Changes in circulating transcobalamin II after injection of cyanocobalamin

Since transcobalamin II is the plasma binder of vitamin B12(cobalamin) that mediates delivery of cobalamin to tissues, we measured circulating levels of transcobalamin II after intravenous injection unlabeled cyanocobalamin in 12 control subjects and, because the liver is a major source of transcobalamin II, in eight patients with decompensated cirrhosis. Thirty minutes after injection of cyanocobalamin (200 ng per kilogram of body weight), total transcobalamin II (unsaturated plus cobalamin bound) declined in all 20 subjects to levels that averaged (+/- S.E.) 53.5 +/- 4.6 per cent of initial values. Subsequently, unsaturated transcobalamin II rapidly increased so that by eight hours, total transcobalamin II approached preinjection levels. Rates of regeneration of transcobalamin II were similar in control subjects and patients with liver disease. Unlike total transcobalamin II, total transcobalamin I was unaltered by injection of cyanocobalamin. Moreover, addition of cyanocobalamin to plasma in vitro did not alter measurements of total transcobalamin II. The rapid response of circulating transcobalamin II to cyanocobalamin injection strongly supports a dynamic role for this polypeptide during transport of a relatively large load of cobalamin.