Involvement of a receptor-mediated component in cellular translocation of riboflavin

This study addresses the transport mechanism of riboflavin (vitamin B-2) ac ross intestinal epithelium in the presence and absence of pharmacologically active compounds. A polarized transport process with a 6-fold higher basol ateral (BL)-to-apical (AP) flux was observed in both a human intestinal cel l model (Caco-2) and rat intestinal tissue. Riboflavin-specific translocati on systems on both the AP and BL cell surfaces were saturable with affinity values close to most receptors (K-m : 9.72 +/- 0.85 and 4.06 +/- 0.03 nM, respectively). Pharmacological agents known to alter intracellular endocyti c events were used to examine the potential involvement of receptor-mediate d events. Nocodazole significantly inhibited AP uptake (58.4%), BL-to-AP ri boflavin (56.7%) and fluorescein isothiocyanate-labeled transferrin (FITC-T f) (31.8%) transport without affecting mannitol or cholic acid transport, w hereas AP-to-BL riboflavin (252.8%) and FITC-Tf (145.1%) transport was incr eased. Brefeldin A significantly enhanced AP-to-BL riboflavin (37.1%) and b idirectional FITC-Tf transport (AP-to-BL: 13-fold; BL-to-AP: 5-fold). witho ut affecting BL-to-AP riboflavin transport. Combined, these data suggest an essential role of microtubule-dependent movement and vesicular sorting com ponent(s) in the bidirectional transport of riboflavin. Dissociation of rib oflavin from the cell surface was pH-dependent with significantly higher su bstrate release at acidic pH, indicating the presence of riboflavin-specifi c cell surface receptors. In summary, our studies provide biochemical evide nce of the involvement of a receptor-mediated mechanism in the cellular tra nslocation of riboflavin.