Urate, a naturally occurring product of purine metabolism, is a scavenger of oxidants. Urate exists primarily as a weak acid (pKa ~5.75). Urate concentration is quite high (~200-500uM) in human due to loss of uricase gene and the presence of an effective reabsorption system. Recently an OAT-related transporter, URAT1, has been identified in the kidney that acts as urate-anion exchanger and effectively regulates urate levels. URAT1/RST/SLC22A12 (mouse 553-aa, human 555-aa, chromosome 11q13, ~40kD) is ~42% identical with OAT4. Like OAT4, it is predicted to display 12 transmembrane domains with a cytoplasmic N- and C-terminus. URAT1 is prominent in epithelial cells of proximal tubule of the renal cortex. Patients with idiopathic renal hypouricemia (lack of blood uric acid) have defects in URAT1 gene.