Some kinetic properties of human red cell uroporphyrinogen decarboxylase

mRNA-based therapy in a rabbit model of variegate porphyria offers new insights into the pathogenesis of acute attacks

Variegate porphyria (VP) results from haploinsufficiency of protoporphyrinogen oxidase (PPOX), the seventh enzyme in the heme synthesis pathway. There is no VP model that recapitulates the clinical manifestations of acute attacks. Combined administrations of 2-allyl-2-isopropylacetamide and rifampicin in rabbits halved hepatic PPOX activity, resulting in increased accumulation of a potentially neurotoxic heme precursor, lipid peroxidation, inflammation, and hepatocyte cytoplasmic stress. Rabbits also showed hypertension, motor impairment, reduced activity of critical mitochondrial hemoprotein functions, and altered glucose homeostasis. Hemin treatment only resulted in a slight drop in heme precursor accumulation but further increased hepatic heme catabolism, inflammation, and cytoplasmic stress. Hemin replenishment did protect against hypertension, but it failed to restore action potentials in the sciatic nerve or glucose homeostasis. Systemic porphobilinogen deaminase (PBGD) mRNA administration increased hepatic PBGD activity, the third enzyme of the pathway, and rapidly normalized serum and urine porphyrin precursor levels. All features studied were improved, including those related to critical hemoprotein functions. In conclusion, the VP model recapitulates the biochemical characteristics and some clinical manifestations associated with severe acute attacks in humans. Systemic PBGD mRNA provided successful protection against the acute attack, indicating that PBGD, and not PPOX, was the critical enzyme for hepatic heme synthesis in VP rabbits.

Erythrocyte uroporphyrinogen decarboxylase activity and therapeutic phlebotomy in porphyria cutanea tarda

On the basis of the uroporphyrinogen decarboxylase ( UD ) activity in the erythrocytes, and the family history of the disease, different types of porphyria cutanea tarda ( PCT ) can be distinguished. In some cases, however, the distinction may involve some uncertainty (overlapping of subgroups). The question arises of whether the current erythrocyte UD activities in the different types of PCT are determined merely genetically. The erythrocyte UD activities in 72 unrelated patients with different forms of PCT (62 with type I PCT and 10 with type II PCT ), in different stages of the disease, were measured in order to test whether the activity exhibits any change during the long period of recovery. In both types the activities were faintly but significantly increased, from 94.9% (in PCT I) or 54.3% (in PCT II) up to 98.4% or 56.1% respectively. In both types the lower activity in the untreated condition can be attributed to a combination of several factors, including oxidative damage to UD , which results in a minor additional inhibition of the genetically determined enzyme activities.

Measurement of uroporphyrinogen decarboxylase activity

Uroporphyrinogen decarboxylase (UROD) catalyzes decarboxylation of the four acetate side chains of urophyrinogen to form coproporphyrinogen. Activity of UROD can be measured using an enzymatically prepared substrate or a chemically prepared one. For the former, bacterial porphobilinogen deaminase is prepared and used to prepare the porphyrinogen substrate for the enzymatic assay. Erythrocyte lysates can be used to measure hemoglobin content as an indicator of UROD activity.

Substrate shuttling between active sites of uroporphyrinogen decarboxylase is not required to generate coproporphyrinogen

Uroporphyrinogen decarboxylase (URO-D; EC 4.1.1.37), the fifth enzyme of the heme biosynthetic pathway, is required for the production of heme, vitamin B12, siroheme, and chlorophyll precursors. URO-D catalyzes the sequential decarboxylation of four acetate side chains in the pyrrole groups of uroporphyrinogen to produce coproporphyrinogen. URO-D is a stable homodimer, with the active-site clefts of the two subunits adjacent to each other. It has been hypothesized that the two catalytic centers interact functionally, perhaps by shuttling of reaction intermediates between subunits. We tested this hypothesis by construction of a single-chain protein (single-chain URO-D) in which the two subunits were connected by a flexible linker. The crystal structure of this protein was shown to be superimposable with wild-type activity and to have comparable catalytic activity. Mutations that impaired one or the other of the two active sites of single-chain URO-D resulted in approximately half of wild-type activity. The distributions of reaction intermediates were the same for mutant and wild-type sequences and were unaltered in a competition experiment using I and III isomer substrates. These observations indicate that communication between active sites is not required for enzyme function and suggest that the dimeric structure of URO-D is required to achieve conformational stability and to create a large active-site cleft.

Non-alcoholic steatohepatitis induced by carbamazepine and variegate porphyria

A 42-year-old woman presented with acute bullous skin lesions and angio-oedema that had developed 3 months after initiation of treatment with carbamazepine for epilepsy. Chromatographic analysis of urinary porphyrins was compatible with variegate porphyria. This was manifested initially by neurological symptoms that were mistaken for epilepsy and later by cutaneous symptoms also. Histological findings excluded hepatic porphyria, but revealed severe fatty changes thought to be caused by idiosyncratic metabolism of carbamazepine. While the porphyrinogenicity of carbamazepine is well known, the presence of variegate porphyria has not been reported. The toxic hepatic effects of the drug on hepatic cytochrome P-450, which is involved in haem metabolism, could have aggravated the pre-existent porphyria, provoking the onset of skin lesions.

Hepatic porphyrin concentration and uroporphyrinogen decarboxylase activity in hepatitis C virus infection

Previous studies have shown a high prevalence of hepatitis C virus (HCV) infection in patients with porphyria cutanea tarda (PCT). The aim of this study was to assess hepatic porphyrin concentrations (HPC) and hepatic uroporphyrinogen decarboxylase (UROD) activity in HCV-infected patients free of PCT. Thirty-two HCV-infected patients (20 M, 12 F, mean age 51 years) and seven control patients (4 M, 3 F, mean age 59 years) free of liver disease, were studied. Knodell's score was determined on liver biopsy by two independent anatomopathologists. Measurement of HPC and hepatic UROD activity levels were carried out on liver biopsy. Relative to controls, HCV-infected patients had high HPC levels (mean +/- SD: 47 +/- 20 vs. 17 +/- 6 pmol/mg protein, P < 0.001) and low hepatic UROD activity levels (514 +/- 95 vs. 619 +/- 125 pmol Copro/h/mg protein, P < 0.05). HPC was not correlated with hepatic UROD activity and the increase was due to coproporphyrin accumulation. No correlation was observed between HPC or hepatic UROD activity values and HCV-RNA concentrations, Knodell's score, hepatic fibrosis, periportal necrosis, periportal inflammation or hepatic iron content in HCV-infected patients. Hepatocellular necrosis was significantly correlated with HPC value (P < 0.005). Hence, in HCV-infected patients, HPC is significantly increased and hepatic UROD activity is very slightly decreased as compared to controls. HPC values and UROD activity are not correlated with HCV-RNA concentrations, hepatic iron content and hepatic fibrosis. The small increase in HPC values in hepatitis C infection is linked with hepatic injury and not with a direct effect on hepatic UROD enzyme.

