Excretion of bikunin and its fragments in the urine of patients with renal stones

Purpose: Bikunin is a Kunitz-type protease inhibitor found in serum and urine. It has been implicated in urinary stone formation. This study was designed to investigate the role of urinary bikunin in stone formation. Materials and Methods: Urinary concentrations of bikunin were measured in 18 male formers of urinary stones 28 to 74 years old and in 77 healthy controls, including 39 males and 38 females, without urological abnormality. A sensitive competitive solid phase enzyme immunoassay was established for urinary bikunin. Bikunin was also qualitatively assessed by Western blot analysis. Results: The mean urinary bikunin-to-creatinine ratio plus or minus standard deviation in stone formers was significantly elevated compared with that in healthy male and female controls (52.3 +/- 46.0 mug./mg. creatinine versus 28.0 +/- 30.4 and 26.5 +/- 21.7, p = 0.005 and 0.006, respectively). By Western blot analysis all urine samples contained authentic 40 kDa. bikunin species. However, a significantly higher proportion of patients was found to have aberrant 25 kDa. bikunin species compared with controls (10 of 18 or 55.6% versus 15 of 77 or 19.5%, p = 0.002). Experiments on de-glycosylation with chondroitinase ABC, amino acid sequencing of the aberrant bikunin species and calcium oxalate crystal growth inhibition assay demonstrated that the 25 kDa. bikunin fragment was identical to de-glycosylated bikunin and less inhibitory on calcium oxalate crystal growth. Conclusions: If urinary bikunin is important in the pathogenesis of urolithiasis, its effect is probably attributable to the concentration and degree of glycosylation.