| Budget Amount *help |
¥2,900,000 (Direct Cost: ¥2,900,000)
Fiscal Year 2004: ¥1,100,000 (Direct Cost: ¥1,100,000)
Fiscal Year 2003: ¥900,000 (Direct Cost: ¥900,000)
Fiscal Year 2002: ¥900,000 (Direct Cost: ¥900,000)
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| Research Abstract |
Urate is reabsorbed and secreted bilaterally at the apical membrane in the tubules. We cloned urate transporter 1(URAT1 encoded by SLC22A12), located at the apical membrane in the proximal tubules. The estimated Michaelis constant (Km) for urate was 371±28mM. We found that the urate transport via URAT1 is inhibited selectively by organic anions such as lactate, nicotinate, acetoacetate, hydroxybutyrate and succinate. Aromatic monocarboxylates such as pyrazinecarboxylic acid, nicotinate and orotate at 1mM inhibited urate uptake more effectively than aliphatic monocarboxylates such as lactate. We also tested the influence of uricosuric substances known to reduce hyperuricemia. Probenecid, phenylbutazone, sulphinpyrazone, nonsteroidal anti-inflammatory drugs, and diuretic drugs at 1mM cis inhibited urate uptake ; benzbromarone was the most potent, completely inhibiting urate uptake at 50mM, with a 50%-inhibitory concentration(IC50) of less than 0.1mM.
We also demonstrated that URAT1 regula
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tes serum urate levels by showing that patients with renal hypouricemia have defects in SLC22A12. Renal hypouricemia is a heterogeneous disorder characterized by impaired tubular urate transport. We elucidated genetic features of renal hypouricemia and the significance of URAT1 in maintaining serum urate levels in vivo using 32 unrelated patients. Thirty out of 32 patients had SLC22A12 mutations (93.8%), while 2 patients had none in the URAT1-coding region. We detected SLC22A12 mutations in 54 out of 64 alleles (84.4 %). Ten mutations in SLC22A12 were identified. Mutations included two nonsense mutations, one splice-site mutation, a 5-bp deletion and 6 missense mutations. The G774A mutation was identified in 40 out of 54 affected SLC22A12 alleles (74.1 %). All of the missense mutations and the 5-bp deletion (1639-1643delGTCCT) were tested for urate transport activity by in vitro expression analysis in Xenopus oocytes. The urate transports of all URAT1 mutants were significantly decreased and similar to that of oocytes that were not injected with URAT1 cRNA. Less
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