Two transport proteins, SLC19A2 (THTR1) and SLC19A3 (THTR2), associated with the plasma membrane, are each able to mediate the transport of extracellular thiamin into the cytosol. In the body, both transporters are widely distributed, and both are abundant in kidney and intestinal epithelia, consistent with their involvement in thiamin uptake under physiological conditions (Ashokkumar et al. 2006; Said et al. 2004; Subramanian et al. 2006 - J Biol Chem). The observation that mutations in SLC19A2 (THTR1) cause a progressive disorder that can be partially reversed by treatment with high doses of thiamin likewise suggests a role for this protein in thiamin uptake under normal conditions (Diaz et al. 1999; Fleming et al. 1999; Labay et al. 1999).
Two features of this transport process remain incompletely understood, however. First, mutations in SLC19A3 cause a progressive disorder that is responsive to biotin treatment (Zhou et al. 2005), although studies of cultured cells indicate that the protein has no affinity for biotin (Subramanian et al. 2006 - Am J Physiol). Also, studies to date provide little information about the mechanism by which thiamin, once taken up by epithelial cells in the intestine and kidney, is released from these cells into the blood.