Correction of Uroporphyrinogen Decarboxylase Deficiency (Hepatoerythropoietic Porphyria) in Epstein-Barr Virus-Transformed B-Cell Lines by Retrovirus-Mediated Gene Transfer: Fluorescence-Based Selection of Transduced Cells

Hepatoerythropoietic porphyria (HEP) is an inherited metabolic disorder characterized by the accumulation of porphyrins resulting from a deficiency in uroporphyrinogen decarboxylase (UROD). This autosomal recessive disorder is severe, starting early in infancy with no specific treatment. Gene therapy would represent a great therapeutic improvement. Because hematopoietic cells are the target for somatic gene therapy in this porphyria, Epstein-Barr virus-transformed B-cell lines from patients with HEP provide a model system for the disease. Thus, retrovirus-mediated expression of UROD was used to restore enzymatic activity in B-cell lines from 3 HEP patients. The potential of gene therapy for the metabolic correction of the disease was demonstrated by a reduction of porphyrin accumulation to the normal level in deficient transduced cells. Mixed culture experiments demonstrated that there is no metabolic cross-correction of deficient cells by normal cells. However, the observation of cellular expansion in vitro and in vivo in immunodeficient mice suggested that genetically corrected cells have a competitive advantage. Finally, to facilitate future human gene therapy trials, we have developed a selection system based on the expression of the therapeutic gene. Genetically corrected cells are easily separated from deficient ones by the absence of fluorescence when illuminated under UV light.

Correction of Uroporphyrinogen Decarboxylase Deficiency (Hepatoerythropoietic Porphyria) in Epstein-Barr Virus-Transformed B-Cell Lines by Retrovirus-Mediated Gene Transfer: Fluorescence-Based Selection of Transduced Cells

Hepatoerythropoietic porphyria (HEP) is an inherited metabolic disorder characterized by the accumulation of porphyrins resulting from a deficiency in uroporphyrinogen decarboxylase (UROD). This autosomal recessive disorder is severe, starting early in infancy with no specific treatment. Gene therapy would represent a great therapeutic improvement. Because hematopoietic cells are the target for somatic gene therapy in this porphyria, Epstein-Barr virus-transformed B-cell lines from patients with HEP provide a model system for the disease. Thus, retrovirus-mediated expression of UROD was used to restore enzymatic activity in B-cell lines from 3 HEP patients. The potential of gene therapy for the metabolic correction of the disease was demonstrated by a reduction of porphyrin accumulation to the normal level in deficient transduced cells. Mixed culture experiments demonstrated that there is no metabolic cross-correction of deficient cells by normal cells. However, the observation of cellular expansion in vitro and in vivo in immunodeficient mice suggested that genetically corrected cells have a competitive advantage. Finally, to facilitate future human gene therapy trials, we have developed a selection system based on the expression of the therapeutic gene. Genetically corrected cells are easily separated from deficient ones by the absence of fluorescence when illuminated under UV light.

Familial porphyria cutanea tarda: characterization of seven novel uroporphyrinogen decarboxylase mutations and frequency of common hemochromatosis alleles

Familial porphyria cutanea tarda (f-PCT) results from the half-normal activity of uroporphyrinogen decarboxylase (URO-D). Heterozygotes for this autosomal dominant trait are predisposed to photosensitive cutaneous lesions by various ecogenic factors, including iron overload and alcohol abuse. The 3.6-kb URO-D gene was completely sequenced, and a long-range PCR method was developed to amplify the entire gene for mutation analysis. Four missense mutations (M165R, L195F, N304K, and R332H), a microinsertion (g10insA), a deletion (g645Delta1053), and a novel exonic splicing defect (E314E) were identified. Expression of the L195F, N304K, and R332H polypeptides revealed significant residual activity, whereas reverse transcription-PCR and sequencing demonstrated that the E314E lesion caused abnormal splicing and exon 9 skipping. Haplotyping indicated that three of the four families with the g10insA mutation were unrelated, indicating that these microinsertions resulted from independent mutational events. Screening of nine f-PCT probands revealed that 44% were heterozygous or homozygous for the common hemochromatosis mutations, which suggests that iron overload may predispose to clinical expression. However, there was no clear correlation between f-PCT disease severity and the URO-D and/or hemochromatosis genotypes. These studies doubled the number of known f-PCT mutations, demonstrated that marked genetic heterogeneity underlies f-PCT, and permitted presymptomatic molecular diagnosis and counseling in these families to enable family members to avoid disease-precipitating factors.

Crystal structure of human uroporphyrinogen decarboxylase

Uroporphyrinogen decarboxylase (URO-D) catalyzes the fifth step in the heme biosynthetic pathway, converting uroporphyrinogen to coproporphyrinogen by decarboxylating the four acetate side chains of the substrate. This activity is essential in all organisms, and subnormal activity of URO-D leads to the most common form of porphyria in humans, porphyria cutanea tarda (PCT). We have determined the crystal structure of recombinant human URO-D at 1.60 A resolution. The 40.8 kDa protein is comprised of a single domain containing a (beta/alpha)8-barrel with a deep active site cleft formed by loops at the C-terminal ends of the barrel strands. Many conserved residues cluster at this cleft, including the invariant side chains of Arg37, Arg41 and His339, which probably function in substrate binding, and Asp86, Tyr164 and Ser219, which may function in either binding or catalysis. URO-D is a dimer in solution (Kd = 0.1 microM), and this dimer also appears to be formed in the crystal. Assembly of the dimer juxtaposes the active site clefts of the monomers, suggesting a functionally important interaction between the catalytic centers.

