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

doi:10.1073/pnas.0601251103

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

Departments of *Nutrition and
^‡Molecular Biosciences,
^‖Department of Entomology and Cancer Research Center, and
**Section of Microbiology, University of California, Davis, CA 95616-5270;
^†Vitalea Science, Woodland, CA 95776;
^§Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, CA 94551-9900; and
^¶Department of Medical Pathology and Laboratory Medicine, University of California, Sacramento, CA 95817

Contributed by John R. Roth, February 22, 2006

Abstract

There is a need for an improved test of human ability to assimilate dietary vitamin B₁₂. Assaying and understanding absorption and uptake of B₁₂ 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 (¹⁴C)-labeled substances after oral ingestion because it is more sensitive than decay counting and can measure levels of ¹⁴C in microliter volumes of biological samples with negligible exposure of subjects to radioactivity. The test we describe employs amounts of B₁₂ in the range of normal dietary intake. The B₁₂ used was quantitatively labeled with ¹⁴C 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 B₁₂ or two of its precursors, cobinamide and DMB. When provided with ¹⁴C-DMB specifically labeled in the C2 position, cells produced ¹⁴C-B₁₂ 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 μg, 2.2 kBq/59 nCi) of purified ¹⁴C-B₁₂ 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 B₁₂ assimilation.

Footnotes

^††To whom correspondence may be addressed. E-mail: pan{at}ucdavis.edu or jrroth{at}ucdavis.edu
Author contributions: C.C., S.R.D., J.W.M., R.G., and P.J.A. designed research; C.C., S.R.D., J.L., B.A.B., and P.J.A. performed research; C.C., S.R.D., and P.J.A. contributed new reagents/analytic tools; C.C., S.R.D., J.L., B.A.B., J.W.M., R.G., B.D.H., J.R.R., and P.J.A. analyzed data; and S.R.D., J.R.R., and P.J.A. wrote the paper.
Conflict of interest statement: A U.S. patent entitled “Assay for Vitamin B₁₂ Absorption and Method of Making Labeled Vitamin B₁₂” has been filed by P.J.A., S.R.D., J.W.M., R.G., J.R.R., C.C., and B.A.B.
Abbreviations:

Abbreviations:

AMS,

accelerator MS;

DMB,

dimethylbenzimidazole;

Th,

Thompson.