Decreased renal clearance of xanthine and hypoxanthine in a patient with renal hypouricemia: A new defect in renal handling of purines

Renal handling of urate, xanthine and hypoxanthine was studied in a hypouricemic patient who had increased plasma concentrations of xanthine and hypoxanthine. The patient, a 50-year-old man, had been suffering from Parkinson's disease, while neither systemic disorders nor particular renal diseases known to affect plasma purine levels were found. His serum urate level was 58 ± 6 μmol/l (healthy controls for males, 310 ± 48 μmol/l, mean ± SD) and the renal uric acid clearance was 3 times higher than that of the controls, establishing a diagnosis of renal hypouricemia. Xanthine and hypoxanthine concentrations in the plasma were elevated to 1.3 ± 0.1 μmol/l (controls, 0.5 ± 0.3) and 5.9 ± 3.5 μmol/l (controls, 1.6 ± 0.4), respectively. Both renal xanthine and hypoxanthine clearance was only half the value of the controls, indicating reduced urinary excretion of xanthine, and hypoxanthine appears to be responsible for their elevation in plasma. A probenecid loading test revealed no response of urinary urate excretion but normal responses of xanthine and hypoxanthine excretion. However, urinary excretion of urate, xanthine or hypoxanthine did not respond at all to pyrazinamide administration. These findings indicate that the patient had a defective renal handling of xanthine and hypoxanthine as well as urate.