Toadfish serum as a binder for in vitro assay of vitamin B12.

Serum from the oyster toadfish, Opsanus tau, has a binding capacity for cyanocobalamin 1,000 times greater than that of human serum. The binding follows the principle of isotope dilution in the physiologic range of vitamin B12 present in human serum. Under proper conditions of storage, this binder is stable for at least 1 year. Standard reagents and techniques used in other vitamin B12 competitive binding assays can be used with the toadfish serum binder. Toadfish serum offers potential advantages over intrinsic factor and human serum, the most commonly used binders in vitamin B12 assays.