Oxidation of Uroporphyrinogens During Heme Synthesis: Role of Iron, Susceptibility and Consequences

In the biosynthesis of heme the tetrapyrrole hydroxymethylbilane is converted enzymatically to uroporphyrinogen III whereas conversion to uroporphyrinogen I occurs spontaneously. Both are substrates for uroporphyrinogen decarboxylase (UROD) but only the III isomer is a precursor of heme. These porphyrinogens are easily oxidised to the respective uroporphyrins and trace amounts occur in the urine of healthy humans and animals. Large quantities of uroporphyrins I and III, as well as other oxidation products, occur in the liver and urine of patients with some porphyrias and after poisoning of people and animals by chemicals, such as hexachlorobenzene (HCB) and 2,3,7,8-tetrachorodibenzo-p-dioxin (TCDD). In the acquired disorder sporadic porphyria cutanea tarda (sPCT) and chemical-induced porphyria, hepatic UROD is inhibited, ostensibly by a partially oxidised uroporphyrinogen. The processes leading to oxidation of the uroporphyrinogens are interactions of a variety of external, internal and genetic factors. In some experimental systems, cytochrome P450 1A2 is implicated in the oxidation of uroporphyrinogens and uroporphyria and many patients and in vivo studies demonstrate the influence of iron. The article reviews the present state of knowledge of uroporphyrinogen oxidation, susceptibility and outcomes, and illustrates areas that require further exploration to explain fully the mechanisms of sPCT and the related uroporphyria caused by chemicals of toxic concern.