Congenital disorders of vitamin B12 transport and their contributions to concepts. II.

Congenital deficiencies of Transcobalamin II (TC II) and R binders of vitamin B12 (B12, cobalamin, Cbl) have been described in several families. The deficiency of TC II exists as at least three variants. The deficiency of TC II is expressed by a profound megaloblastic pancytopenia during the first few weeks of life, but the serum Cbl is normal. In contrast, the deficiency of R binder is asymptomatic, tissues are replete in Cbl, but the serum Cbl is low. All of the R binder in the several body sources is under the same genetic control. Studies of the congenital deficiency TC II suggest the following: (1) The function of TC II is the promotion of cell uptake of physiologic amounts of Cbl, which can also be accomplished by very large amounts of Cbl, and not in any intracellular process. (2) TC II is essential for the absorption, postabsorptive distribution, and recycling of TC II. (3) The metabolic consequences of TC II deficiency are expressed primarily in rapidly dividing cells probably because they are dependent upon the constant need for new Cbl.