On the chemistry of 'black' pigment stones from the gallbladder

Radiolucent (33 cases) and radiopaque (17 cases) black pigment gallstones from patients who underwent cholecystectomy were studied using several spectroscopic and chromatographic methods. Radiolucent pigment stones (mean Ca percentage 2.1%) are composed chiefly of degenerated tetrapyrrolic bile pigments (mean 85.3%) deriving from bilirubin and bilirubinates. Degeneration includes both polymerization and bacterial reduction and leads to products of different grade of polymerization. Final extraction residues (mean 55.5%), called the 'black pigments' are considered to be degenerated bile pigments of high molecular weight. The mean percentage of bilirubin (free and inorganic bound bilirubin) was 8.5%, while the percentage of lipids was very low (mean of total lipids approximately 2.7%). Radiopaque black pigment stones (Ca: 12.4%) were composed of 'black pigments', too, but contained large amounts of calcium phosphate (carbonate apatite) and/or calcium carbonate. 65% of the radiopaque stones were calcified by calcium phosphate. 'Black pigments' were degraded by chromate to maleimides and 2,5-pyrroledialdehyde. These degradation products can be prepared in the same way from normal bile pigments with a tetrapyrrole structure. Polymerized dipyrrolic bile pigments like polymer propentdyopent or 'mesobilifuscin' did not give 2,5-pyrroledialdehydes during chromate oxidation. Thus we conclude that the formation of 'black pigments' starts from the polymerization of tetrapyrrolic, but not from dipyrrolic units. Accumulation of unconjugated bilirubin and bilirubinates within the gallbladder will precede the development of 'black pigments' which play an important role in pigment gallstone formation.