Hepatic uroporphyrinogen decarboxylase activity in porphyria cutanea tarda patients: the influence of virus C infection

Porphyria cutanea tarda (PCT) is caused by a decreased activity of the hepatic enzyme uroporphyrinogen decarboxylase (URO-D). This deficiency causes overproduction, hepatic deposition, and increased excretion of uroporphyrin. Iron overload and hepatic viral infections are considered aggravating factors of the disease. Two forms of PCT have been described, as follows: a familial one with an inherited decrease of URO-D activity in all tissues and a sporadic one with a decreased activity of URO-D restricted to the liver. To assess whether the hepatic URO-D returns to normal during a remission of the disease, this activity was measured in liver biopsy samples in 24 sporadic PCT patients. The hepatic and urinary porphyrin concentrations were also measured. Viral status and histopathological findings were analyzed to assess their involvement in PCT. Six patients treated by phlebotomy to reduce hepatic iron and who were considered to be in clinical remission, characterized by a disappearance of cutaneous lesions, showed higher hepatic URO-D activities and lower hepatic porphyrin concentrations than did patients with overt PCT. The medians of these variables, however, did not achieve normal values. The hepatic URO-D activity showed a significant inverse relationship with both hepatic porphyrins and urinary uroporphyrin excretion. Hepatic URO-D activity was not reduced by hepatitis C virus (HCV) infection and liver damage. We conclude that the achievement of remission in PCT largely depends on the transient normalization of hepatic URO-D activity. A small increase in hepatic coproporphyrin in nonporphyric patients could reflect hepatic injury/iron/alcohol-induced oxidative stress oxidizing the accumulated heme precursors rather than a direct effect on hepatic URO-D enzyme.

A conserved cysteine residue in yeast uroporphyrinogen decarboxylase is not essential for enzymatic activity

Uroporphyrinogen decarboxylase catalyzes the fifth step of heme biosynthesis in Saccharomyces cerevisiae. Studies utilizing sulfhydryl-specific reagents suggest that the enzyme requires a cysteine residue within the catalytic site. This hypothesis was tested directly by site-directed mutagenesis of highly conserved cysteine-52 to serine or alanine. Plasmids containing these mutations were able to complement a hem6 mutant strain. In addition, properties associated with decreased uroporphyrinogen decarboxylase activity were not detected in the mutant strain transformed with these mutant plasmids. These results suggest that the conserved cysteine-52 by itself is not essential for enzymatic activity.

Mutational analysis of human uroporphyrinogen decarboxylase

Uroporphyrinogen decarboxylase (URO-D), a heme biosynthetic enzyme, catalyzes the multi-step decarboxylation reaction converting uroporphyrinogen I or III to coproporphyrinogen I or III. The URO-D protein has been purified from several sources and its gene has been cloned from many organisms. In spite of this, little is known about the active site(s) of the enzyme. Inhibitor studies suggest that cysteine and histidine residues are important for enzyme activity. We employed the Kunkel method of site-directed mutagenesis to convert each of the six cysteines in human URO-D to serine and each of the three conserved histidines to asparagine. Recombinant mutant URO-D's were expressed in Escherichia coli, partially purified, and their kinetic properties compared to recombinant wild-type URO-D. All cysteine mutants retained approx. 40% wild-type enzyme activity, indicating that no single cysteine is absolutely critical for the integrity of the catalytic site. The three histidine mutants also retained significant enzyme activity and one, (H339N), displayed unique properties. The H339N mutation resulted in an enzyme with high residual activity but decarboxylation of intermediate reaction products of the I isomer series was markedly abnormal. The histidine at residue 339 is likely important in imparting isomer specificity.

Uroporphyrinogen decarboxylase

Uroporphyrinogen decarboxylase (EC 4.1.1.37) catalyzes the decarboxylation of uroporphyrinogen III to coproporphyrinogen III. The amino acid sequences, kinetic properties, and physicochemical characteristics of enzymes from different sources (mammals, yeast, bacteria) are similar, but little is known about the structure/function relationships of uroporphyrinogen decarboxylases. Halogenated and other aromatic hydrocarbons cause hepatic uroporphyria by decreasing hepatic uroporphyrinogen decarboxylase activity. Two related human porphyrias, porphyria cutanea tarda and hepatoerythropoietic porphyria, also result from deficiency of this enzyme. The roles of inherited and acquired factors, including iron, in the pathogenesis of human and experimental uroporphyrias are reviewed.

Studies on the active centre(s) of rat liver porphyrinogen carboxy-lyase. In vivo effect of hexachlorobenzene on decarboxylation site(s) of porphyrinogens

1. The role of histidine on the decarboxylation of porphyrinogens of 7-, 6-, and 5-COOH III brought about by porphyrinogen carboxy-lyase (PCL) was studied. 2. For this purpose hepatic PCL from normal and hexachlorobenzene (HCB) treated rats were modified with diethylpyrocarbonate. 3. The results indicated that the enzyme from both normal and porphytic animals had histidine at the binding sites of all the porphyrinogens assayed. 4. Comparative studies between the enzyme from normal and porphyric rats suggested that in vivo HCB treatment affected the active site for the decarboxylation of 7-, 6- and 5-COOH porphyrinogens III at histidine residues. 5. On the other hand arginine modification by 2,3-butanedione treatment altered 5-COOH porphyrinogen III decarboxylation for both enzymes. However this amino acid was not involved at the binding site of this substrate.

