The synthesis and excretion of non-erythroid bilirubin in the dog

Between 10 and 20% of the bilirubin excreted in the bile is not derived from the breakdown of hemoglobin. When delta-aminolevulinic acid is given as a bilirubin precursor, 99% of the bilirubin formed is of this non-erythroid variety. The non-erythroid bilirubin has been supposed to be synthetized exclusively in the liver. Sequential samples of arterial, portal and liver vein blood, as well as of bile and thoracic duct lymph, were analyzed for bilirubin and non-bilirubin radioactivity following the injection of labeled delta-aminolevulinic acid in dogs. Radioactive bilirubin appeared in blood before it could be demonstrated in bile or lymph. The concentration differences between aortic, portal and liver vein blood displayed a considerable extrahepatic non-erythroid bilirubin synthesis. Most non-erythroid bilirubin synthetized in the liver is excreted directly into the biliary canaliculi.