Surface Scintillation Measurements in Humans of the Uptake of Parenterally Administered Radioactive Vitamin B₁₂

1. The distribution of radioactive vitamin B₁₂ in humans was studied by scintillation counting of radioactivity over various skin projections of underlying organs in five individuals, following parenteral administration of radioactive vitamin B₁₂. The results of these investigations showed that the scintillation surface measurements of the radioactivity following parenteral administration of radioactive vitamin B₁₂ may be profitably applied to the study of the metabolic turnover of vitamin B₁₂ under normal and pathological conditions in humans.

2. In two young normal control subjects over 96 per cent of the parenterally administered radioactive vitamin B₁₂ (5 and 10 µg. containing 0.925 µc. of Co⁶⁰) disappeared from the site of the intramuscular injection within three to four hours, and during that time radioactivity rose to its peak value over the areas corresponding to kidney, spleen and iliac crest, and somewhat later over the muscles of the extremities. Subsequently, a gradual decline of radioactivity was observed over the spleen, kidney and extremities, so that at the end of three months in the normal subject from ⅓ to ½ of radioactivity as compared to peak values was observed over the spleen and left kidney and about ¼ over the calf muscle.

The projection areas of the liver differed from all the other areas of the body in requiring about five to six days to build up the peaks of radioactivity. These exceeded counts over the kidney and spleen about 2½ times, counts over the iliac crest approximately 7 times, and those over the calves about 17 times. The decline of radioactivity over the liver and iliac crest was very slow and small.

3. The patterns of the uptake of injected radioactive vitamin B₁₂ by the totally gastrectomized patient were grossly similar to those observed in normal controls, except for possibly slightly faster absorption of the injected material from the site of injection, and somewhat faster and larger accumulation, but also faster discharge of radioactivity from the liver area.

4. In a patient with pernicious anemia in partial remission who received three parenteral doses of 2 respectively 10 µg. radioactive vitamin B₁₂ (0.185-0.925 µc. Co⁶⁰), the uptake of the radioactive vitamin B₁₂ by the kidney and spleen area was slightly higher than that of the normal controls, that of the liver similar to the normal, and that over the iliac crest showing a significantly faster decline than in normals during the course of study. The uptake of radioactive vitamin B₁₂ in the same patient following intravenous injection of the same dose of radioactive vitamin B₁₂ was grossly similar to that following intramuscular injection of the same dose of radioactive material, except for the faster accumulation of radioactivity over the kidney area.

5. From 65 to 86 per cent of the peak radioactivity persisted over the liver 2-3 months after parenteral administration of radioactive vitamin B₁₂ to two control subjects. After five months 60 per cent of the initial peak of radioactivity was still observed over the liver in the patient with anemia of sprue in remission, 85 per cent in a patient with pernicious anemia in partial remission, and at the end of eight months still 35 per cent of the peak liver count in a patient with total gastrectomy without anemia. This does not take into account the normal decay of Co⁶⁰ which is about 1 per cent per month.

6. If the presence of Co⁶⁰ in the liver indicates the deposition of Co⁶⁰-B₁₂ in this organ, which is most probable, then the long storage of vitamin B₁₂ in the liver may explain the long time needed in humans for depletion of hepatic stores of vitamin B₁₂, as well as for long remissions observed in pernicious anemia following parenteral treatment with liver extracts or vitamin B₁₂.