Effects of Tamm-Horsfall protein with normal and reduced sialic acid content upon the crystallization of calcium phosphate and calcium oxalate in human urine

Objective To examine the effects of Tamm-Horsfall protein (THP) of normal and low sialic acid content on urinary crystallization, and establish whether there are changes conducive to the formation of kidney stones. Materials and methods Purified samples of THP were recovered from the urine of non-stone forming individuals. A portion of each THP sample was treated with the enzyme neuraminidase to yield the low sialic acid form of the protein. The two forms of THP were added separately to ultrafiltered urine and crystallization was then induced in the urine by evaporation at 37 degrees C. Two types of experiment were then conducted with the crystals that formed; the rate at which the resulting calcium phosphate or calcium oxalate crystals sedimented in the evaporated urine was determined and the proportion of these crystals and protein which was retained when the urine was passed through a 75 mu m sieve was measured. Results Calcium phosphate and calcium oxalate crystals remained in stable colloidal suspension in ultrafiltered urine when in the presence of normal THP; these suspensions passed freely through the 75 mu m sieves. When crystals formed in the presence of low sialic acid THP, the sedimentation was rapid and the crystals were readily retained with protein on the sieves. Conclusions These results indicate that whilst normal THP inhibits urinary crystal aggregation, the properties of the low sialic acid form are consistent with the promotion of crystal aggregation and hence stone formation.