Hepatic Vitamin B₁₂ Release and Transcobalamin II Synthesis in the Rat

1. The release of ⁵⁷Co-labelled vitamin B₁₂ ([⁵⁷Co]B₁₂) and synthesis of transcobalamin II (TCII) by the isolated perfused rat liver were studied 10–42 days after the parenteral administration of a trace dose of 15 pmol (approximately 20 ng) of radioactive cyanocobalamin.

2. The rate of release of [⁵⁷Co]B₁₂ into plasma and bile was linear and constituted approximately 0.9% and 0.3% respectively of the initial hepatic radioactivity per hour of perfusion.

3. [⁵⁷Co]B₁₂ released into plasma was bound to TCII. Saturation of the total TCII by the addition of cyanocobalamin before perfusion resulted in the appearance of the hepatic [⁵⁷Co]B₁₂ in the free form.

4. These data were found to be compatible with the following observations in vivo: (i) rates of [⁵⁷Co]B₁₂ release as measured by urinary [⁵⁷Co]B₁₂ excretion after saturation of plasma binders with non-labelled cyanocobalamin; (ii) rates of biliary excretion of [⁵⁷Co]B₁₂.

5. Liver damage produced by hypoxaemia was associated with a fall in the rate of release of [⁵⁷Co]B₁₂.

6. TCII release occurred at a linear rate of almost twenty times that required for the binding of newly released hepatic vitamin B₁₂.

7. Cycloheximide at a dose sufficient to inhibit release of TCII did not prevent the release of [⁵⁷Co]B₁₂ from the liver into plasma or bile.

8. Alteration of perfusate composition to contain either high plasma concentrations of vitamin B₁₂ and low concentrations of unsaturated TCII or high plasma concentrations of vitamin B₁₂ and high concentrations of unsaturated TCII had no effect on the rate of [⁵⁷Co]B₁₂ release into plasma or bile.

9. It is concluded that the fluxes of hepatic vitamin B₁₂ and TCII are very rapid and that the release of vitamin B₁₂ by the rat liver is controlled in the short term by factors other than the synthesis of TCII and the concentration of vitamin B₁₂ or unsaturated transcobalamin in the plasma.