Identification of the amino acid residues involved in the species-dependent differences in the pyridoxine transport function of SLC19A3.

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.

Imaging of hepatocellular carcinoma: qualitative and quantitative analysis of postvascular phase contrast-enhanced ultrasonography with sonazoid. Comparison with superparamagnetic iron oxide magnetic resonance images.

PURPOSE: To evaluate the usefulness of vascular phase images of contrast-enhanced ultrasonography (CE-US) with Sonazoid for hepatocellular carcinomas (HCCs), a retrospective, comparative study was conducted of images of HCCs obtained by CE-US and superparamagnetic iron oxide (SPIO) magnetic resonance imaging (MRI) and evaluated qualitatively and quantitatively. METHODS: Seventy-seven patients with 88 HCCs who received CE-US and SPIO-MRI were reviewed. The ratio of the echogenicity of the tumor and nontumor areas was calculated with postvascular phase CE-US (postvascular phase ratio). The ratio of the intensity of the nontumor to tumor areas on SPIO-enhanced MRI (SPIO intensity index) was also calculated. The Pearson correlations were calculated for all values between the postvascular phase ratio and SPIO intensity index for quantitative comparison. These images were also compared qualitatively for the detection rate of the tumors. RESULTS: The sensitivities of CE-US and SPIO-MRI in detecting tumors were 98 and 95%, respectively (nonsignificant, chi(2) test). The postvascular phase ratio correlated with the SPIO intensity index for HCCs (Pearson r = 0.803, p < 0.05). The image conformity of the result from the liver parenchymal phase CE-US and SPIO-MRI was 92%. Dedifferentiation spots of nodule-in-nodule HCCs were detected in 4 (80%) of 5 on postvascular phase images of CE-US, and in 2 (40%) of 5 on SPIO-MRI (nonsignificant, chi(2) test). CONCLUSIONS: Postvascular phase images of CE-US with Sonazoid appear promising as an alternative to SPIO-enhanced MRI. Further study cases are needed to confirm the usefulness of postvascular phase images of CE-US compared to SPIO-MRI for the detection of dedifferentiation foci in hepatic tumors.