Ascorbate conversion to oxalate in alkaline milieu and Proteus mirabilis culture

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 o f ascorbate may be influenced by urinary pH and environment, so we studied ascorbate conversion to oxalate in standard buffer solution and in urine. A bout 10% of infection stones associated with Proteus mirabilis are reported ly composed of calcium oxalate, and their pathogenesis is not well explaine d. Therefore, we studied whether a pH change induced by P mirabilis contrib utes 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 sp ecimens, Two-hour exposure to pH 10 in a urinary milieu containing approxim ately 3 mM ascorbate converted approximately 40% of the ascorbate to oxalat e, whereas 24-hour exposure to pH 8 in a urinary milieu that was approximat ely 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 as corbate concentration in the culture medium significantly decreased at 12 h ours and 24 hours, and the oxalate concentration increased significantly at 24 hours. Conclusion: Urinary ascorbate, if present at a high concentration in associ ation with Proteus mirabilis infection, appears to be locally degraded to o xalate, potentially leading to calcium oxalate deposition on infection ston es.