FORMS OF VITAMIN-B12 IN RADIOISOTOPE DILUTION ASSAYS

Since the presence of analogs of vitamin B12 (cobalamin, Cbl) has been postulated as the basis for the high values obtained by some radioisotope dilution assays (RIDA) of human serum Cbl, serum was examined for analogs. None could be demonstrated in the extracts of serum prepared for RIDA as sought by both direct and indirect techniques. The natural forms of serum Cbl were converted to cyanocobalamin by this process of extraction which included cyanide. The correctly performed RIDA for Cbl based on R binder gave higher values than a RIDA based on intrinsic factor or than Euglena gracilis bioassay. By exclusion, the difference appeared to be due to unidentified factors rather than the presence of analogs.