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, ␦-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.heme ͉ porphyrins ͉ iron ͉ oxidation

Porfiria cutánea tarda en un paciente VIH: ¿papel secundario o protagonista del virus?

“…Es un hecho frecuente observar siderosis en las biopsias hepáticas de estos pacientes y elevación en los valores séricos de hierro y ferritina, como en el caso que presentamos. La inactivación exclusiva de la UROD hepática podría estar mediada por un inhibidor competitivo (uoroporphomethene) resultado de la oxidación parcial de uroporfirinogeno en el citocromo P450 por un mecanismo oxidativo dependiente del hierro 9,10 . Las manifestaciones clínicas de la enfermedad se delimitan a la piel expuesta al sol y son consecuencia de un exceso de fragilidad cutánea que produce la aparición de ampollas, localizadas con frecuencia en el dorso de las manos, que al romperse producen costras y pequeñas pápulas blancas llamadas milios que dejan finalmente zonas de piel atrófica y de color marrón.…”

Porphyria Cutanea Tarda is a Biochemical and Not Histological Diagnosis

“…Genetic mutation of the UROD enzyme is a susceptibility factor in about 20% PCT cases, also known as familial PCT [1,2]. Iron overload of mild to moderate degree even in the absence of HFE gene mutations is present in PCT, which is required for the generation of the UROD inhibitor, uroporphomethene [4]. In a large series of PCT cases, about 70% of PCT cases had three or more susceptibility factors [7].…”

“…Porphyria cutanea tarda (PCT), the most common human porphyria, is acquired due to deficient activity of hepatic uroporphyrinogen decarboxylase (UROD) in the heme biosynthetic pathway, secondary to production of its inhibitor uroporphomethene, which is produced in the presence of mild to moderate iron overload [1][2][3][4]. Decreased activity of UROD leads to increased accumulation of uroporphyrinogen and the partially decarboxylated intermediates hepta-, hexa-and pentacarboxylate porphyrinogens, which are auto-oxidized to the corresponding porphyrins.…”

CYP1A2*1F and GSTM1 Alleles Are Associated with Susceptibility to Porphyria Cutanea Tarda

“…Therefore, it has been suggested that inhibition of UROD activity is the primary factor causing disease manifestations. An inhibitor of UROD activity has been found in cytosolic extracts from livers of PCT patients and has been identified in a mouse model as a uroporphomethene (9). In mice, cytochrome P450 1a2 (Cyp1a2) plays a major role in the development of uroporphyria and may be involved in the partial oxidation of a heme pathway intermediate (most likely uroporphyrinogen or hydroxymethylbilane) to form the uroporphomethene inhibitor of UROD activity (9)(10)(11).…”