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°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 S_N, 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, S_A, is derived from the maximum decrease in OD₆₂₀ with time. S_N and S_A 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 S_N and S_A in a concentration-dependent manner (P<0.01 for all comparisons vs control). Chondroitin-6-sulfate at 6.25–25 mg/l moderately lowered S_N, 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).