Isotopic Determination of Vitamin B12 Binding Capacity and Concentration.

Abstract

A method for measuring additional binding capacity (ABC) of Vit. B12 in serum with the aid of radio-B12 is described. Fixed amounts of untreated serum (fresh or frozen) and of Co60b12 are incubated for 30-60 minutes at 37[degree]C and the excess of unbound vitamin adsorbed on charcoal; the radioactivity of the charcoal is counted directly in a well-type scintillator. ABC values in various physiological and pathological conditions are given. The ABC assay was also applied to measure Vit. B12 concentrations in biological material, by first releasing the vitamin from the carrier protein and incubating it together with a "standard serum" (of known ABC) with Co60B12 Vitamin content is determined by the increase in radioactivity in the charcoal over that found in the control containing "standard serum" only. Both methods are simple, rapid, and particularly suitable for serial determinations. The results obtained with these assays showed good agreement with microbiological and other radiometric methods. Various aspects of the assays are discussed.