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1 Department of Medicine, West Side Veterans Affairs Medical Center and University of Illinois at Chicago, Chicago, Illinois 60612; and 2 Departments of Medicine and Physiology/Biophysics, University of California Irvine, and Veterans Affairs Medical Center, Long Beach, California 90822
Thiamine, a water-soluble vitamin, is essential for
normal cellular functions, growth and development. Thiamine deficiency leads to significant clinical problems and occurs under a variety of
conditions. To date, however, little is known about the mechanism of
thiamine absorption in the native human small intestine. The objective
of this study was, therefore, to characterize the mechanism of thiamine
transport across the brush-border membrane (BBM) of human small
intestine. With the use of purified BBM vesicles (BBMV) isolated from
the jejunum of organ donors, thiamine uptake was found to be
1) independent of Na+ but markedly stimulated by
an outwardly directed H+ gradient (pH 5.5in/pH
7.5out); 2) competitively inhibited by the
cation transport inhibitor amiloride (inhibitor constant of 0.12 mM);
3) sensitive to temperature and osmolarity of the incubation medium; 4) significantly inhibited by thiamine structural
analogs (amprolium, oxythiamine, and pyrithiamine), but not by
unrelated organic cations (tetraethylammonium,
N-methylnicotinamide, or choline); 5) not
affected by the addition of ATP to the inside and outside of the BBMV;
6) potential insensitive; and 7) saturable as a
function of thiamine concentration with an apparent Michaelis-Menten constant of 0.61 ± 0.08 µM and a maximal velocity of 1.00 ± 0.47 pmol · mg protein
1 · 10 s1. Carrier-mediated thiamine uptake was also found in
BBMV of human ileum. These data demonstrate the existence of a
Na+-independent, pH-dependent, amiloride-sensitive,
electroneutral carrier-mediated mechanism for thiamine absorption in
native human small intestinal BBMV.
thiamine transporter; human small intestine; brush-border membranes
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