Evidence for a carrier-mediated mechanism for thiamine transport to human jejunal basolateral membrane vesicles

Recent studies from our laboratory have demonstrated the presence of a pH-dependent, amiloride-sensitive, electroneutral carrier-mediated exchange for thiamine absorption in the human small intestinal brush-border membrane vesicles. However, the mechanism of thiamine transport across the human small intestinal basolateral membrane is not understood. The present study was aimed to characterize the mechanism of thiamine transport across the basolateral membranes of the human jejunum. Basolateral membrane vesicles (BLMV) were purified from mucosal scrapings of organ donors, utilizing a Percoll continuous density gradient centrifugation technique. The results showed [³H] thiamine uptake into BLMV to be: (1) markedly stimulated in the presence of an outwardly directed H⁺ gradient (pH 5.5_in/7.5_out); (2) significantly inhibited by amiloride in a dose-dependent manner; (3) sensitive to temperature and medium osmolarity and insensitive to changes in membrane potential; (4) not influenced by the addition of 1 mM Mg²⁺-ATP, inside and outside the vesicles in the presence of Na⁺ and K⁺; (5) inhibited by structural analogs—amprolium, oxythiamin, and unlabeled thiamine (100 M); (6) not affected by organic cations, eg, TEA, N-methyl-nicotinamide (NMN), and choline. (7) saturable as a function of concentration (apparent K_m of 0.76 ± 0.21 M and a V_max of 1.38 ± 0.35 pmol/mg protein/10 sec). These results indicate the presence of a proton gradient-dependent specialized carrier-mediated exchange mechanism for thiamine transport across the human jejunum basolateral membranes.