Cobalamin analogues in plasma. An in vitro phenomenon?

In the course of investigations on the source of cobalamin analogues in human plasma six kits for measuring serum cobalamins were studies to determine if analogues can arise in vitro. High pressure liquid chromatography (HPLC) were used to estimate tracer stability during extraction procedures. Results showed that up to 60 percent of the radioactive cyanocobalamin was degraded. Alkaline pH increases the degradation; this effect is further intensified by addition of dithiothreitol. Implications for the cobalamin assay is discussed. Determination of cobalamin analogues by the difference of concentrations determined from assays using haptocorrin ("total corrinoids") and instrinsic factor ("true cobalamins") as binders may be misleading because the affinity of possible analogues for the assay binder is unknown. Thus quantification of the analogues could be erroneous. The need for definitive assay methods for cobalamin and its analogues is stressed.