Glycosylation of prothrombin fragment 1 governs calcium oxalate crystal nucleation and aggregation, but not crystal growth.

Urinary glycoproteins play an important role in the modulation of calcium oxalate crystallisation. In several cases, this has been attributed to glycosylation of the proteins as evidenced by urinary prothrombin fragment 1 where there is a correlation between sialylation and calcium oxalate kidney stone disease. In the present study, plasma-derived prothrombin fragment 1 (PTF1) was enzymatically modified in order to generate its asialo and aglyco forms. The parent glycoprotein and its two glycoforms were used in calcium oxalate crystallisation studies to assess the role of the carbohydrate moeity in PTF1's potent inhibitory activity. The glycans inhibited crystal aggregation and promoted crystal nucleation, but had no effect on crystal growth. The terminal sialic acid residues had a small effect on inhibition of crystal aggregation whereas they contributed significantly to promotion of nucleation. These results indicate that glycosylation of PTF1 governs calcium oxalate crystal nucleation and aggregation but it does not affect the protein's role in inhibiting crystal growth. Since promotion of nucleation and inhibition of aggregation are both regarded as protective mechanisms against calcium oxalate urinary stone formation, the kringle domain on which the glycans are located is implicated in PTF1's inhibitory activity. It is speculated that modifications in the glycosylation of urinary PTF1 in stone-formers may regulate its capacity to protect against calcium urolithiasis.