Biliverin is a green pigment formed as a byproduct of heme breakdown. It consists of four linearly connected pyrrole rings (a tetrapyrrole). Biliverdin is formed when a heme prosthetic group is cleaved at its α-methine bridge. The resulting water-soluble biliverdin (C33H34N4O6) is then converted to bilirubin (C33H36N4O6) by biliverdin reductase in macrophages of the reticuloendothelial system. The changing color of a bruise from deep purple to yellow over time is a graphical indicator of this reaction.