Identification of amino acid changes affecting yeast uroporphyrinogen decarboxylase activity by sequence analysis of hem12 mutant alleles

The molecular basis of the uroporphyrinogen decarboxylase defect in eleven yeast 'uroporphyric' mutants was investigated. Uroporphyrinogen decarboxylase, an enzyme of the haem-biosynthetic pathway, catalyses the decarboxylation of uroporphyrinogen to coproporphyrinogen and is encoded by the HEM12 gene in the yeast Saccharomyces cerevisiae. The mutations were identified by sequencing the mutant hem12 alleles amplified in vitro from genomic DNA extracted from the mutant strains. Four mutations leading to the absence of enzyme protein were found: one mutation caused the substitution of the translation initiator Met to Ile, a two-base deletion created a frameshift at codon 247 and two nonsense mutations were found at codons 50 and 263. Four different point mutations were identified in seven 'leaky' mutants with residual modified uroporphyrinogen decarboxylase activity; each of three mutations was found in two independently isolated mutants. The nucleotide transitions resulted in the amino acid substitutions Ser-59 to Phe, Thr-62 to Ile, Leu-107 to Ser, or Ser-215 to Asn, all located in or near highly conserved regions. The results suggest that there is a single active centre in uroporphyrinogen decarboxylase, the geometry of which is affected in the mutant enzymes.

Characterization of a new mutation (R292G) and a deletion at the human uroporphyrinogen decarboxylase locus in two patients with hepatoerythropoietic porphyria

A deficiency in the activity of uroporphyrinogen decarboxylase (UROD), the fifth enzyme of the haem biosynthetic pathway, is found in familial porphyria cutanea tarda (F-PCT) and hepatoerythropoietic porphyria (HEP). A new mutation (R292G) and a deletion have been found in a pedigree with two HEP patients (two sisters). The R292G mutation was not detected in 13 unrelated affected patients with F-PCT, so it appears to be uncommon. The possibility that the arginine 292 may participate at the active site of the enzyme is discussed. A summary of the 7 mutations/deletions found in the UROD gene with their frequency is presented.

Uroporphyrinogen decarboxylase from mouse mammary carcinoma and liver of normal and tumor-bearing mouse

1. URO-D was investigated in crude extracts from mouse mammary carcinoma, normal mouse (NM) liver and tumor-bearing mouse (TBM) liver. 2. URO-D from TBM liver and tumor appears to be more sensitive to increasing concentrations of UROgen than the NM liver enzyme. 3. In tumor the rate-limiting step seems to be the decarboxylation of the first carboxyl group, but this was not so clear for the NM and the TBM liver URO-D. 4. URO-D activity was enhanced when incubated at higher temperatures in the presence of its substrate, suggesting that UROgen might afford some protection of the enzyme against heat inactivation. 5. The optimum pH for all three sources is around 7.0.

Uroporphyrinogen decarboxylase in Saccharomyces cerevisiae. HEM12 gene sequence and evidence for two conserved glycines essential for enzymatic activity

The HEM12 gene from Saccharomyces cerevisiae encodes uroporphyrinogen decarboxylase which catalyzes the sequential decarboxylation of the four acetyl side chains of uroporphyrinogen to yield coproporphyrinogen, an intermediate in protoheme biosynthesis. The gene was isolated by functional complementation of a hem12 mutant. Sequencing revealed that the HEM12 gene encodes a protein of 362 amino acids with a calculated molecular mass of 41,348 Da. The amino acid sequence shares 50% identity with human and rat uroporphyrinogen decarboxylase and shows 40% identity with the N-terminus of an open reading frame described in Synechococcus sp. We determined the sequence of two hem12 mutations which lead to a totally inactive enzyme. They correspond to the amino acid changes Gly33----Asp and Gly300----Asp, located in two evolutionarily conserved regions. Each of these substitutions impairs binding of substrates without affecting the overall conformation of the protein. These results argue that a single active center exists in uroporphyrinogen decarboxylase.

Uroporphyrinogen decarboxylases from human erythrocytes: purification, complete separation and partial characterization of two isoenzymes

1. Two distinct molecular forms of uroporphyrinogen decarboxylase have been completely separated and highly purified from human erythrocytes. 2. Each protein, with molecular masses of about 52-54 kDa and 35 kDa, are apparently composed of a single polypeptide chain. 3. They may form a functional decarboxylating complex for heme biosynthesis.

Studies on the active centre of rat liver porphyrinogen carboxylase in vivo effect of hexachlorobenzene

1. Porphyrinogen carboxylase from the liver of normal and hexachlorobenzene porphyric rats was subjected to chemical modification using photo-oxidation with methylene blue, diethylpyrocarbonate, butane-2,3-dione, and phenylglyoxal. 2. All of these chemicals inactivated the enzyme from both sources. 3. Reversion of the diethylpyrocarbonate reaction with hydroxylamine as well as protection of the enzymes with uroporphyrinogen III indicated that histidine is involved at least in the first decarboxylation active site of the porphyrinogen carboxylyase, and perhaps in one or more sites where the removal of the other carboxyl groups take place. 4. Arginine seems not to be at the active site of the enzyme but at its environment since two diketones alter the enzyme activity, however the substrate did not protect the enzyme from the butane-2,3-dione modification. 5. Comparative studies between the enzyme from normal and porphyric animals suggest that the low enzyme activity from intoxicated animals could be due to alterations of its active centre environment produced by hexachlorobenzene treatment. This treatment seems to partially protect the active site of the porphyrinogen carboxylase from the modification reactions.

Purification and properties of uroporphyrinogen decarboxylase from Saccharomyces cerevisiae. Yeast uroporphyrinogen decarboxylase

Uroporphyrinogen decarboxylase (EC 4.1.1.37) was purified about 14000-fold to homogeneity from the yeast Saccharomyces cerevisiae with a 70% overall yield. The purification included affinity chromatography on uroporphyrin-I-Affi-Gel 102. The specific activity of the final preparation was 1750 nmol coproporphyrinogen formed.h-1.(mg protein)-1 at pH 7.5 and 37 degrees C using 4 microM uroporphyrinogen I as substrate. The purified enzyme has a minimum molecular mass of 38 kDa by sodium dodecyl sulfate/polyacrylamide gel electrophoresis and 46 kDa by gel filtration, suggesting that yeast uroporphyrinogen decarboxylase is a monomer. Chromatofocusing gave a pI of 6.0. Enzyme activity was inhibited by metals, such as Cu2+, Zn2+, Fe2+, Fe3+ and by sulfhydryl-specific reagents, but no cofactor requirement could be demonstrated. The optimum pH was pH 5.7 for uroporphyrinogens I and III and heptacarboxylate porphyrinogen I as estimated by coproporphyrinogen formation. The optimum pH for substrate decarboxylation was pH 5.7 for uroporphyrinogen I, but pH 6.8 for the two other substrates. The Km values at pH 5.7 were 10 nM for uroporphyrinogen I, 6 nM for uroporphyrinogen III and 7 nM for heptacarboxylate porphyrinogen I as measured by coproporphyrinogen formation. The pattern of accumulation of intermediate and final decarboxylation products and the rates of the successive decarboxylations were determined for the three substrates at different concentrations at pH 5.7 and pH 6.8. The rate-limiting step at 4 microM substrate concentration was the elimination of the second carboxyl group of uroporphyrinogen III and the fourth carboxyl of uroporphyrinogen I. An antiserum to purified yeast uroporphyrinogen decarboxylase was used to characterize the protein in several mutants.

