Placental Transfer and Biological Half-life of Radioactive Vit. B 12 in the Dog. ∗

Summary

Placental transfer of radioactive Vit. B₁₂ in the dog following pregestational administration, was found in the entire litter to be 20% of the residual radioactive vitamin in the bitch at time of delivery. Because the proportional distribution of radioactive B₁₂ among major organs of the mother and pups was found to be similar, it was possible to calculate placental transfer from this characteristic as well as by postpartum “calibration” of the hepatic radioactivity in pups. Biological half-life of the radioactive Vit. B₁₂ averaged 2 months in pups and bitch during postpartum period. A new explanation is suggested for the low maternal and high fetal plasma levels of Vit. B₁₂.