Biological Half-life of Vitamin B₁₂ in Plasma

Abstract

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.