Porphyrin biosynthesis. VII. Porphyrinogen carboxy-lyase from avian erythrocytes. Purification and properties

Feather content of porphyrins in Eurasian eagle owl (Bubo bubo) fledglings depends on body condition and breeding site quality

Porphyrins are pigments produced in most animal cells during the synthesis of heme, but their importance for external coloration is unclear. Owls (Order Strigiformes) are among the few animals that accumulate porphyrins in the integument, where it could serve as a means of signaling. Here we hypothesized that the porphyrin content of feathers may depend on body condition and breeding site quality in Eurasian eagle owl (Bubo bubo) fledglings and, thus, constitute amplifiers of the quality of the area where they are born. Using high-performance liquid chromatography, we found 2 porphyrins (protoporphyrin IX and coproporphyrin III) in the body feathers of 19 eagle owl fledglings from 7 breeding territories. Coproporphyrin III, but not protoporphyrin IX feather concentration, was positively associated with the body mass of fledglings and with the quality of the breeding sites where they were reared with respect to food quality and availability. As coproporphyrin III is produced under oxidative stress, we suggest that good breeding sites may lead to fledglings in good condition. This, in turn, may make fledglings induce a certain level of free radical and coproporphyrin III production to signal to conspecifics their site-mediated capacity to cope with oxidative stress. This is the first time that porphyrin content in the integument has been found to be related to individual quality, opening a new scenario for studying evolution of animal coloration.

The modification of acetate and propionate side chains during the biosynthesis of haem and chlorophylls: mechanistic and stereochemical studies

In the conversion of uroporphyrinogen III into protoporphyrin IX and thence into chlorophylls, all eight carboxylic side chains, as well as the four meso positions, are modified, and four enzymes are involved. In the uroporphyrinogen decarboxylase-catalysed reaction all four acetate side chains are converted into methyl groups by the same mechanism, to produce coproporphyrinogen III. Both methylene hydrogen atoms remain undisturbed and the reaction occurs with the retention of stereochemistry. Several questions regarding the enzymology of the decarboxylase are posed. Do all the decarboxylations occur at the same active site and, if so, are the four acetate chains handled in a particular sequence? Is the decarboxylation reaction aided by the transient formation of an electron-withdrawing functionality in the pyrrole ring? Coproporphyrinogen oxidase converts the two propionate side chains of rings A and B into vinyl groups, with an overall anti-periplanar removal of the carboxyl group and the Hsi from the neighbouring position. Evidence is examined to evaluate whether a hydroxylated compound acts as an intermediate in the oxidative decarboxylation reaction. Protoporphyrinogen oxidase then converts the methylene-interrupted macrocycle of protoporphyrinogen IX into a conjugated system. The conversion has been suggested to involve three consecutive dehydrogenation reactions followed by an isomerization step. The face of the macrocycle from which the three meso hydrogen atoms are removed in the dehydrogenation reaction is thought to be opposite to that from which the fourth meso hydrogen is lost during the prototropic rearrangement. In an investigation of the in vivo mechanism for the esterification of the ring D propionic acid group with a C20 isoprenyl group 5-aminolaevulinic acid was labelled with 13C and 18O at C-1 and incorporated into bacteriochlorophyll a. The 18O-induced shift of the 13C resonance in the NMR spectrum showed that both oxygen atoms of the carboxyl group are retained in the ester bond. This and other results suggest that the reaction occurs by the nucleophilic attack of the ring D carboxylate anion on the activated form of an isoprenyl alcohol.

