Investigation of the Possible Role of Sialic Acid in Calcium Oxalate Urolithiasis

Objective: It is considered that sialic acid in urine comprises two different types of sialic acid, that is, the free sialic acid and the bound one which exists as a basic component of urinary macromolecules. The present studies were performed to clarify the role of sialic acid in urine and compare the excretion and concentrations of sialic acid in urine between calcium oxalate (CaOx) stone formers and healthy subjects. Methods: First, we prepared the crystal surface binding substances from whole human urine. The crystal surface binding substances were digested with sialidase and both the inhibitory activity on CaOx crystal growth and concentrations of sialic acid before and after sialidase digestion were measured. The inhibitory activity of free sialic acid in ultrafiltered urine was also measured. The excretion and concentration of sialic acid in urine from patients with CaOx urolithiasis and healthy controls were determined and compared. Results: The inhibitory activity of crystal surface binding substances on CaOx crystal growth did not show any significant difference before and after sialidase digestion. The free sialic acid had also no effect on CaOx crystal growth. There was no statistically significant difference between CaOx stone formers and healthy controls on the excretion and concentration of sialic acid in urine. The free:bound sialic acid ratio in urine was considered 1:0.97 and 1:1.33 for healthy controls and CaOx stone formers respectively (p = NS). Conclusions: It is suggested that neither the free sialic acid nor the bound one in urine has a significant inhibitory effect on CaOx crystal growth.