Studies on uroporphyrinogen decarboxylase of etiolated Euglena gracilis Z

1. A 423-fold purified fraction of uroporphyrinogen decarboxylase (EC 4.1.1.37) showing a specific activity of 770 units/mg protein has been employed in order to study some properties in etiolated Euglena gracilis Z. 2. Uroporphyrinogen decarboxylase has a relative molecular mass of 54,000, an optimum pH of 7.2 and exhibits Michaelis-Menten kinetics, employing both uroporphyrinogen I and uroporphyrinogen III as substrates. 3. Anaerobic conditions seem not to be necessary for uroporphyrinogen decarboxylase activity. Neither EDTA nor cysteine affected enzyme activity, whereas dithiothreitol produced a remarkable activation of coproporphyrinogen formation. 4. Kinetic data employing both substrates showed an accumulation of porphyrinogen (i.e. hexa- and hepta-porphyrin) containing six or seven COOH groups, depending on the uroporphyrinogen concentration used. 5. An unusual elution profile of the intermediates on Sephacryl S-200 was found.

A simplified method for determination of uroporphyrinogen decarboxylase activity in human blood

The determination of erythrocyte uroporphyrinogen decarboxylase activity is essential for differentiating familial (type II) porphyria cutanea tarda from the sporadic (type I) form of the disease. A new technique for the determination of uroporphyrinogen decarboxylase activity in human blood is described. Haemolysate is incubated with uroporphyrinogen III as substrate. Uroporphyrinogen unconverted during the reaction is oxidised to uroporphyrin and measured directly as free acid by HPLC. The enzyme activity is then calculated from the amount of substrate consumed. The technique is simple, rapid and highly reproducible. It is recommended as a clinical assay.

Hepatic uroporphyrin accumulation and uroporphyrinogen decarboxylase activity in cultured chick-embryo hepatocytes and in Japanese quail (Coturnix coturnix japonica) and mice treated with polyhalogenated aromatic compounds

The relationship between hepatic uroporphyrin accumulation and uroporphyrinogen decarboxylase (EC 4.1.1.37) activity was investigated in cultured chick-embryo hepatocytes, Japanese quail (Coturnix coturnix japonica) and mice that had been treated with polyhalogenated aromatic compounds. Chick-embryo hepatocytes treated with 3,3',4,4'-tetrachlorobiphenyl accumulated uroporphyrin in a dose-dependent fashion without a detectable decrease in uroporphyrinogen decarboxylase activity when either pentacarboxyporphyrinogen III or uroporphyrinogen III were used as substrates in the assay. Other compounds, such as hexachlorobenzene, parathion, carbamazepine and nifedipine, which have been shown previously to cause uroporphyrin accumulation in these cells, did not decrease uroporphyrinogen decarboxylase activity. Japanese quail treated with hexachlorobenzene for 7-10 days also accumulated hepatic uroporphyrin without any decrease in uroporphyrinogen decarboxylase activity. In contrast, hepatic uroporphyrin accumulation in male C57BL/6 mice treated with iron and hexachlorobenzene was accompanied by a 20-80% decrease in uroporphyrinogen decarboxylase activity, demonstrating that the assay used for uroporphyrinogen decarboxylase, using pentacarboxyporphyrinogen III as substrate, could detect decreased enzyme activity. Our results with chick hepatocytes and quail, showing uroporphyrin accumulation without a decrease in uroporphyrinogen decarboxylase activity, are consistent with a new two-stage model of the uroporphyria: initially uroporphyrinogen is oxidized by a cytochrome P-450-mediated reaction, followed in rodents by a progressive decrease in uroporphyrinogen decarboxylase activity.

The effects in vivo of mutationally modified uroporphyrinogen decarboxylase in different hem12 mutants of baker's yeast (Saccharomyces cerevisiae)

Nine new hem12 haploid mutants of baker's yeast (Saccharomyces cerevisiae), totally or partially deficient in uroporphyrinogen decarboxylase activity, were subjected to both genetic and biochemical analysis. The mutations sites studied are situated far apart within the HEM12 gene located on chromosome IV. Uroporphyrinogen decarboxylase activity in the cell-free extracts of the mutants was decreased by 50-100%. This correlated well with the decrease of haem formation and the increased accumulation and excretion of porphyrins observed in vivo. The pattern of porphyrins (uroporphyrin and its decarboxylation products) accumulated in the cells of mutants partially deficient in uroporphyrinogen decarboxylase activity did not differ significantly, although differences in vitro were found in the relative activity of the mutant enzyme at the four decarboxylation steps. The excreted porphyrins comprised mainly dehydroisocoproporphyrin or pentacarboxyporphyrin. In heterozygous hem12-1/HEM12 diploid cells, a 50% decrease in decarboxylase activity led to an increased accumulation of porphyrins as compared with the wild-type HEM12/HEM12 diploid, which points to the semi-dominant character of the hem12-1 mutation. The biochemical phenotypes of both the haploid and the heterozygous diploid resembles closely the situation encountered in porphyria cutanea tarda, the most common human form of porphyria.

