Releasing Factor and Endogenous Vitamin B 12 ∗

Summary

Experiments were performed for the purpose of grossly identifying the substance responsible for the release, by rat intestinal segment extracts and human organ extracts, of vitamin B₁₂ bound to rat stomach extract or normal human gastric juice. It has been found that the active principle in the proximal area and mid-ileum of the rat small intestine is dialyzable and that its distribution parallels closely that of endogenous vitamin B₁₂. Non-radioactive vitamin B₁₂ has been found to be very active in the release of radioactive vitamin B₁₂ bound to rat or human intrinsic factor concentrate. This appears to be due to its ability to equilibrate with the bound form. In a complex equilibrium system, endogenous vitamin B₁₂ appears to account for substantially all of the activity displayed by the proximal area and mid-ileum of the rat small intestine, and by mucosa homogenates from beef small intestine and various human organ extracts. Vitamin B₁₂ bound to hog intrinsic factor on the other hand shows little tendency to equilibrate with unbound vitamin B₁₂.