1. The long-term clearance of radioactive vitamin B 12 from serum and from whole body was measured in control subjects and patients with treated pernicious anaemia and the data were analysed by the occupancy principle to provide estimates of dietary intake, daily requirements and whole-body mass of vitamin B 12 . 2. In six cases the daily requirement for vitamin B 12 ranged from 0·15 to 1·9 μg with an average of 1·0 μg. The whole-body vitamin B 12 in four cases ranged from 780 to 1350 μg with an average of 1060 μg. 3. Estimates of the whole-body vitamin B 12 on the assumption of single-pool equilibrium gave constantly changing results, which, together with discrepant half-lives for serum radioactivity and whole-body radioactivity, constitute evidence against the concept of single-pool equilibrium.
1. The transcobalamins from normal serum were obtained in two fractions. One contained transcobalamin I and transcobalamin III: the other contained transcobalamin II. The forms of vitamin B 12 in the two fractions were then examined. 2. Methylcobalamin and adenosylcobalamin were found in both fractions. Hydroxocobalamin was found in the fraction containing transcobalamin I and transcobalamin III. Cyanocobalamin was found in both fractions in two cases, in the transcobalamin III fraction only in one case and was absent in one case.
1. The absorption of radioactive cobalamin was measured by a whole-body counting technique in control subjects and in patients with pernicious anaemia. 2. The absorption of cyanocobalamin by patients with pernicious anaemia was decreased by charcoal but not by bile or saliva. 3. The absorption of cyanocobalamin by control subjects was not affected by food but was significantly increased by pentagastrin. With pentagastrin the absorption of cyanocobalamin was significantly greater than that of hydroxocobalamin. The hog intrinsic factor-mediated absorption of cyanocobalamin by patients with pernicious anaemia was significantly depressed by pentagastrin. 4. The effect of increasing the mass of hog intrinsic factor concentrate on the absorption of cyanocobalamin by patients with pernicious anaemia could be described by a function relating the amount absorbed, the mass of intrinsic factor and two constants. The relationship implies that when the mass of intrinsic factor is small the amount of cyanocobalamin absorbed is directly proportional to the mass of intrinsic factor but that absorption approaches a saturation value with increasing mass of intrinsic factor. In physiological terms the function implies that absorption is proportional to the amount of cyanocobalamin attached to receptor sites but that cyanocobalamin attached to receptor sites may become detached and either reattached or lost to absorption. 5. With oral doses of 25 μg and 50 μg, control subjects absorbed more cyanocobalamin and hydroxocobalamin than patients with pernicious anaemia. At both dose levels control subjects absorbed more cyanocobalamin than hydroxocobalamin but no difference was observed in patients with pernicious anaemia. The intrinsic factor mechanism therefore influences amounts absorbed at such dose levels and appears to be a factor in the differences in absorption of cyanocobalamin and hydroxocobalamin. 6. The use of double-tracer techniques makes it possible for each subject to act as his own control in studies of vitamin B 12 absorption. The value of this technique is stressed.
1. Values for total body vitamin B 12 were calculated for eighteen patients by giving a tracer dose of radioactive cyanocobalamin and measuring the radioactivity and microbiological activity in liver biopsies obtained at laparotomy. 2. The values for total body vitamin B 12 ranged from 960 to 5984 μg with a mean of 2528 μg. Five values were greater and thirteen less than the mean. 3. Significant correlations were found between the serum vitamin B 12 and total body vitamin B 12 , the serum vitamin B 12 and the hepatic vitamin B 12 and the hepatic B 12 and the total body vitamin B 12 .