Radioiodination of human intrinsic factor

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Abstract
Human intrinsic factor (IF) saturated with 60Co-labeled cyanocobalamin (60CoB12) was purified and then iodinated with 125I to yield 125I-labeled IF-60CoB12 preparations of high specific activity. Sephadex G200 and DEAE-cellulose chromatography of the iodinated IF-60CoB12 complex showed coincidence of the major 125I and the 60Co radioactivity peaks. During starch-gel electrophoresis 60Co radioactivity from noniodinated and iodinated complexes migrated to the same extent while 125I radioactivity from the iodinated complex migrated slightly further anodally than did the 60Co radioactivity. After the iodinated complex was mixed with antibody to the IF-B12 complex (antibody II) the 125I and 60Co radioactivity were: (a) precipitated in similar amounts by antiglobulin serum. (b) eluted coincidentally in the 19S region on Sephadex G200, and (c) excluded to the same extent from starch gel during electrophoresis. After equilibrium exchange of IF “blocking” antibody (antibody I) for 60Co-vitamin B12 on 125I-labeled IF. 125I radioactivity from the IF-antibody I complex: (a) was precipitated by antiglobulin serum, (b) was eluated in the 19S region on Sephadex G200 gel filtration, and (c) migrated slowly towards the anode on starch-gel electrophoresis. Urinary excretion of 60Co radioactivity in pernicious anemia patients after oral administration of 60Co-vitamin B12 bound to freshly prepared 125I-labeled IF was similar to that obtained with noniodinated intrinsic factor. These results show that iodination of IF-60CoB12 complex does not markedly alter the chromatographic, electrophoretic, antigenic, or absorption-promoting properties of IF.