Isolation and characteristics of human urinary sialoglycoproteins

Three sialoglycoproteins (S1, S2, and S3) have been isolated from normal human urine by ultrafiltration, zone electrophoresis, gel chromatography, and ion-exchange chromatography. The average yields per litre of urine were 0.39 mg (S1), 0.40 mg (S2), and 1.9 mg (S3). The isolated substances were homogeneous on ultracentrifugation both at neutral and acid pH with sedimentation coefficients of 3.6 S (S1), 2.4S (S2), and 0.93S (S3). Equilibrium ultracentrifugation gave molecular weights of 77900 (S1), 37000 (S2), and 5300 (S3). All three components were rich in carbohydrate (64 to 76%) and contained 28 to 35% of sialic acid. Alkaline borohydride degradation of component S3 yielded two sialylated oligosaccharides which were partially characterized. The origin of the isolated substances is unknown. The molecular size of S3 (Stokes' radius of 2.0 nm) is compatible with passage from the blood by glomerular filtration whereas the size of S1 (Stokes' radius of 6.5 nm) would suggest a renal origin.