Abstract

The radioactive material in urine after parenteral radioactive hydroxocobalamin is composed of radioactive cyanocobalamin, radioactive hydroxocobalamin, and a radioactive anionic complex. Studies in vitro suggest that these materials have a similar microbiological activity. Comparison of the amounts of radioactive and microbiologically active material in urine after parenteral radioactive hydroxocobalamin show a significant correlation both in the first and second 24 hours after injection showing that the amount of radioactivity is an acceptable measure of the loss of cobalamin in urine. The slope of the regression lines obtained from cobalamin-deficient and normal subjects implies that there is no equilibration of injected radioactive hydroxocobalamin and body stores: this may be due to the absence of hydroxocobalamin from tissues or to the fact that tissue hydroxocobalamin is bound and incapable of equilibration.