Abstract
Hydroxocobalamin (OH-B_<12>) and cyanocobalamin (CN-B_<12>) both labeled with ^<57>Co were compared with respect to binding by serum proteins and retention in blood following intravenous administration in rats. There was no difference in blood retention following injection of physiological doses, but disappearance of OH-B_<12> from blood was retarded compared with CN-B_<12> when given in supraphysiological doses. More OH-B_<12> was bound to serum as determined by dialysis method when its amount to be mixed with serum was increased, whereas no such increase ion binding occurred in CN-B_<12> under the same condition. There was practically no difference between human and rat sera in behavior toward these analogues.
