The effect of cyanide on serum cobalamin and on cobalamin analogues when measured by radioassay using R-protein and intrinsic factor.

Concentrations of cobalamin measured in human serum by radioassay differ when R-protein or intrinsic factor is used as the binding determinant. Experiments were carried out to determine whether the concentration of cyanide used to extract the serum cobalamin could account for this difference. When either R-protein or intrinsic factor was used, the cobalamin measured in serum extracted in the absence of cyanide was low. As the concentration of cyanide was raised in the extraction mixture, the cobalamin concentration that was measured increased when either protein binder was used, but the absolute increase was greater with R-protein. R-protein or intrinsic-factor-reactive cobalamin could be recovered from serum proteins precipitated in the absence of cyanide and reextracted in the presence of cyanide. There was low recovery of hydroxo, methyl, or adenosyl cobalamins added to serum when the serum was extracted in the absence of cyanide, but recovery was quantitative when using either protein binder with cyanide in the extraction step. With 0.25 microgram of potassium cyanide/ml of extraction mixture and R-protein as the binding determinant in the radioassay, sera with normal, moderately low, and very low cobalamin concentrations could by separated. These experiments indicate that the concentration of cyanide used to extract cobalamin from the serum may account for some but not all of the differences observed when R-protein and intrinsic factor are used in the radioassay. The range of normal and low serum cobalamin are method-dependent and must be established by each laboratory for the radioassay procedure used.