The reduction in urinary glutamate excretion is responsible for lowering urinary pH in pink urine syndrome

We frequently encounter brownish-red, cloudy urine in some obese subjects, which occurs due to pink urine syndrome (PUS). PUS is a phenomenon in which uric acid precipitates into the urine due to reduced urinary pH (UpH). The mechanism underlying urinary acidification has not been elucidated so far. UpH level is adjusted by urinary excretion of ammonia synthesized from glutamate or glutamine, suggesting that renal synthesis of ammonia from glutamate or glutamine is decreased in PUS. However, this hypothesis has not been examined yet. We therefore examined the changes in the urinary excretion of these amino acids in PUS. One-hundred-fifty male students who had undergone a physical examination were enrolled. To determine the presence [PUS (+), n = 72] or absence [PUS (-), n = 78] of PUS, urinary amino acid excretion and UpH were evaluated. Independent risk factors of lower UpH were determined using multiple regression analyses. The PUS (+) subjects, who had lower UpH values than PUS (-) subjects, showed lower urinary excretion of glutamate and some other glucogenic amino acids. Thus, UpH correlated positively with the urinary excretion of glutamate in the PUS (+) subjects. A reduction in urinary glutamate but not in glutamine excretion proved to be an independent risk factor for reduced UpH. In conclusion, PUS appears to occur when a reduction in the synthesis of ammonia from glutamate causes a decrease in UpH. Our results showed that urinary glutamate excretion was reduced in PUS because renal glutamate was consumed by a reaction different from ammonia production.