The waste product bilirubin is produced from the heme molecules lacking iron. Bilirubin is a highly insoluble antioxidant that is produced by the metabolism of heme. It is a brownish yellow pigment of bile, secreted by the liver in vertebrates, which gives to solid waste products (feces) their characteristic color. It is produced in bone marrow cells and in the liver as the end product of red blood cell breakdown.
