Vitamin B12-Binding Proteins of Man

Body proteins that bind vitamin B12 are now finding multiple uses in clinical medicine. Intrinsic factor in the intestinal tract and transcobalamins I and II in the plasma participate in normal vitamin B12 transport. Several other binding proteins have been identified in various cells and body fluids, but the reasons for their existence are not known. The absence of intrinsic factor, the basis for the vitamin B12 deficiency in pernicious anemia, is obviously useful in differential diagnosis. The different degrees of increase in transcobalamin I in myeloid metaplasia, chronic myeloid leukemia, and polycythemia vera are useful in the differential diagnosis of those diseases. In addition, there is a large amount of a binder of unknown function in the plasma of polycythemia vera. Some of the natural substances that bind vitamin B12 have been used as a reagent in isotope-dilution assays of the vitamin.