The Uptake and Turnover of Radioactive Vitamin B12 in Rabbit Tissues

Abstract

In rabbits the rate of turnover of Co⁶⁰ vitamin B₁₂ during a 26-day period is negligible in liver, kidney, heart, skeletal muscle and brain, while in plasma and spleen the rate of vitamin turnover is rapid. The plasma disappearance curve of Co⁶⁰ vitamin B₁₂ exhibits two components with biological half lives of 4.4 days and approximately 50 days. Erythrocytes of normal rabbits fail to incorporate injected Co⁶⁰ vitamin B₁₂ but the young reticulocytes of animals made anemic with phenylhydrazine or hemorrhage incorporate the vitamin rapidly. The radioactivity of the reticulocytes is transient and leaves the cells as the reticulocytes mature. The uptake of the injected vitamin by other tissues is not altered in the anemic animals. The accumulation of radioactivity in kidney tissue is markedly elevated above that of control animals when rabbits are starved for 10 days or fortified with injections of non-radioactive vitamin B₁₂ prior to the injection of Co⁶⁰ vitamin B₁₂. The incorporated Co⁶⁰ vitamin B₁₂ may be mobilized from the tissues by 10 days of starvation. This mobilization is variable, and differs for different tissues suggesting that vitamin B₁₂ is intimately associated with the specific metabolism of individual tissues. The data also suggest a more fundamental role of the kidney in vitamin B₁₂ metabolism in addition to the excretory function of the kidney.