Metal alteration of uroporphyrinogen decarboxylase and coproporphyrinogen oxidase

Both UD and CO are susceptible to alteration by sulfhydryl-directed binding agents including a variety of trace metals. UD apparently requires a functional SH group or groups for catalytic activity, and the various steps of decarboxylation catalyzed by the enzyme can be differentially inhibited by divalent cations such as Hg2+ at very low concentrations. There is evidence that tissue-specific factors such as the endogenous GSH concentration may influence the susceptibility of UD in some tissues to metal inhibition, and this circumstance could be highly relevant to the etiology of porphyrinopathies or porphyrinurias that arise during prolonged metal exposures. CO does not appear to have a requirement for functional SH groups at the active site, but several SH groups on the enzyme appear to be involved in maintaining the protein's noncovalent structural characteristics. CO appears to be substantially more readily inhibited by metals in vivo than in vitro. This observation may reflect effects of metals on both the structural integrity of the enzyme is functionally associated in the intact cell. Finally, it seems reasonable to suggest that tissues, such as the kidney, that ordinarily contribute only sparingly to total excreted porphyrin levels may assume increased importance in this regard when challenged by specific porphyrinogenic chemicals such as trace metals. Advantage might be taken of such chemical- and organ-specific changes in porphyrin metabolism and porphyrin excretion patterns in monitoring prolonged, subclinical exposure to such chemicals in human populations.

Evidence for two uroporphyrinogen decarboxylase isoenzymes in human erythrocytes

In animals and plants, uroporphyrinogen decarboxylase catalyzes the stepwise decarboxylations of uroporphyrinogen, the precursor of heme and chlorophyll. To better understand its metabolic roles, we characterized the enzyme purified to electrophoretic homogeneity (about 11,000-fold) from human erythrocytes by a novel uroporphyrin-sepharose affinity chromatographic method. Native polyacrylamide disc gel electrophoresis of the purified enzyme preparation showed two bands detected by staining either for protein or with uroporphyrin-I. Each individual protein eluted from the gel when subjected to re-electrophoresis on SDS-polyacrylamide gel, appeared as a single protein band with molecular masses of approximately 54,000 and approximately 35,000 daltons respectively. Both proteins were able to catalyze all four decarboxylation steps, though the ratios of enzyme activity using octa-, hepta-, hexa- to pentacarboxylic porphyrinogen substrates were distinctly different. Also, their kinetic analysis with heptacarboxylic porphyrinogen-I substrate provided distinctly different apparent Michaelis constants. This provides the first evidence that decarboxylations of uroporphyrinogen to coproporphyrinogen are catalyzed by two isoenzymes.

Liver porphyrinogen carboxylase in hexachlorobenzene porphyric rats. Studies with intermediate porphyrinogens of series III and with uroporphyrinogen I

The present work studies the action of hexachlorobenzene (HCB) on the decarboxylation of uroporphyrinogen (Urogen) I and III and also on the decarboxylation of intermediate porphyrinogens of series III under different conditions using liver of normal and porphyric rats as enzyme source. The same enzyme is involved in the Urogen decarboxylation of both isomeric series I and III and catalyses the four steps in both cases. HCB affects all of them. HCB blocks the four steps of Urogen III decarboxylation to the same degree, as a function of intoxication time. HCB leads, in general, to an increase in the efficiency (Km/Vmax) of the porphyric system. These data can be interpreted as a reaction of the organism to overcome the enzymatic blockade.

In vitro studies of the mechanism of inhibition of rat liver uroporphyrinogen decarboxylase activity by ferrous iron under anaerobic conditions

Human porphyria cutanea tarda (PCT) is an unusual consequence of common hepatic disorders such as alcoholic liver disease and iron overload, where hepatic iron plays a key role in the expression of the metabolic lesion, i.e., defective hepatic decarboxylation of porphyrinogens. In this investigation, kinetic studies on a partially purified rat liver uroporphyrinogen decarboxylase have been conducted under controlled conditions to determine how iron perturbs porphyrinogen decarboxylation in vitro. The enzyme, assayed strictly under anaerobic conditions in the dark, was inhibited progressively by ferrous iron. Approximately 0.45 mM ferrous ammonium sulfate was required to observe about 50% inhibition of enzyme activity measured with uroporphyrinogen I as substrate. We showed that (a) all the steps of enzymatic decarboxylation (octa-, hepta-, hexa-, and pentacarboxylic porphyrinogen of isomer I series) were inhibited by ferrous iron. The inhibition was competitive with respect to uroporphyrinogen I and III substrates; (b) the cations, e.g., Fe3+ and Mg2+, had no effect, whereas sulfhydryl group specific cations and compounds such as Hg2+, Zn2+, p-mercuribenzoate, and 5,5'-dithiobis(2-nitrobenzoate) all inhibited the enzyme; (c) the enzyme could be protected from inhibition by Fe2+ and p-mercuribenzoate by preincubation with pentacarboxylic porphyrinogen, a natural substrate and competitive inhibitor. These data suggest for the first time a direct interaction of ferrous iron with cysteinyl residue(s) located at the active site(s) of the enzyme.

Photodynamic inactivation of red cell uroporphyrinogen decarboxylase by porphyrins

The effects of light and porphyrins on the activity of red cell uroporphyrinogen decarboxylase were studied. Photoinactivation of uroporphyrinogen decarboxylase was dependent on uroporphyrin concentration, irradiation time and temperature. Using 40 W/m2 of UV light intensity, 40-45% decreased activity was produced with 200 microM uroporphyrin I, at 37 degrees C and after 2 hr of illumination. It has been demonstrated that porphyrins photoinactivate uroporphyrinogen decarboxylase and a mechanism for this action in relation to skin lesions is proposed.

