BY kinetic analysis of isotope experiments using tracer doses of radioactive vitamin B12, Reizenstein1 has deduced that the biological half-life of vitamin B12 in the plasma is about 6 days. Confirmation of this finding has been obtained by a different approach. Working with injected doses of radioactive vitamin B12 in the range 50–1,000 µg, a linear relationship with a coefficient of correlation differing significantly from zero was found between the logarithm of the amount of injected vitamin B12 retained in the body and the time in days for the serum vitamin B12 level to fall to the pre-injection level, the lower limit of normal, in patients with pernicious anæmia2. The regression equation was Y= 17.0886x − 0.4270 when Y = the time in days and x = the logarithm of the retained dose. From this information it can be calculated that the biological half-life in the plasma of the radioactive vitamin B12 retained in the body was 5.14 days and that the velocity constant (k) is 0.1347 days−1. The closeness of the estimates is satisfying particularly as they were obtained by totally different experimental methods. The short biological half-life in plasma contrasts sharply with that in the liver, which has been calculated to average about 12 months.
Reizenstein, P. G., Acta Med. Scand., 165 (1959).
Adams, J. F., Vitamin B 12 and Intrinsic Factor; Second European Symp., Hamburg, 628 (1962).
Glass, G. B. J., J. Lab. Clin. Med., 52, 875 (1958).
Schloesser, L. L., Deshpande, P., and Schilling, R. F., Arch. Int. Med., 101, 306 (1958).
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ADAMS, J. Biological Half-life of Vitamin B12 in Plasma. Nature 198, 200 (1963). https://doi.org/10.1038/198200a0