Non-mucin proteins in the matrix of human cholesterol gallstones

Human cholesterol gallstones contain a pigmented organic matrix that may originate from biliary sludge. The cholesterol gallstone matrix contains mucin, bile pigments, and calcium salts. The goal of this study was to examine whether non-mucin proteins are present in the matrix of cholesterol gallstones. Matrix was prepared from cholesterol gallstones from 18 patients. Proteins were identified by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and by molecular sieve high-performance liquid chromatography (HPLC). Two proteins were present in each gallstone and migrated with or just adjacent to standards of bovine serum albumin on SDS-PAGE. Several additional lower molecular weight proteins were identified, but not in every gallstone. Protein fractions contained visible pigment after chloroform extraction, and pigment co-eluted with proteins on HPLC, suggesting binding of pigments to proteins in the matrix. We conclude that low molecular weight proteins are present in the cholesterol gallstone matrix. The major protein appears to be serum albumin, although definitive identification has not been established. The origin of these matrix proteins and their possible significance in the pathogenesis of cholesterol cholelithiasis is unknown.