The role of iron in the pathogenesis of porphyria cutanea tarda. II. Inhibition of uroporphyrinogen decarboxylase.

Abstract

A B S T R A C T Porphyria cutanea tarda is characterized biochemically by excessive hepatic synthesis and urinary excretion of uroporphyrin I and 7-carboxyl porphyrins. This pattern of excretion suggests an impaired ability to decarboxylate uroporphyrinogen to the 4-carboxyl porphyrinogen, coproporphyrinogen, a reaction catalyzed by the enzyme uroporphyrinogen de-carboxylase.Because clinical evidence has implicated iron in the pathogenesis of porphyria cutanea tarda, these experiments were designed to study the effect of iron on uroporphyrinogen decarboxylase in porcine crude liver extracts.Mitochondria-free crude liver extracts were preincubated with ferrous ion and aliquots were assayed for uroporphyrinogen decarboxylase activity. Uroporphyrinogens I and III, the substrates for the decarboxylase assay, were prepared enzymatically from [3H]porphobilinogen. The products of the decarboxylase reaction were identified and quantitated by three methods: (a) extraction into ethyl acetate at pH 4.0, back extraction into 1.5 N HCO and spectrophotometric quantitation; (b) adsorption onto talc, esterification, paper chromatographic identification, and quantitation by liquid scintillation counting; and (c) adsorption onto talc, esterification, thin-layer chromatographic identification on silica gel, and quantitation by liquid scintillation counting. The thin-layer scintillation method proved most sensitive as it was the only method which accurately identified and quantitated the 7-carboxyl porphyrin reaction product. Uroporphyrinogens I and III were decarboxylated at the same rate by porcine hepatic uroporphyrinogen decarboxylase, and the addition of iron induced marked inhibition of the decarboxylase activity. Orthophenanthroline blocked the inhibitory effect of iron.The inhibition of uroporphyrinogen decarboxylase by ferrous ion, coupled with its previously reported inhibitory effect on uroporphyrinogen III cosynthetase, provides a possible biochemical explanation for the pattern of urinary porphyrin excretion observed in patients with porphyria cutanea tarda and the clinical association with disordered iron metabolism.

An inherited enzymatic defect in porphyria cutanea tarda: decreased uroporphyrinogen decarboxylase activity.

“…We have reported that iron, an inhibitor of URODECARB, exerts its major inhibitory effect upon the removal of the second carboxyl group, leading to the accumulation of uroporphyrin and 7-carboxyl porphyrin in an in vitro hepatic model of PCT (25). Histologic evidence of excessive hepatocellular iron was found in specimens from all of the patients studied here, a finding that is virtually uniform in PCT (16).…”

“…The concept that there may be a genetically determined contribution to the pathogenesis of PCT has been strengthened by occasional reports describing a familial occurrence of the disease (19,20). In addition, several reports have described abnormalities of porphyrin metabolism in apparently unaffected relatives of patients with PCT (21,22 URODECARB was assayed by a previously published method by utilizing enzymatically generated, tritiated uroporphyrinogen as the substrate and thin layer chromatographic identification and liquid scintillation quantitation of the reaction products (25).…”

The role of transferrin in the mechanism of cellular iron uptake

“…Turnbull et al [131] and Reizenstein et al [132] reported increased iron absorption in overt PCT and this was found also in the presence of excess liver iron. As to the relationship between decreased uroporphyrinogen decarboxylase activity and excess liver iron, most studies suggest an inverse relationship between enzyme activity and liver iron [133][134][135] although Blekkenhorst et al [136] claimed that ferrous iron enhances uroporphyrinogen decarboxylase activity. According to Elder & Sheppard [1371, in PCT there is loss of enzyme activity but no loss of enzyme protein.…”

Iron and the liver: acute effects of iron-loading on hepatic heme synthesis of rats. Role of decreased activity of 5-aminolevulinate dehydrase.

“…Decreases in this heme pool may arise as the result of enhanced heme breakdown or of decreased heme synthesis. It has been proposed by some investigators that iron decreases hepatic heme synthesis by inhibiting uroporphyrinogen decarboxylase (6). However, others have reported no effect (7) or activation (8) of the decarboxylase by ionic iron.…”