Human urine is often supersaturated with respect to monosodium urate and uric acid. To study the stability of the supersaturated state, monopotassium urate crystals were incubated at 37°C in buffer solutions containing 150 mM sodium and various concentrations of potassium, in the pH range of 4.6∼8.2. The original pH of the solutions was maintained by frequent titration, and the concentration of total dissolved urate (undissociated uric acid + urate anion) was measured after 24 hours and 7 days. The effect of urinary macromolecules on the solubility of uric acid and urates in this system was also studied. The solubility of monopotassium urate was inversely related to the potassium concentration. In the pH range below 6.0, crystallization of uric acid started within 24 hours, and the concentration of total dissolved urate (UT) decreased to the solubility of anhydrous uric acid after 7 days incubation. In the pH range over 6.0, the total dissolved urate was stable for 24 hours up to 80mg/dl, approximately 10 times as high as the solubility of monosodium urate. With further incubation, the UT decreased with monosodium urate crystallization. The higher the pH or the higher the UT at 24 hours incubation, the more marked the decrease. Despite the crystallization, the UT remained considerably high when compared with the solubility of monosodium urate even after 7 days incubation. When urinary macromolecules were added, the UT remained higher than the solubility of anhydrous uric acid in the pH range under 6.0after 7 days incubation. These findings well explain why monosodium urate crystals seldom formed in the normal urine. On the contrary, urinary stasis and/or abnormally high urinary pH may cause monosodium urate crystallization. Urinary macromolecules contribute an effect to maintaining the supersaturation of uric acid by inhibiting the formation of uric acid crystals. However they do not increase the solubility of uric acid and urates.
A method for the determination of uric acid in human serum by HPLC-ECD was reexamined. Eluate from a column was amperometrically monitored by aid of an electrochemical detector with a plate-type glassy carbon electrode or a rod-type one as a working electrode. The upper detection limit of the detector with the plate-type electrode was found to be lower than that with the rod-type one. Since it was found that uric acid levels in human serum samples were very high, use of the detector with the rod-type electrode was recommended. In addition, we optimized a procedure for the treatment of human serum prior to chromatography. When the optimized procedure was applied for the pretreatment of human serum and an aliquot of the treated serum was injected onto a column, uric acid in human serum was able to be determined by aid of both electrochemical detectors. Comparison of the HPLC-ECD and HPLC-UV detection methods for the determination of human serum uric acid was done in order to provide some significant informations for routine use. We stressed that the HPLC-ECD method was useful for the determination of uric acid in mammalian tissues as well as in body fluids because the method was found to be highly selective, sensitive and reliable for the determination of uric acid. The HPLC-UV detection method allowed to determine uric acid not in mammalian tissues, but in human serum.
We investigated uric acid metabolism in two patients with primary hyperparathyroidism in relation to parathyroid function. Patient 1 was a 56-year-old man, who visited our clinic for headache and insomnia in March,1987. Two years earlier he noticed right flank pain and red-colored urine. Urological examination revealed a right ureter stone. On examination, hypercalcemia (5.6mEq/l), hypophosphatemia (2.1 mg/dl) and reduced % tubular reabsorption of phosphate (%TRP) were found, and the serum uric acid level was 8.0-9.5mg/dl. Patient 2 was a 44-year-old woman, who visited our clinic because of epigastral discomfort. She also exhibited hypercalcemia(6.0mEq/l), hypophosphatemia (2.5mg/dl)and decreased %TRP (71.8%), although the serum uric acid level was normal. Surgery of the neck revealed adenoma of the parathyroid gland, which was confirmed by histological examination. In these two patients, we investigated uric acid metabolism by urate clearance study and pyrazinamide (PZA) suppression test. Patient 1, who had hyperuricemia, showed reduced urate clearance, although the results of PZA suppression test were normal. On the other hand, Patient 2, whose serum urate level was normal, had almost normal results, although there was no difference in the serum level of PTH between the two. We also examined the changes of serum and urinary urate following calcium or EDTA loading. In Patient 1 there was a significant correlation between serum urate level and calcium and phosphate levels. On the other hand, there was no correlations in Patient 2 between these parameters. After successful parathyroidectomy, serum urate level did not show any significant changes in Patient 1. These results indicate that PTH dose not affect human uric acid metabolism, especially in the renal tubular transport system.