ThDP, either from the diet or from production by the intestinal microflora of the colon, is hydrolyzed in the intestinal lumen to free thiamine. Movement of thiamine into and through cells is primarily through transport carriers because it has a quaternary nitrogen, which limits diffusion. Thiamine is transported into the intestinal enterocytes via the human thiamine transporters, (hTHTR)-l and hTHTR-2.141 These transporters are now known as solute carrier family 19, member 2 (SLC19A2) and member 3 (SLC19A3), respectively, and are related to the reduced folate carrier (RFC) for folates, which is member 1 (SLC19A1). This carrier-mediated process is saturable at relatively low micromolar concentrations, and higher concentrations can be absorbed through passive diffusion.142 Once inside the cell, some thiamine is phosphorylated to form ThDP, and the rest is transported across the basolateral membrane into the blood where it freely distributes to other tissues. Phosphorylation of thi-
CH3 2 Cl Thiamine hydrochloride.
amine in the cell is thought to be the driving force behind the transport mechanism. Tissues requiring thiamine use SLC19A2 and SLC19A3 to transport thiamine into the cell where it is then phosphorylated. These same transporters also explain entry of thiamine into the CNS. Excess thiamine is readily excreted by the kidneys.
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