A Radioisotope Dilution Assay for Unlabelled Vitamin B12‐Intrinsic Factor Complex Employing the Binding Intrinsic Factor Antibody: Probable Evidence for Two Types of Binding Antibody

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Abstract
A new radioisotope dilution assay for vitamin B12-intrinsic factor complex was described. The method was based on use of human serum binding type intrinsic factor antibody (the binding reagent), which when combined with the intrinsic factor-vitamin B12 complex (labeled ligand) was quantitatively adsorbed onto zirconium phosphate gel at pH 6.25. The new assay apparently provided a measure of intrinsic factor comparable with other intrinsic factor assays, but had the important advantage of being able to measure the unlabeled vitamin B12-intrinsic factor complex (unlabeled ligand), and would be valuable in the study of physiological events in the gastrointestinal tract. There apparently was some evidence for at least 2 types of binding intrinsic factor antibody: one which combined preferentially with the intrinsic factor-vitamin B12 complex and one which combined equally well with this complex or with free intrinsic factor.