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This Article Full Text Full Text (PDF) All Versions of this Article: 294/5/C1192    most recent 00015.2008v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Said, Z. M. Articles by Said, H. M. PubMed PubMed Citation Articles by Said, Z. M. Articles by Said, H. M.

MEMBRANE TRANSPORTERS, ION CHANNELS, AND PUMPS

Pyridoxine uptake by colonocytes: a specific and regulated carrier-mediated process

¹Departments of Medicine and Physiology/Biophysics, University of California, Irvine, California; and ²Department of Medical Research, Veterans Affairs Medical Center, Long Beach, California

Submitted 10 January 2008 ; accepted in final form 4 March 2008

The water-soluble vitamin B₆ (pyridoxine) is important for normal cellular functions, growth, and development. The vitamin is obtained from two exogenous sources: a dietary source, which is absorbed in the small intestine, and a bacterial source, where the vitamin is synthesized in significant quantities by the normal microflora of the large intestine. Evidence exists to suggest the bioavailability of the latter source of the vitamin, but nothing is known about the mechanism involved and its regulation. In this study, we addressed these issues using young adult mouse colonic epithelial (YAMC) cells and human colonic apical membrane vesicles (AMV) as models and using [³H]pyridoxine as the uptake substrate. The results showed the initial rate of [³H]pyridoxine uptake by YAMC cells to be 1) energy- and temperature- (but not Na-) dependent and to occur without metabolic alteration in the transported substrate; 2) saturable as a function of concentration with an apparent K_m and V_max of 2.1 ± 0.5 µM and 53.4 ± 4.3 pmol·mg protein^–1·3 min^–1, respectively; 3) cis-inhibited by unlabeled pyridoxine and its structural analogs, but not by the unrelated compounds theophylline, penicillamine, and isoniazid; 4) trans-stimulated by unlabeled pyridoxine; 5) amiloride sensitive; and 6) regulated by extracellular and intracellular factors. Uptake of pyridoxine by native human colonic AMV was also found to involve a carrier-mediated process. These studies demonstrate, for the first time, the functional existence of a specific and regulatable carrier-mediated process for pyridoxine uptake by mammalian colonocytes.

colonic transport; transport mechanism; transport regulation