Analysis of the soluble organic matrix of five morphologically different kidney stones

Our aims were to analyze the protein composition of the organic matrix of urinary stones and to investigate the role of albumin in its constitution. Five different morphological types of stones were studied. Proteins extracted from the stone were submitted to sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and analyzed by immunoblotting with antibodies to 13 urinary proteins. Nine of the 13 proteins were found in all types of stone: human serum albumin (HSA), α1-acid glycoprotein (α1-GP), α1-microglobulin (α1-M), immunoglobulins (Igs), apolipoprotein A1 (apo-A1), transferrin (Tr), α1-antitrypsin (α1-T), retinol-binding protein (RBP) and renal lithostathine (RL). The β2-microglobulin (β2-M) was present only in calcium oxalate and uric acid stones. In contrast, ceruloplasmin, haptoglobin and Tamm-Horsfall protein (THP) were detected in none of them. Because HSA appeared as the major protein component in all stones, we wondered whether it might play a specific role in the constitution of the stone matrix. Association of HSA with urinary proteins that were present in stones was demonstrated by showing that proteins present in the matrix comigrated with HSA on gel filtration, whereas proteins that were absent did not. Moreover, HSA induced the binding of stone matrix proteins to an albumin-specific affinity column. Finally, we evidenced HSA binding to calcium oxalate monohydrate (COM) crystals in a solution similar to urine. It was concluded that (1) only a subset of urinary proteins is present in stone matrix, (2) the same proteins are found in all types of stones, (3) HSA shows significant affinity for several proteins of the matrix, but not for proteins absent from stones and, (4) HSA also displays significant affinity for COM crystals.