Transcytosis and coenzymatic conversion of [Co-57]cobalamin bound to either endogenous transcobalamin II or exogenous intrinsic factor in Caco-2 cells

We have examined the intracellular route, coenzyme conversion and transcyto sis rate of [Co-57]-labeled cobalamin (Cbl) in function of its presentation to the apical side of Caco-2 cells, either free or bound to intrinsic fact or (IF). The free-presented Cbl was progressively bound to endogenous trans cobalamin II (TCII) which may stem, in part, from a basolateral to apical p assage. Its transcytosis was TCII-mediated as it was abolished when antibod ies to TCII were added to the apical medium. The apparent permeability coef ficient (P-app) was estimated at 20.8+/-3.6, 103.5+/-17.7, 0.9+/-0.3 x 10(- 5) cm/h for TCII-Cbl, IF-CM and haptocorrin-Cbl, respectively. Chloroquine inhibited the transcytosis rate of both TCII and If-bound Cbl in a dose-dep endent manner. Approximately 80% of apical Cbl, bound to either exogenous I F or endogenous TCII, was transported to the basolateral side as intact cya no[57Co]Cbl whereas the remainder was converted into Ado-Cbl and CH3-Cbl wi thin the cells, as shown by HPLC analyses of a 1,000-g pellet and a 12,000- g supernatant. Coenzymatic conversion was virtually abolished by chloroquin e. In conclusion, we suggest that apically presented free Cbl is internaliz ed via TCII-dependent transport. The apically internalized CN-Cbl, bound to either IF or TCII, is processed via an acidic vesicle and part of it is co nverted to coenzymes, whereas bulk of CN-Cbl is transcytosed intact. Copyri ght (C) 2000 S. Karger AG, Baser.