Abstract
Methylcobalamin(CH_3-B_<12>)preparations labeled with ^<57>Co and ^<14>CH_3 were used in rats as well as in man to study its absorption in comparison with cyanocobalamin (CN-B_<12<)-^<57>Co. The preparations were more than 90% pure, and when eluted with distilled water from a carboxymethylcellulose column, a small unidentified fraction was seen preceding the major peak of CH_3-B_<12> labeled with ^<57>Co, but not with ^<14>C. Absorption of CH_3-B_<12>-^<57>Co by oral administation and in an intestinal loop in rats was quite similar to that of CN-B_<12> with respect to amount and speed at varying dose levels. The behavior toward intrinsic factor concentrate, both hog and homologous, was also the same. Intrinsic factor bound CH_3-B_<12> was also photosensitive, and ^<14>CH_3 Was readily released and dialyzed following photolysis. In man, the urinary excretion in 24 hours of ^<51>Co following oral administration of 0.5μg of CH_3-B_<12>-^<57>Co and flushing with 1 mg of CN-B_<12>-^<57>Co(Schilling test) was about 1/3 that obtained with CN-B_<12>. The liver uptake of ^<57>Co was greater with CH_3-B_<12> than with CN-B_<12> when net absorption was the same in rats, suggesting conversion of the former to either hydroxocobalamin or other coenzyme forms in tissues.
