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. Author manuscript; available in PMC: 2024 Apr 1.
Published in final edited form as: Trends Mol Med. 2023 Feb 22;29(4):315–328. doi: 10.1016/j.molmed.2023.01.007

Figure I. Heme catabolism to bilirubin.

Figure I.

Heme is catabolized by heme oxygenase to biliverdin while releasing oxygen (O2), iron (Fe2+), and carbon monoxide (CO). Then, biliverdin reductase reduces biliverdin to bilirubin IXalpha by an NADPH to NADP mechanism. Bilirubin is cleared by hepatocytes (liver cells) via the UGT1A1 UDP-glucuronosyltransferase (UGT1A1, OMIM *191740) in microsomes by conjugation with glucuronide giving rise to bilirubin diglucuronide (also named bilirubin diglucuronoside according to IUBMB Enzyme Nomenclature), which is then excreted into the bile.