Preparation of Radioactive Labelled (57Co) Sulfitocobalamin

Farquharson and Adams (Br. J. Nutr. 36, 127-135 (1976)) have identified sulfitocobalamin (S0 3 −Cbl) as one of the naturally occurring cobalamins (Cbls) in foods. We have devised a method of making radioactive labelled S0 3 −Cbl for invivo and in vitro studies of this form of Cbl. 57 Co labelled cyanocobalamin ( 57 Co CN-Cbl) was acid photolyzed to 57 Co hydroxocobalamin ( 57 Co OH-Cbl) followed by ligand substitution with S0 3 −2 ion from aqueous sodium (meta) bisulfite in the dark. The resulting 57 Co SO 3 −Cbl was purified by organic extraction and cation ex-change chromatography. The final preparation was >99% Co 57 S0 3 −Cbl with an overall yield of >70%, stable for up to four weeks at 20°C in the dark, and capable of binding to the human Cbl binding proteins Transcobalamin II (TC II), Intrinsic factor (IF) and Salivary R. This method allows a simple 1 day preparation of high specific activity labelled 57 Co S0 3 −Cbl for biological studies.