Relationships between protein and urate in avian urine.

Abstract

Uric acid is a nitrogenous compound that is produced as an end product of the deamination of amino acids. Many insects and all reptiles and birds excrete the majority of excess nitrogen as solid uric acid granules that are suspended in the urine. However, the solid uric acid is not in crystalline form, but is packaged into small, spherical structures that are usually 3 - 15 μm in diameter. Further, the uric acid that is dissolved in the urine exceeds its aqueous solubility limit and the solubility limits of the potassium and sodium urate salts. Thus, a factor(s) in the urine is facilitating urate in exceeding its solubility limits and in causing urate to form spheres rather than crystals. I have reported that avian urine contains unusually high amounts of protein (1-3 mg/ml), and that the uric acid spheres also contain protein. Linear regression analysis shows that the amount of uric acid and protein in urine are positively correlated (r = 0.80, p<0.005), suggesting a functional relationship between the two. Equilibrium dialysis of urine before and after its treatment with protease shows the urinary protein capable of binding urate (and calcium). After isolating and purifying a sphere protein, I was able to generate sheep anti-sphere protein antibodies. By using these and commercially obtained anti-chicken serum albumin antibodies in a series of western blot analyses, I have identified serum albumin as the sphere protein and as a major urinary protein in birds. In summary, my data show serum albumin to playa significant role in the excretion of uric acid in birds. Because urine from all uricotelic species I have examined contains similar spheres, I am suggesting that all uricoteles may use like, or analogous, proteins to facilitate the excretion of urates.