Ion binding by small molecular weight substances (1,000-10,000 daltons) in urine.

The reliability of calculated activity products as an index of the urinary saturation of calcium oxalate was tested by comparing the solubility of calcium oxalate in various urine preparations. Urine samples were collected from six control subjects and nine patients with nephrolithiasis. From each sample four urine preparations were prepared: centrifugation at 1400 g. 0.22-mu filtration, ultrafiltration through a 10,000 daltons cutoff filter, and ultrafiltration through a 1000 daltons cutoff filter. Each preparation was incubated with an excess of calcium oxalate to steady-state, and the activity product of calcium oxalate was calculated for the ambient solution. Our findings suggest that most calcium and/or oxalate conplexes in urine that are not recognizable by available computational techniques for activity products probably reside in the fraction of 1000 to 10,000 daltons.