title: A porphomethene inhibitor of uroporphyrinogen decarboxylase causes porphyria cutanea tarda.
abstract: Porphyria cutanea tarda (PCT), the most common form of porphyria in humans, is due to reduced activity of uroporphyrinogen decarboxylase (URO-D) in the liver. Previous studies have demonstrated that protein levels of URO-D do not change when catalytic activity is reduced, suggesting that an inhibitor of URO-D is generated in hepatocytes. Here, we describe the identification and characterization of an inhibitor of URO-D in liver cytosolic extracts from two murine models of PCT: wild-type mice treated with iron, delta-aminolevulinic acid, and polychlorinated biphenyls; and mice with one null allele of Uro-d and two null alleles of the hemochromatosis gene (Uro-d(+/-), Hfe(-/-)) that develop PCT with no treatments. In both models, we identified an inhibitor of recombinant human URO-D (rhURO-D). The inhibitor was characterized by solid-phase extraction, chromatography, UV-visible spectroscopy, and mass spectroscopy and proved to be uroporphomethene, a compound in which one bridge carbon in the uroporphyrinogen macrocycle is oxidized. We synthesized uroporphomethene by photooxidation of enzymatically generated uroporphyrinogen I or III. Both uroporphomethenes inhibited rhURO-D, but the III isomer porphomethene was a more potent inhibitor. Finally, we detected an inhibitor of rhURO-D in cytosolic extracts of liver biopsy samples of patients with PCT. These studies define the mechanism underlying clinical expression of the PCT phenotype, namely oxidation of uroporphyrinogen to uroporphomethene, a competitive inhibitor of URO-D. The oxidation reaction is iron-dependent.
authors.affiliation: Department of Medicine, University of Utah School of Medicine, Salt Lake City, UT 84132, USA. john.phillips@hsc.utah.edu
authors.forename: John D
authors.initials: JD
authors.lastname: Phillips
authors.forename: Hector A
authors.initials: HA
authors.lastname: Bergonia
authors.forename: Christopher A
authors.initials: CA
authors.lastname: Reilly
authors.forename: Michael R
authors.initials: MR
authors.lastname: Franklin
authors.forename: James P
authors.initials: JP
authors.lastname: Kushner
mesh_terms: D000818:Animals; D002851:Chromatography, High Pressure Liquid; D003600:Cytosol / Q000187:drug effects / Q000201:enzymology; D006801:Humans; D008099:Liver / Q000187:drug effects / Q000201:enzymology; D008110:Liver Extracts; D013058:Mass Spectrometry; D051379:Mice; D008810:Mice, Inbred C57BL; D017119:Porphyria Cutanea Tarda / Q000139:chemically induced / Q000209:etiology*; D011166:Porphyrins / Q000032:analysis / Q000737:chemistry / Q000494:pharmacology*; D011994:Recombinant Proteins / Q000378:metabolism; D014575:Uroporphyrinogen Decarboxylase / Q000037:antagonists & inhibitors*; D014577:Uroporphyrinogens / Q000737:chemistry
chemical_list: D008110:Liver Extracts; D011166:Porphyrins; D011994:Recombinant Proteins; D014577:Uroporphyrinogens; D014575:Uroporphyrinogen Decarboxylase