Abstract
This study addresses the transport mechanism of riboflavin (vitamin B2) across intestinal epithelium in the presence and absence of pharmacologically active compounds. A polarized transport process with a 6-fold higher basolateral (BL)-to-apical (AP) flux was observed in both a human intestinal cell model (Caco-2) and rat intestinal tissue. Riboflavin-specific translocation systems on both the AP and BL cell surfaces were saturable with affinity values close to most receptors (Km: 9.72 ± 0.85 and 4.06 ± 0.03 nM, respectively). Pharmacological agents known to alter intracellular endocytic events were used to examine the potential involvement of receptor-mediated events. Nocodazole significantly inhibited AP uptake (58.4%), BL-to-AP riboflavin (56.7%) and fluorescein isothiocyanate-labeled transferrin (FITC-Tf) (31.8%) transport without affecting mannitol or cholic acid transport, whereas AP-to-BL riboflavin (252.8%) and FITC-Tf (145.1%) transport was increased. Brefeldin A significantly enhanced AP-to-BL riboflavin (37.1%) and bidirectional FITC-Tf transport (AP-to-BL: 13-fold; BL-to-AP: 5-fold). without affecting BL-to-AP riboflavin transport. Combined, these data suggest an essential role of microtubule-dependent movement and vesicular sorting component(s) in the bidirectional transport of riboflavin. Dissociation of riboflavin from the cell surface was pH-dependent with significantly higher substrate release at acidic pH, indicating the presence of riboflavin-specific cell surface receptors. In summary, our studies provide biochemical evidence of the involvement of a receptor-mediated mechanism in the cellular translocation of riboflavin.
Footnotes
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Send reprint requests to: Dr. Peter W. Swaan, Division of Pharmaceutics, College of Pharmacy, 500 West 12th Ave., Columbus OH 43210-1291. E-mail: swaan.1{at}osu.edu
- Abbreviations:
- RME
- receptor-mediated endocytosis
- FITC-Tf
- fluorescein isothiocyanate-labeled transferrin
- TfR
- transferrin receptor
- AP
- apical
- BL
- basolateral
- BFA
- brefeldin A
- Is.c.
- short-circuit current
- Received December 16, 1999.
- Accepted March 10, 2000.
- The American Society for Pharmacology and Experimental Therapeutics
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