尿酸の尿中溶解度に関する研究

尿酸ナトリウムおよび尿酸の過飽和状態について-

抄録

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 (U_T) 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 U_T decreased with monosodium urate crystallization. The higher the pH or the higher the U_T at 24 hours incubation, the more marked the decrease. Despite the crystallization, the U_T remained considerably high when compared with the solubility of monosodium urate even after 7 days incubation. When urinary macromolecules were added, the U_T 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.