Bilirubin analysis--the state of the art and future prospects.

Progress in separating and identifying different bile pigments in serum has led to the recognition of a bilirubin fraction (delta) distinct from unconjugated bilirubin and its (mono- and di-) sugar conjugates. Delta bilirubin reacts directly diazo-positive and is strongly linked to an albumin-like protein, presumably via an amide bond between a propionic acid side-chain of the tetrapyrrole and a functional group (e.g., epsilon-amino group of lysine) on the protein backbone. Because of its unusual properties and its wide occurrence in icteric sera, the delta fraction may have important analytical and clinical implications. We examine here some of these implications and discuss the prospects for a better understanding of the molecular basis of jaundice.