Recent studies have shown that human solute carrier SLC19A3 (hSLC19A3) can transport pyridoxine (vitamin B6) in addition to thiamine (vitamin B1), its originally identified substrate, whereas rat and mouse orthologs of hSLC19A3 can transport thiamine but not pyridoxine. This finding implies that some amino acid residues required for pyridoxine transport, but not for thiamine transport, are specific to hSLC19A3. Here, we sought to identify these residues to help clarify the unique operational mechanism of SLC19A3 through analyses comparing hSLC19A3 and mouse Slc19a3 (mSlc19a3). For our analyses, hSLC19A3 mutants were prepared by replacing selected amino acid residues with their counterparts in mSlc19a3, and mSlc19a3 mutants were prepared by substituting selected residues with their hSLC19A3 counterparts. We assessed pyridoxine and thiamine transport by these mutants in transiently transfected human embryonic kidney 293 cells. Our analyses indicated that the hSLC19A3-specific amino acid residues of Gln⁸⁶, Gly⁸⁷, Ile⁹¹, Thr⁹³, Trp⁹⁴, Ser¹⁶⁸, and Asn¹⁷³ are critical for pyridoxine transport. These seven amino acid residues were found to be mostly conserved in the SLC19A3 orthologs that can transport pyridoxine but not in orthologs that are unable to transport pyridoxine. In addition, these residues were also found to be conserved in several SLC19A2 orthologs, including rat, mouse, and human orthologs, which were all found to effectively transport both pyridoxine and thiamine, exhibiting no species-dependent differences. Together, these findings provide a molecular basis for the unique functional characteristics of SLC19A3 and also of SLC19A2.

最近的研究表明，人类溶质载体 SLC19A3 (hSLC19A3) 除了最初鉴定的底物硫胺素（维生素 B1）外，还可以转运吡哆醇（维生素 B6），而 hSLC19A3 的大鼠和小鼠直系同源物可以转运硫胺素，但不能转运吡哆醇。这一发现意味着吡哆醇转运而非硫胺素转运所需的一些氨基酸残基对 hSLC19A3 具有特异性。在这里，我们试图通过比较 hSLC19A3 和小鼠 Slc19a3 (mSlc19a3) 的分析来识别这些残基，以帮助阐明 SLC19A3 的独特运行机制。对于我们的分析，hSLC19A3 突变体是通过用 mSlc19a3 中的对应物替换选定的氨基酸残基来制备的，而 mSlc19a3 突变体是通过用它们的 hSLC19A3 对应物替换选定的残基来制备的。我们评估了这些突变体在瞬时转染的人胚胎肾 293 细胞中的吡哆醇和硫胺素转运。我们的分析表明 Gln 的 hSLC19A3 特异性氨基酸残基⁸⁶、Gly ⁸⁷、Ile ⁹¹、Thr ⁹³、Trp ⁹⁴、Ser ¹⁶⁸和 Asn ¹⁷³对吡哆醇转运至关重要。发现这七个氨基酸残基在可以转运吡哆醇的 SLC19A3 直系同源物中大部分是保守的，但在不能转运吡哆醇的直系同源物中则不是。此外，还发现这些残基在几个 SLC19A2 直系同源物中是保守的，包括大鼠、小鼠和人类直系同源物，它们都被发现可以有效地运输吡哆醇和硫胺素，没有表现出物种依赖性差异。总之，这些发现为 SLC19A3 和 SLC19A2 的独特功能特征提供了分子基础。