Reduced inhibitory activity of uronic-acid-rich protein in urine of stone formers.

We recently reported that human urine contains a newly identified urinary glycoprotein acting as a potent inhibitor against calcium oxalate crystallization. This inhibitor is a uronic-acid-rich protein (UAP) with a molecular weight of approximately 35 kDa. In the present study, UAP was isolated from urine of stone formers and of subjects without a stone history, and its inhibitory activity was tested in a calcium oxalate crystallization system in vitro. Our results show a weaker inhibitory activity of UAP extracted from the urine of stone formers than that extracted from the urine of healthy subjects. Preliminary analyses of amino acid and carbohydrate content showed some differences between the two groups. The main difference was the reduction in sialic acid in UAP isolated from the urine of stone formers. We suggest that UAP contributes significantly to total urinary inhibitory activity of calcium oxalate crystallization and that the decrease in this activity in the urine of recurrent stone formers is due, in part, to the weak inhibitory activity of UAP. A structural abnormality of UAP could explain the diminution of its inhibitory activity in the urine of stone formers.