Tissue Distribution of Radio-B₁₂ after Intravenous Injection of Gastric Juice, Saliva and Plasma-Bound Co⁵⁷ B₁₂: I. In Man

Co⁵⁷B₁₂ bound to pernicious anemia gastric juice was administered intravenously to 4 normal individuals, and Co⁵⁷B₁₂ bound to autologous plasma was administered to 2 of these. Hepatic, arm and blood radioactivity were measured for one week and compared with that following the intravenous injection of unbound Co⁵⁷B₁₂. Pernicious anemia gastric juice increased hepatic uptake, delayed blood clearance and decreased arm uptake of Co⁵⁷B₁₂ as compared with Co⁵⁷B₁₂ injected unbound or bound to plasma. Thus, the B₁₂ bound to gastric juice is retained longer in the intravascular space with greater uptake by liver.

These results suggest that gastric juice B₁₂ binders, but not plasma B₁₂ binders, enhance hepatic B₁₂ uptake either by acting at specific receptor sites on the liver cell membrane or by retaining B₁₂ longer in circulation. This property of gastric juice B₁₂ binders is not related to IF activity but is rather dependent upon some structural or functional feature common to both IF-related and nonrelated B₁₂ binders of gastric juice.