Simultaneous measurements of calcium oxalate crystal nucleation and aggregation: impact of various modifiers.

Rates of nucleation and aggregation of calcium oxalate crystals were derived from 20-min time course measurements of OD620 after mixing solutions containing CaCl2 and K2C2O4 at 37 degrees C, pH 5.7, ionic strength (IS) 0.21, with constant stirring (500 rpm); final assay concentrations were 4.25 mM calcium and 0.5 mM oxalate, respectively. The maximum increase of OD620 with time, termed SN, mainly reflects maximum rate of formation of new particles and thus crystal nucleation. After equilibrium has been reached, OD620 progressively decreases despite ionized calcium staying constant and no new particles being formed, due to crystal aggregation. Rate of aggregation, SA, is derived from the maximum decrease in OD620 with time. SN and SA are not independent, as indicated by a positive correlation (r = 0.844, P = 0.0001). Among the modifiers studied, citrate at 0.5-2.5 mM lowered both SN and SA in a concentration-dependent manner (P < 0.01 for all comparisons vs control). Chondroitin-6-sulfate at 6.25-25 mg/l moderately lowered SN, whereas it strongly inhibited aggregation (P < 0.01 vs control). At 6.8-20.4 mg/l, albumin did not affect nucleation, whereas it inhibited aggregation in a concentration-dependent manner (P < 0.005 vs control for all comparisons).