Enzymatic dissolution of calcium and struvite crystals: in vitro evaluation of biochemical requirements

To evaluate the factors that affect the enzymatic dissolution rate of calcium oxalate monohydrate (COM), calcium phosphate (brushite), and magnesium ammonium phosphate (struvite) crystals as enzymatic digestion of kidney stones could enhance lithotripsy or provide alternatives to surgical removal.

At pH 4.2, pelleted COM crystals were combined with oxalate decarboxylase (ODC from Bacillus subtilis), oxalate oxidase (from Hordeum vulgare), or control. Crystal dissolution was followed by measuring increases in solution calcium ion concentration. For phosphate-based crystals, the rates of phosphorolysis by the enzyme purine nucleoside phosphorylase (PNP, assay form) were compared to the control solution using spectrophotometry.

The addition of ODC to COM crystals resulted in production of highly soluble calcium formate and a 15-fold increase in COM solubility. By adding a formate-catabolizing enzyme (formate dehydrogenase), dissolution increased 47-fold compared with controls with nearly one half of the mineral dissolved. Oxalate oxidase showed much lower activity than ODC in COM dissolution. Using inorganic phosphate as a substrate, PNP was able to dissolve both brushite and struvite minerals in water at concentrations near saturation. Measuring dissolution by adding more PNP was not possible because of equilibrium and assay detection restraints.

Stone dissolution using enzymes appears to be viable, particularly for oxalate-based minerals. In a closed system, product inhibition by calcium formate appeared to limit the extent of COM crystal dissolution using ODC. Although phosphate-containing minerals appear to be suitable phosphate sources for PNP, the reversibility of the reaction limits the use of this enzyme.