Bilirubin conjugates of human bile. The excretion of bilirubin as the acyl glycosides of aldobiouronic acid, pseudoaldobiouronic acid and hexuronosylhexuronic acid, with a branched-chain hexuronic acid as one of the components of the hexuronosylhexuronide.

Structure elucidations have been performed on the bilirubin conjugates isolated from human hepatic bile as the phenylazo derivatives. The major bilirubin conjugates are excreted, not as was formerly thought in the form of glucuronides, but as the acyl glycosides of aldobiouronic acid, pseudoaldobiouronic acid and hexuronosylhexuronic acid. The isolated aldobiouronides are proposed to have the structures of an acyl 6-O-hexopyranosyluronic acid-hexopyranoside, an acyl 4-O-hexofuranosyluronic acid-d-glucopyranoside, and an acyl 4-O-beta-d-glucofuranosyluronic acid-d-glucopyranoside respectively, with the acyl radicals being those of the phenylazo derivative of bilirubin. The pseudoaldobiouronide is suggested to be the acyl 4-O-alpha-d-glucofuranosyl-beta-d -glucopyranosiduronic acid, with the acyl radical being that of the phenylazo derivative of vinylneoxanthobilirubinic acid. The hexuronosylhexuronide presumably is the acyl 4-O-(3-C-hydroxymethylribofuranosyluronic acid)-beta-d-glucopyranosiduronic acid, with the acyl radical being that of the phenylazo derivative of bilirubin. The 3-C-hydroxymethylriburonic acid, isolated as one of the components of the hexuronosylhexuronide, is the first natural branched-chain hexuronic acid to be detected, and the first branched-chain sugar ever detected in humans.