Background and purpose: Ascorbate breakdown reportedly accounts for 30% to 55% of urinary oxalate excreted. Three potential degradation routes can be postulated: bowel, endogenous, and urinary. Because the pH of normal urine ranges from 4.5 to 8.0, the urinary oxalate concentration in the presence of ascorbate may be influenced by urinary pH and environment, so we studied ascorbate conversion to oxalate in standard buffer solution and in urine. About 10% of infection stones associated with Proteus mirabilis are reportedly composed of calcium oxalate, and their pathogenesis is not well explained. Therefore, we studied whether a pH change induced by P. mirabilis contributes to ascorbate conversion to oxalate in vitro.
Results: Oxalate production from ascorbate increased as a function of pH (7.0-10.0) and incubation time (30 minutes-24 hours) in standard and urine specimens. Two-hour exposure to pH 10 in a urinary milieu containing approximately 3 mM ascorbate converted approximately 40% of the ascorbate to oxalate, whereas 24-hour exposure to pH 8 in a urinary milieu that was approximately 3 mM ascorbate converted approximately 20% of the ascorbate to oxalate. The pH in Proteus cultures increased to 9.0 at 24 hours of culture. The ascorbate concentration in the culture medium significantly decreased at 12 hours and 24 hours, and the oxalate concentration increased significantly at 24 hours.
Conclusion: Urinary ascorbate, if present at a high concentration in association with Proteus mirabilis infection, appears to be locally degraded to oxalate, potentially leading to calcium oxalate deposition on infection stones.