers of Caco-2 cells

Caco-2 cells grown on 0.45-micron filters, in Millicell chambers, form intact monolayers with many of the properties of polarized intestinal epithelial cells. It is reported here that these cells bind and internalize intrinsic factor-cobalamin complexes and that after 14-28 days in culture this specific binding is exclusively located on the apical membrane. Caco-2 cells also synthesize and secrete a protein with properties similar to transcobalamin II. This protein is secreted from the basolateral side of the cells after 20 days in culture. Specific apical-to-basolateral transcellular transport of [57Co]cobalamin also occurs between 20 and 28 days in culture. Thus, Caco-2 cells provide the first polarized human cell system for studying the transepithelial transport of cobalamin.