Human vitamin B12 absorption measurement by accelerator mass spectrometry using specifically labeled 14C-cobalamin

There is a need for an improved test of human ability to assimilate dietary vitamin B-12. Assaying and understanding absorption and uptake of B-12 is important because defects can lead to hematological and neurological complications. Accelerator mass spectrometry is uniquely suited for assessing absorption and kinetics of carbon-14 (C-14)-Iabeled substances after oral ingestion because it is more sensitive than decay counting and can measure levels of C-14 in microliter volumes of biological samples with negligible exposure of subjects to radioactivity. The test we describe employs amounts of B-12 in the range of normal dietary intake. The B-12 used was quantitatively labeled with C-14 at one particular atom of the dimethylbenzimidazole (DMB) moiety by exploiting idiosyncrasies of Salmonella metabolism. To grow aerobically on ethanolamine, Salmonella enterica must be provided with either preformed B-12 or two of its precursors, cobinamide and DMB. When provided with C-14-DMB specifically labeled in the C2 position, cells produced C-14-B-12 of high specific activity (2.1 GBq/mmol, 58 mCi/mmol) (1 Ci = 37 GBq) and no detectable dilution of label from endogenous DMB synthesis. In a human kinetic study, a physiological dose (1.5 mu g, 2.2 kBq/59 nCi) of purified C-14-B-12 was administered and showed plasma appearance and clearance curves consistent with the predicted behavior of the pure vitamin. This method opens new avenues for study of B-12 assimilation.