Intracellular Distribution of Radioactive Vitamin B12 in Rat Liver

When radioactive cyanocobalamin (CN-[57Co]B12) was administered to rats by intracardiac injection, 98% of the CN-[57Co]B12 bound to transcobalamin II immediately upon injection and was transported to the tissues in this form. The radioactive compounds extracted from liver 20 min and 72 hours after the administration of CN-[57Co]B12 were identified either as CN-B12 or as a derivative of CN-B12. Five minutes after injection of CN-[57Co]B12, 46% of the radioactivity of the liver sedimented with the microsomal fraction (P). The radioactivity in this P was associated primarily with vesicles containing 5'-nucleotidase (marker enzyme for plasma membranes) rather than glucose 6-phosphatase (marker enzyme for endoplasmic reticulum). The amount of radioactivity in P then decreased progressively to 9% of the total liver radioactivity by 72 hours, whereas the amount of radioactivity in the heavy mitochondrial and soluble fractions of the liver increased to 33% and 32%, respectively, at 72 hours. Highly purified lysosomal fractions contained little radioactivity 5 min and 72 hours after the injection of CN-[57Co]B12; however, at intermediate times these lysosomal fractions contained significant amounts of radioactivity (15% of the total liver radioactivity at 30 min post-injection). Approximately 80% of the radioactivity in these lysosomal fractions was present as CN-[57Co]B12. Mitochrondial fractions containing approximately 10% of the 5'-nucleotidase of the liver were fractionated on sucrose gradients, and the preliminary data suggest that a portion of this 5'-nucleotidase may be associated with lysosomes.