Uroporphyrinogen decarboxylase

An enzyme involved in heme biosynthesis, uroporphyrinogen decarboxylase has been found to be inhibited by a variety of xenobiotics. This assay measures the conversion of pentacarboxylporphyrinogen I to coproporphyrinogen I. 

Coproporphyrinogen I was separated from other assay components by chromatography on a Partisil 5 column (4.6 mm x 250 mm) column. The mobile phase was composed of heptane-ethyl acetate-dichloromethane-methanol (60 25 14 1). Quantitation was by a UV fluorescence detector (excitation, 405 nm detection, 605 nm). At a flow rate of 1.1 mL/min, the 

The enzyme preparation was a low speed homogenate prepared from livers of 17-day-old chicken embryos. 

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Deficiencies of other enzymes in the heme pathway produce porphyrias in which photosensitivity is a common finding. Chronic inflammation to overt blistering and shearing in exposed areas of the skin characterize these porphyrias. The most common is porphyria cutanea tarda (deficiency of uroporphyrinogen decarboxylase). 

Affected enzyme ALA-dehy- dratase Hydroxymethyl-bilane synthase Uroporphyrinogen III synthase Uroporphyrinogen decarboxylase Coproporphyrinogen oxidase Protoporphyrinogen oxidase Ferrochelatase... 

The first porphyrin intermediate of the biosynthetic pathway is uroporphyrinogen, which is stepwise decarboxylated by uroporphyrinogen decarboxylase to heptacarboxy-, hexacarboxy-, pentacarboxy-, and coproporphyrinogen. This latter compound proceeds, as indicated in Fig. 7.3.1, to protoporphyrinogen and protoporphyrin. The oxidized uroporphyrin and its decarboxylation products up to coproporphyrin are assayed in urine. Coproporphyrin and the further downstream intermediaries can be recovered from feces as described below. 

As the next enzymes of the heme pathway are active, uroporphyrinogen is converted to heptacarboxyporphyrinogen during the enzymatic reaction. This can be overcome if the incubation temperature is raised to 45°C, which inactivates uroporphyrinogen decarboxylase. Yet, 37°C is the standard temperature for enzyme activity determinations. Despite our neglect of the additionally formed porphyrins by uroporphyrinogen decarboxylase in the activity calculations, we found a highly reliable test performance. 

A chronic disease caused by a deficiency in uroporphyrinogen decarboxylase. 

The first of these decarboxylations is catalyzed by the cytoplasmic uroporphyrinogen decarboxylase, which removes the carboxylate groups of the four acetate side chains sequentially from the D, A, B, and C rings.395-3963 A possible mechanism, utilizing a tautomerized ring, is illustrated in the accompanying structural formula. 

Jones KG, Sweeney GD. 1977. Association between induction of aryl hydrocarbon hydroxylase and depression of uroporphyrinogen decarboxylase activity. Res Commun Chem Pathol Pharmacol 17 631-637. 

Uroporphyrin I and coproporphyrin I will be present in the urine. In the absence of uroporphyrinogen cosynthase, hydroxymethyl bilane will spontaneously cyclyze into uroporphyrinogen I. Some of it will be oxidized to uroporphyrin I. Some will be decarboxylated by uroporphyrinogen decarboxylase to coproporphyrinogen I and then oxidized to coproporphyrin I. None of these will be degraded by heme oxidase. 

Uroporphyrinogen decarboxylase 1 Coproporphyrinogen III Porphyria cutanea tarda (AD,1q34) J... 

Figure 3 The synthesis of heme from glycine and sucdnyl-CoA. The enzymes are ALAS, S-aminolevulinic acid (ALA) synthase ALAD, S-aminolevulinic acid dehydratase PBGD, porphobilinogen deaminase UROIIIS, uroporphyrinogen III synthase UROD, uroporphyrinogen decarboxylase CPO, coproporphyrinogen oxidase PPO, protoporphyrinogen oxidase and FECH, ferrochelatase.

Disorder of haem synthesis Inhibition of hepatic coproporphy-rinogen-oxidase and uroporphyrinogen-decarboxylase can give... 

This is a very rare form of chronic hepatic porphyria. As with PCT, the enzymatic defect is a deficiency of uroporphyrinogen decarboxylase. However, the genetic defect is homozygous. It may also be caused by exogenous factors. HEP manifests in early childhood with high photosensitivity, sclerodermia, hypertrichosis and anaemia. The liver shows red fluorescence. Histologically, siderosis and non-specific hepatitis are found. Development of cirrhosis is possible. No effective therapy is known. (314, 316)... 

In porphyria cutanea tarda, iron deposition in the liver is generally low to moderate, predominantly in the hepa-tocytes of the acinar periphery. The cause is thought to be reduced activity of uroporphyrinogen decarboxylase resulting in decreased synthesis of porphyrin and haem. However, this variant of hepatic siderosis only becomes manifest as a result of various triggering factors. Treatment is also based on venesection. 

Egger, N.G., Goeger, D.E., Payne, D.A., Miskovsky, E.P., Weinman, S.A., Anderson, K.E. Porphyria cutanea tarda. Multiplicity of risk factors including HFE mutations, hepatitis C, and inherited uroporphyrinogen decarboxylase deficiency. Dig. Dis. Sci. 2002 47 419-426... 

The genes for all the enzymes of human heme biosynthesis have been characterized (Table 32-2), and the structures of 5-aminolevulinic acid dehydratase (ALAD), hydroxymethyl-bilane synthase (HMBS), uroporphyrinogen-III synthase (UROS), uroporphyrinogen decarboxylase (UROD), and ferrochelatase (FECH) have been determined by x-ray crys-tallography. - - ... 

Christiansen L, Ged C, Hombrados I, Brons-Poulsen J, Fontanellas A, de Verneuil H, et al. Screening for mutations in the uroporphyrinogen decarboxylase gene using denaturing gradient gel electrophoresis. Identification and characterization of six novel mutations associated with familial PCT. Hum Mutat 1999 14 222-32. 

Elder GH, Roberts AG, Uroporphyrinogen decarboxylase. J Bioenerg Biomembr 1994 27 207-14,... 

Elder GH, Wyvfil PC. Measurement of uroporphyrinogen decarboxylase using porphyrinogens prepared by chemical reduction. Enzyme 1982 28 186-95. 

Luo J, Lim CK. Order of uroporphyrinogen-ill decarboxylations on incubation of porphobilinogen and uroporphyrinogen-III with erythrocyte uroporphyrinogen decarboxylase. Biochem J 1993 289, 519-523. 

Mendez M, Sorkin L, Rossetti MV, Astrin KH, del C Batlle AM, Parera VE, et al. Familial porphyria cutanea tarda characterization of seven novel uroporphyrinogen decarboxylase mutations and frequency of common hemochromatosis aUeles. Am J Hum Genet 1998 63 1363-75. 

Mukerji SK, Pimstone NR. Uroporphyrinogen decarboxylases from human erythrocytes purification, complete separation and partial characterization of the two isoenzymes. Int J Biochem 1992 24 105-19. 

Phillips JD, Parker TL, Schubert HL, Whitby EG, HiU CP, Kushner JP. Functional consequences of naturally occurring mutations in human uroporphyrinogen decarboxylase. Blood 2001 98 3179-85. 

Whitby FG, Phillips JD, Kushner JP, HiH CP. Crystal structure of human uroporphyrinogen decarboxylase. EMBO Journal 1998 17 2463-71. 

Cytosolic uroporphyrinogen decarboxylase catalyzes successive decarboxylation of the four acetic groups to yield four methyl groups (Figure 29-6). 

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