Identification of bikunin isolated from human urine inhibits calcium oxalate crystal growth and its localization in the kidneys.
BACKGROUND
We previously reported a high molecular weight substance purified from human urine that strongly inhibited calcium oxalate (CaOx) crystal growth in vitro. In the study present herein, we identified and investigated a protein purified from human urine that strongly inhibits CaOx crystal growth using a column chromatography series.


METHODS
The protein was identified by amino acid sequencing and was investigated by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), western blotting and immunohistochemical staining.


RESULTS
The molecular weight of this protein was approximately 35 KDa, and it also had another band around 20 KDa. We determined that the amino acid sequence of the protein was homologous with that of bikunin, the light chain of the inter-alpha-trypsin inhibitor, which is known as a strong CaOx crystallization inhibitor in vitro. On western blotting analysis, the molecular weight was also found to be around 35K Da, the same as that of bikunin. Immunohistochemical staining revealed that it was mainly located in the epithelial cells of the proximal tubules and the thin descending segment near the loop of Henle, but not in the glomeruli, distal tubules or the collecting ducts.


CONCLUSIONS
In the present study, the protein extracted from human urine was identical to bikunin, which may be expressed mainly in the proximal tubules and the thin descending segment near the loop of Henle, and which prevents CaOx crystallization in vitro.
D000818:Animals; D002129:Calcium Oxalate / Q000037:antagonists & inhibitors*; D003460:Crystallization; D006801:Humans; D007669:Kidney Calculi / Q000517:prevention & control*; D008297:Male; D008562:Membrane Glycoproteins / Q000494:pharmacology* / Q000652:urine*; D051379:Mice; D008807:Mice, Inbred BALB C; D014360:Trypsin Inhibitor, Kunitz Soybean / Q000494:pharmacology* / Q000652:urine*
D008562:Membrane Glycoproteins; C109148:SPINT2 protein, human; D002129:Calcium Oxalate; D014360:Trypsin Inhibitor, Kunitz Soybean