ABSTRACT
In mammals, bilirubin-IXα is excreted by liver cells as carbohydrate conjugates. Initial evidence, indicating an ester glucuronoside structure for a conjugated azo pigment fraction, was based on its hydrolysis with β-glucuronidase and with dilute alkali. The presence in bile of "alkali-stable" noncarbohydrate conjugates of bilirubin remains controversial. Evidence adduced in favor of the existence of a sulfate conjugate includes chromatographic and optical-spectrum similarities between azo pigments supposedly derived from a natural bilirubin sulfate and those prepared from a synthetic bilirubin sulfate of unknown structure, and observation of a 1:1 molar ratio between purified azo pigment and sulfate liberated from it by acid. In view of the heterogeneity and labile nature of bilirubin conjugates, several criteria should be satisfied for their successful analytical and preparative separation. Mineral acids, oxygen, and light must be excluded to prevent free-radical and acid-catalyzed disproportionation of bilirubin and oxidative breakdown.