Hematologic and hepatic manifestations of the cutaneous porphyrias

This chapter has dealt with five photocutaneous forms of human porphyria. The forms are a diverse group of disorders with many different hematologic, hepatologic, and neurologic manifestations. In essence, most photocutaneous porphyrias occurring in childhood will relate to congenital erythropoietic porphyria or protoporphyria. The nature of the skin lesions and a study of the heme precursor profile in red cells, plasma, urine, and feces should easily distinguish these two conditions. CEP is a disease wherein photomutilation is a dominant concern and aggressive new approaches of therapy also have been discussed. In protoporphyria, the dermatologic problem is less severe and the dermatologist should be aware that a subset of patients could develop active liver disease that may lead to fatal cirrhosis. Novel approaches of therapy have been briefly alluded to. With regard to postpubertal photocutaneous porphyria, the classic porphyria cutanea tarda syndrome is associated with liver disease, usually alcoholic with siderosis, and the treatment by phlebotomy to reduce hepatic iron is highly effective. The potential danger of liver carcinoma has been discussed. In subsets of porphyria cutanea tarda, this can be an endemic disease relating to environmental factors, ie, ingestion of polyhalogenated hydrocarbons. The biochemical diagnosis can be attained by fairly straight-forward solvent extraction analyses of urine and feces, showing the dominance of uroporphyrin excretion in the urine and coproporphyrin in the feces. Chromatographic techniques in plasma, bile, and feces reveal a PCT-specific porphyrin: isocoproporphyrin. Rare subtypes with hematologic manifestations, ie, hepatoerythropoietic porphyria and CEP, indicate the wide spectra of disorders that might be associated with a spontaneous deficiency of uroporphyrinogen decarboxylase activity. These latter syndromes are, however, rare. Two hereditary hepatic porphyrias, ie, autosomal dominantly inherited VP and HCP, have been briefly discussed. The hepatic lesion is metabolic, not morphologic, and its expression by the liver relates to its adaptive response to induction of microsomal hemoproteins by a variety of exogeneous and endogeneous compounds, eg, drugs and hormones. Photocutaneous lesions of HCP and VP are identical to PCT, the latter having no neurologic sequelae. In the former two, however, exposure of persons to drugs, such as the hydantoins and barbiturates, can lead to potentially fatal acute porphyric attacks.(ABSTRACT TRUNCATED AT 400 WORDS)

Reduced substrate affinity for human erythrocyte uroporphyrinogen decarboxylase constitutes the inherent biochemical defect in porphyria cutanea tarda

The pathogenesis of human porphyria cutanea tarda (PCT) is associated with an intrinsic abnormality of the uroporphyrinogen decarboxylase enzyme. To characterize this, we studied the kinetic properties of the red cell enzyme procured from patients with various forms of PCT and non-porphyric controls. The enzyme activity (units/mg hemoglobin) in the red cell hemolysate was close to normal in sporadic PCT but about 75% diminished in the familial PCT. The Michaelis constants (Km) of 200-fold purified red cell enzyme preparations, determined by using pentacarboxylic porphyrinogen I and uroporphyrinogen I as substrates, were more than 3.8-4.0 times higher, and the maximum velocity (Vmax) was about 70% diminished in familial PCT, whereas the Km was about 1.7-1.9 times higher and the Vmax was more or less normal for sporadic PCT. These observations suggest for the first time that the primary lesion in familial PCT is a genetically determined kinetic abnormality of uroporphyrinogen decarboxylase which appears to be different from the sporadic form of the disease.

Exposure to toxic agents: the heme biosynthetic pathway and hemoproteins as indicator

The heme biosynthetic pathway is closely controlled by levels of the end product of the pathway, namely, heme, and porphyrins are normally formed in only trace amounts. When control mechanisms are disturbed by xenobiotics, porphyrins accumulate and serve as a signal of the interaction between a xenobiotic and the heme biosynthetic pathway. For example, an increase in erythrocyte protoporphyrin is a useful measurement for early detection of exposure to lead and porphyrinuria was an early manifestation of a hexachlorobenzene-induced porphyria in Turkey. In recent years a variety of additional xenobiotics has been shown to interact with the heme biosynthetic pathway, namely, halogenated aromatic hydrocarbons, pesticides, sulfides, and a variety of metals. Moreover, different xenobiotics (e.g., dihydropyridines and compounds containing unsaturated carbon-carbon bonds) interact with the prosthetic heme of cytochrome P-450 forming novel N-alkylporphyrins.

Assay of mouse liver uroporphyrinogen decarboxylase by reverse-phase high-performance liquid chromatography

A method for the estimation of hepatic uroporphyrinogen decarboxylase activity employing reverse-phase HPLC is described. Mouse liver homogenate in 0.25 M sucrose was pretreated with a suspension of cellulose phosphate and then centrifuged to remove hemoglobin and debris. The supernatant was used as the enzyme source. Incubations were acidified, oxidized, and centrifuged only before analysis of the porphyrins formed, using a Spherisorb ODS column and a gradient solvent system constructed from methanol/lithium citrate mixtures. Coproporphyrinogen formation by BALB/c mouse liver supernatant was estimated as about 5.0 and 9.1 pmol/min/mg protein from uroporphyrinogens I and III, respectively, at 10 microM substrate concentration and pH 6.8. Decarboxylation of pentacarboxyporphyrinogens (the last step in coproporphyrinogen formation) proved to be easily measured. Coproporphyrinogen formation from pentacarboxyporphyrinogen III abd (20 microM) at pH 6.8 was about 109 pmol/min/mg protein. Pentacarboxyporphyrinogen I was not as good a substrate as III abd but was decarboxylated faster at pH 5.4 than at 6.8, and at the lower pH and at 10 microM concentration of substrate 42 pmol of coproporphyrinogen was formed/min/mg protein. These results compared favorably with those obtained by previously published procedures involving time-consuming extraction and esterification steps.

Modified uroporphyrinogen decarboxylase activity in a yeast mutant which mimics porphyria cutanea tarda

The isolation of a new mutant Sm1 strain of yeast, Saccharomyces cerevisiae, is described: this strain was partially defective in haem formation and accumulated large amounts of Zn-porphyrins. Genetic analysis showed that the porphyrin accumulation was under the control of a single nuclear recessive mutation. Biochemical analysis showed that the main porphyrins accumulated in the cells were uroporphyrin and heptacarboxyporphyrin, mostly of the isomer-III type. The excreted porphyrins comprised mainly dehydroisocoproporphyrin. Analysis of uroporphyrinogen decarboxylase activity in the cell-free extract revealed a 70-80% decrease of activity in the mutant and showed that the relative rates of the different decarboxylation steps were modified with the mutant enzyme. A 2-3-fold increase in 5-aminolaevulinate synthase activity was measured in the mutant. The biochemical characteristics of the Sm1 mutant are very similar to those described for porphyria cutanea tarda.

