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Background: Previous studies demonstrated that crystals of uric acid (UA) and sodium urate (NaU) can induce the precipitation of calcium oxalate (CaOx) from its inorganic metastable solutions, but similar effects have not been un- equivocally shown to occur in urine. The aim of this in- vestigation was to determine whether preincubation of these seeds with urine alter their ability to induce deposi- tion of CaOx from solution and thus provide a possible explanation for discrepancy of results obtained from aque- ous inorganic solutions and undiluted urine. Materials and Methods: The effects of commercial seed crystals of UA, NaU and CaOx (6 mg/100 ml) on CaOx crys- tallization were compared in a solution with the same crys- tals that had been preincubated for 3 hours with healthy male urine. A Coulter Counter was used to follow the crys- tallization and decrease in soluble 14C-oxalate was mea- sured to determine the deposition of CaOx. The precipitated particles were examined by scanning electron microscopy (SEM). The preincubated seeds were demineralized and proteins released were analyzed by sodium dodecylsulphate polyacrylamide gel electrophoresis (SDS-PAGE). Results: Analysis of 14C-oxalate data revealed that while treated UA seeds did not affect CaOx deposition, those of NaU and CaOx inhibited the process by 51.9 (p  0.05) and 8.5% (p  0.05) relative to their respective untreated counterparts. Particle size analysis showed that the average modal sizes of particles precipitated in the presence of treated seed crystals of UA, NaU, and CaOx were very sim- ilar to those deposited in the presence of their respective untreated controls. These findings were confirmed by SEM which also showed that seed crystals of UA and NaU, un- treated and treated, were attached like barnacles upon the surfaces of CaOx crystals which themselves were bigger than those precipitated in the presence of CaOx seeds. SDS-PAGE analysis of the demineralized treated seeds showed that they all selectively adsorbed urinary proteins, and perhaps other urinary macromolecules and low mole- cular weight components, on their surface. Conclusions: It was concluded that preincubation with urine, such as occurs in vivo, only slightly reduces the ability of seed crystals of CaOx, but not of UA, to cause deposition of CaOx. The most striking effect was on NaU seeds where the preincubation quite dramatically atten- uated their promotory effect on the mineral deposition. This may explain the discrepancy between findings of studies carried out in inorganic solutions and undiluted human urine. This stresses the invalidity of directly ex- trapolating results obtained in inorganic solutions to likely effects in urine and more importantly, on stone formation.

molecular medicine

The Effect of Preincubation of Seed Crystals of Uric Acid and Monosodium Urate with Undiluted Human Urine to Induce Precipitation of Calcium Oxalate in Vitro: Implications for Urinary Stone Formation