Transcobalamin II synthesized in the intestinal villi facilitates transferof cobalamin to the portal blood

This study was designed to identify the cellular component of the intestina l villus where transcobalamin II (TCII) is synthesized, because this protei n provides an essential function in the intestinal absorption of vitamin B- 12 (cobalamin, Cbl). When a segment of proximal or distal small intestine o f the guinea pig is cultured in medium containing [Co-57]Cbl, TCII[Co-57]Cb l appears within 15 min. Northern blot analysis of RNA from both proximal a nd distal small intestine identified the TCII transcript. In situ hybridiza tion of the distal ileum with S-35-labeled TCII antisense transcript locali zed grains predominantly in crypts and in the lower third and central core of the villi. Grains were also evident at the base of the enterocytes in cl ose apposition with the vascular network, whereas few grains appeared in th e apical region of the columnar cells. This study provides evidence that TC II is constitutively expressed in the intestinal villi where vascular endot helium is abundant. In the distal ileum, where the intrinsic factor (IF) re ceptor is expressed, after uptake of IF-Cbl and the subsequent binding of f ree Cbl to TCII synthesized in the villi, the TCII-Cbl complex enters the m icrocirculation and passes into the portal blood.