Case report of symptomatic porphyria cutanea tarda associated with histiocytic lymphoma

A fifth case is reported of symptomatic porphyria cutanea tarda (PCT) associated with lymphoma (of the histiocytic type) in a 45-year-old female. The PCT was documented in detail histologically and biochemically. The lymphoma was complicated by two paraneoplastic phenomena: inappropriate antidiuretic hormone secretion and peripheral neuropathy. It is possible that the PCT might also be a paraneoplastic phenomenon of the lymphoma. Search for an occult lymphoma may be of diagnostic and therapeutic value in patients presenting with PCT.

Decarboxylation of uroporphyrinogen I and III in 2,3,7,8-tetrachlorodibenzo-p-dioxin induced porphyria in mice

The decarboxylation of uroporphyrinogens I and III by porphyrinogen carboxy-lyase (EC 4.1.1.37) in mouse liver supernatant was compared in relation to substrate concentrations. In this species uroporphyrinogen III was the best substrate judging by the criteria of Km/Vmax (estimated for total porphyrinogens) and was converted into coproporphyrinogen faster than its series I isomer. The difference between the two isomers was mainly due to the first decarboxylation. This difference was confirmed by calculation of the Hill coefficient and of Lineweaver-Burk plot which suggested that isomer I induced negative cooperativity in the active centre of the enzyme. After treatment with a porphyrogenic dose of TCDD (25 micrograms/kg/week for 9 weeks) differences between uroporphyrinogen I and III as substrate were maintained. In addition treatment reduced Vmax and Km (estimated for total porphyrinogens) of liver porphyrinogen carboxy-lyase to about half control values for both isomers. Vmax was reduced mainly because of the formation of smaller amounts of all products of decarboxylation, and Km because more heptaporphyrinogen was formed than coproporphyrinogen. Values of the Hill coefficient and Lineweaver-Burk plots suggested TCDD induced altered substrate affinity for isomer III too. Treatment with TCDD did not affect the decarboxylation of uroporphyrinogen III by RBC porphyrinogen carboxy-lyase, estimated from Km and Vmax for total porphyrinogens formed.

High-performance liquid chromatography of naturally occurring 8-, 7-, 6-, 5- and 4-carboxylic porphyrin isomers

Naturally occurring 8-, 7-, 6-, 5- and 4-carboxylic porphyrin isomers are separated on C18 reversed-phase columns with various proportions (13-31%, v/v) of acetonitrile in 1 M ammonium acetate buffer (pH 5.16) as the mobile phases. Hydrophobic interaction between the porphyrin side chain substituents and the C18 hydrophobic surface is the main retention mechanism. Ion-exchange behaviour is also observed, but this does not influence the relative retention of the isomers. All possible forms of the decarboxylation intermediates of uroporphyrinogen III are detected in normal and porphyric urine, and the results provide conclusive evidence for the existence of decarboxylation pathways other than the currently accepted clockwise sequence, starting at the ring D acetic acid group of uroporphyrinogen III.

Purification of uroporphyrinogen decarboxylase from human erythrocytes. Immunochemical evidence for a single protein with decarboxylase activity in human erythrocytes and liver

Uroporphyrinogen decarboxylase (EC 4.1.1.37) has been purified 4419-fold to a specific activity of 58.3 nmol of coproporphyrinogen III formed/min per mg of protein (with pentacarboxyporphyrinogen III as substrate) from human erythrocytes by adsorption to DEAE-cellulose, (NH4)2SO4 fractionation, gel filtration, phenyl-Sepharose chromatography and polyacrylamide-gel electrophoresis. Progressive loss of activity towards uroporphyrinogens I and III occurred during purification. Experiments employing immunoprecipitation, immunoelectrophoresis and titration with solid-phase antibody indicated that all the uroporphyrinogen decarboxylase activity of human erythrocytes resides in one protein, and that the substrate specificity of this protein had changed during purification. The purified enzyme had a minimum mol.wt. of 39 500 on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. Gel filtration gave a mol.wt. of 58 000 for the native enzyme. Isoelectric focusing showed a single band with a pI of 4.60. Reaction with N-ethylmaleimide abolished both catalytic activity and immunoreactivity. Incubation with substrates or porphyrins prevented inactivation by N-ethylmaleimide. An antiserum raised against purified erythrocyte enzyme precipitated more than 90% of the uroporphyrinogen decarboxylase activity from human liver. Quantitative immunoprecipitation and crossed immunoelectrophoresis showed that the erythrocyte and liver enzymes are very similar but not identical. The differences observed may reflect secondary modification of enzyme structure by proteolysis or oxidation of thiol groups, rather than a difference in primary structure.

Investigations of rat liver uroporphyrinogen decarboxylase. Comparisons of porphyrinogens I and III as substrates and the inhibition by porphyrins

1. The decarboxylations of uroporphyrinogens, hepta-, hexa- and penta-carboxyporphyrinogens I and III by porphyrinogen carboxy-lyase (EC 4.1.1.37) in rat liver supernatant have been compared as functions of substrate concentrations. Although Km and Vmax. (for total porphyrinogens formed) were estimated, prophyrinogens and CO2 produced at 1 microM were considered to be a better indication of real relative rates, owing to substrate/product inhibitions. Uroporphyrinogen III was the best substrate by the criteria of Km/Vmax. and decarboxylation at 1 microM and was converted into coproporphyrinogen more quickly than its series-I isomer. 2. The difference between uroporphyrinogens I and III as substrates was confirmed by using a mixture of [14C8]uroporphyrinogens, the discrimination occurring principally in the first decarboxylation. 3. Porphyrins, especially oxidation products of the substrates, inhibited the enzyme. Heptacarboxyporphyrin III was the most effective inhibitor of both uroporphyrinogen III and heptacarboxyporphyrinogen III conversion into coproporphyrinogen. 4. Rapid analysis of the livers from rats made porphyric with hexachlorobenzene demonstrated that substantial quantities of the tetrapyrroles were present in vivo as the porphyrinogens (21-42%). 5. Enzymic decarboxylation of uroporphyrinogen III in 2H2O-containing buffer gave [2H4]coproporphyrinogen. 6. Rats treated with cycloheximide for 10h showed no decrease in uroporphyrinogen decarboxylase activity/mg of protein, suggesting a relatively slow turnover of the enzyme.