Hippuric acid as a modifier of calcium oxalate crystallisation

Hippuric acid (HA) originating from the conjugation of benzoic acid with gl ycine is a physiological component of human urine. Findings suggest that HA inhibits calcium oxalate (CaOx) growth and considerably enhances the CaOx solubility in artificial urine. Thus, it is assumed that HA is a major modi fier of CaOx formation. However, only a slight CaOx growth inhibition of 1- 8 % was also reported. These values were also derived from artificial urine . The key mechanism, which led HA to be of interest in urolithiasis researc h is the fact that in presence of Ca2+ ions HA can form a hippurate complex . By forming such a complex, Ca2+ concentration in urine decreases, and as a consequence, CaOx formation is inhibited. This study was performed in order to clarify the role of HA in native and a rtificial urine. Biochemical analyses to calculate the relative CaOx supers aturations and crystallisation experiments using an in-line laser probe wer e examined. BONN Risk Indices indicating the risk of CaOx crystallisation w ere calculated from the results of the crystallisation experiments. The results obtained from artificial as well as from native urines showed t hat HA has no significant effects on CaOx formation. We suggest that HA pla ys only a minor role as a crystallisation modifier in human urine.