Vitamin B12-binding proteins in serum and plasma in various disorders. Effect of anticoagulants.
Because of recent developments in the study of vitamin B12-binding proteins, the levels of the three serum binders were compared in serum and plasma samples from subjects with various disorders. The results allow the following conclusions: (1) As previously reported, transcobalamin (TC) III and to a lesser extent TC I are artifactually elevated in serum. The appear to be released in vitro during the clotting process, presumably from granulocytes. (2) Blood cells of patients with polycythemia vera release exceedingly large amounts of TC I and TC III in vitro. (3) The above findings support, but do not prove, at least a partial granulocytic source of TC I. Nevertheless, factors other than granulocytes influence TC I levels, as disorders characterized by increased TC I (most prominently chronic myelogenous leukemia but also several cases of cancer) manifest relatively little cellular release of TC I in vitro. (4) Despite the serum artifact, the serum abnormalities described in various conditions were seen in plasma also, even though the actual values of themselves were lower in plasma. The chief exception was TC III, which was elevated in plasma only in polycythemia vera (and in a few cases of leukocytosis). (5) EDTA-NaF anticoagulant is not suitable, as it causes plasma dilution, thus explaining previous reports of TC II level differences between serum and plasma. EDTA is therefore a preferable anticoagulant for vitamin B12-binding protein studies, although it too may not be ideal.