An isotopic method for the determination of vitamin B12 levels in blood.

By RUSSEL M. BARAKAT AND ROGER P. EKINS F OR A PERIOD of about a decade, microbiological assay with Euglena gracilis1 and other microorganisms2 has been accepted as a standard method of vitamin B,2 determination in serum and other body fluids. However, this method is time-consuming, entails many sources of error and is not fully reliable. Other methods of assay3 require relatively large quantities of material which preclude their application to the determination of the concentrations found in serum. With the introduction of technics for labeling the vitamin at high specific activity with radioactive isotopes of cobalt and the reported observations4#{176} of specific vitamin B,2 binding protein in normal plasma, the basis has been established for the development of an alternative technic possessing sensitivity sufficient for the determination of blood levels of this compound. The fundamental approach employed by the authors has been that of “saturation analysis.”7 Recently, this technic has been applied to the assay of serum concentrations of thyroxme,5 and, in slightly modified form, to the assay of serum insulin.9’#{176} The assay may be considered to consist of two stages: 1 ) Extraction of the vitamin from the plasma under investigation ( henceforth referred to as the “test” plasma ) after heat coagulation of plasma pro-