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

摘要

We have examined the intracellular route, coenzyme conversion and transcytosis 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 factor (IF). The free-presented Cbl was progressively bound to endogenous transcobalamin II (TCII) which may stem, in part, from a basolateral to apical passage. Its transcytosis was TCII-mediated as it was abolished when antibodies to TCII were added to the apical medium. The apparent permeability coefficient (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-dependent manner. Approximately 80% of apical Cbl, bound to either exogenous IF or endogenous TCII, was transported to the basolateral side as intact cyano[57Co]Cbl whereas the remainder was converted into Ado-Cbl and CH3-Cbl within 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 chloroquine. In conclusion, we suggest that apically presented free Cbl is internalized 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 converted to coenzymes, whereas bulk of CN-Cbl is transcytosed intact. Copyright (C) 2000 S. Karger AG, Baser. [References: 36]