Figure 1.

Schematic illustration of the biosynthesis and catabolism of heme. Rectangles indicate the enzymes involved in heme metabolism. Heme molecules are synthesized from glycine and succinyl Co-A via eight-stepped enzymatic reactions. ALA: γ-aminolevulinic acid; PBG: porphobilinogen. Heme, thus formed, is catabolized to biliverdin by the microsomal enzyme heme oxygenase 1. Biliverdin is subsequently metabolized to bilirubin by biliverdin reductase. ABC transporter ABCB6 is considered to be responsible for the import of coproporphyrinogen III into mitochondria, whereas ABCG2 transports porphyrins across the plasma membrane to maintain intracellular porphyrin homeostasis. ABCG2-mediated porphyrin transport can be inhibited by drugs, such as protein kinase inhibitors, as shown in Figure 5.