Calcium oxalate crystallization in urine: Role of urate and glycosaminoglycans. Increasing the concentration of urate promotes the crystallization of calcium oxalate in human urine. In this study the possibility that this effect might be attributable to the attenuation of the inhibitory activity of urinary glycosaminoglycans (GAGs) was explored. Urine samples were collected from 20 men with no history of urolithiasis and the intact GAGs removed by 10 kDa ultrafiltration. Ten of these specimens, designated type A, spontaneously precipitated calcium oxalate crystals when the median urate concentration was increased from 3.13 to 7.33 mmol/liter by the addition of a saturated solution of sodium urate. In the remaining more dilute urines, which were designated type B, spontaneous calcium oxalate crystallization did not occur when the median urate concentration was raised from 2.20 to 6.40 mmol/liter. In these samples crystallization was induced by a standard load of oxalate above the empirically determined metastable limit. Addition of urate significantly reduced the median metastable limit from the control value of 125 to 46 µmol oxalate, and the volume of calcium oxalate deposited was increased fourfold from 25,000 to 104,000 µm³/µl. The median size of the precipitated particles was also increased in the presence of urate from 12.06 µm to 14.3 µm; this was confirmed by scanning electron microscopy, which demonstrated that the crystals precipitated in the presence of added urate, though individually smaller, were markedly more numerous and more highly aggregated than those deposited in the control. Re-ultrafiltration of the urines to which urate had been added did not alter the urate concentration, and SEM examination of the ultrafiltration membranes did not reveal the presence of any particulate material. It was concluded that urate does not exist in urine in a colloidal or crystalline form, and that the promotion of calcium oxalate crystallization by urate is not a consequence of its reducing the inhibitory activity of GAGs or other urinary macromolecules.