Quantification of osteopontin in the urine of healthy and stone-forming men

Osteopontin (OPN) is one of the most important components in calcium stone matrix, but its role in stone formation is not clear. Since quantitative data regarding the excretion of OPN are necessary to assess its role, we have developed a quantitative enzyme-linked immunosorbent assay (ELISA) for OPN, and measured the urinary OPN concentrations in urolithiasis patients. Forty-seven men with urinary stones composed chiefly of calcium oxalate participated in the study. The controls were 13 normal healthy male volunteers. Urine samples were collected early in the morning and analyzed by a quantitative ELISA employing purified polyclonal antibodies to synthesized OPN aminopolypeptides. The urinary ratio of the concentrations of OPN and creatinine (OPN/Cre) in the urolithiasis patients (0.039 +/- 0.029) was significantly lower than that in the control subjects (0.062 +/- 0.030) (P < 0.05). Single stone formers(n = 26; 0.050 +/- 0.020) had significantly higher OPN/Cre ratios compared with recurrent stone formers (n = 21; 0.031 +/- 0.021) (P < 0.05). The results show that OPN excretion in urolithiasis patients was lowered, presumably because of the incorporation of OPN by kidney stones.