Abnormal kinetic behavior of uroporphyrinogen decarboxylase obtained from rats with hexachlorobenzene-induced porphyria

Uroporphyrinogen decarboxylase is an essential enzyme in all organisms and functions in the heme biosynthetic pathway, catalyzing the decarboxylation of the four acetate groups of uroporphyrinogen to form coproporphyrinogen. This work examines whether the four sequential decarboxylations occur at the same active site, and explores whether hexachlorobenzene-induced porphyria affects the behavior of the enzyme. For this purpose, kinetic competition studies were done with mixtures of uroporphyrinogen III and pentacarboxyporphyrinogen III. With the enzyme from normal rats, a constant velocity was obtained with all the mixtures, indicating that uroporphyrinogen and pentacarboxy-porphyrinogen react at the same active site, i.e. the first and fourth decarboxylations occur at the same site. In contrast, in experiments with enzyme from rats with hexachlorobenzene-induced porphyria, the total rate for mixtures was always lower than the reference rate; and a curve with a deep minimum was obtained, indicating that the two reactions occur at functionally different sites, but with cross-inhibition. This suggests that the modifications induced in the enzyme by hexachlorobenzene cause the two active sites to become nonequivalent and functionally different. The question is discussed how the hexachlorobenzene treatment may produce this abnormal kinetic behavior, and alternative hypotheses are considered.

Studies on the mechanism of uroporphyrinogen decarboxylase inhibition in hexachlorobenzene-induced porphyria in the female rat

Hexachlorobenzene (HCB)-induced porphyria occurs in female, but not male, rats after a delay of 35 days following HCB treatment. Uroporphyrinogen decarboxylase (UROD) inhibition has been proposed as a primary causative event. To determine whether there also exists a delay phase and a sexual dimorphism for UROD inhibition, groups of male and female rats were given HCB (100 mg/kg/day) from Days 1 to 5. Hepatic uroporphyrin III was markedly increased only after Day 33. Liver cytosol UROD activity in HCB-treated female rats with porphyria at Days 33, 40, 47, 54, and 100 was decreased by over 70% compared to concurrent control, whereas treated male rats as well as nonporphyric female rats had UROD activity comparable to control levels at Days 6, 12, 19, 26, 33, 40, 47, and 54. Level of immunoreactive UROD in cytosol of porphyric rats was not modified by HCB. No gender-related differences in liver cytosol radiolabel level ([14C]HCB given as the fifth dose) were found at Days 6 and 30. Chromatography of liver cytosol showed nonspecific binding of radiolabel to proteins for males, porphyric and nonporphyric females, and loss of UROD activity did not correlate with the amount of radiolabel in the UROD-containing fractions. Thus, the gender-specific decrease in UROD activity observed when porphyria develops in female rats (delay of about 4 weeks), as well as the persistence of low activity and porphyria for months, suggests that UROD inhibition was causally related to porphyria.

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.

Molecular analysis of HEM6 (HEM12) in Saccharomyces cerevisiae, the gene for uroporphyrinogen decarboxylase

HEM6 (HEM12) in Saccharomyces cerevisiae encodes uroporphyrinogen decarboxylase, the fifth enzyme in the heme biosynthetic pathway. The HEM6 (HEM12) gene was cloned by complementation of heme auxotrophy of a hem6 mutant. Sequence analysis revealed an open reading frame of 1086 nucleotides. The predicted amino acid sequence of HEM6 (HEM12) shows extensive homology to those reported for uroporphyrinogen decarboxylase from mammalian sources. Expression of HEM6 (HEM12) was investigated and was found to increase two-fold in a non-fermentable carbon source. However, HEM6 (HEM12) transcription was unaffected by heme or by intermediates in the heme biosynthetic pathway. In addition, HEM6 (HEM12) expression is not regulated by the transcriptional activator complex HAP2-3-4, as has been shown for some genes encoding heme biosynthetic enzymes.

Order of uroporphyrinogen III decarboxylation on incubation of porphobilinogen and uroporphyrinogen III with erythrocyte uroporphyrinogen decarboxylase

The isomeric compositions of the heptacarboxylic, hexacarboxylic and pentacarboxylic porphyrinogens formed by incubation of porphobilinogen with human red-cell haemolysates have been analysed and compared with those derived from incubation with chemically prepared uroporphyrinogen III as substrate. The results indicated that when supplied with an excess (3.7 microM) of exogenous uroporphyrinogen III, uroporphyrinogen decarboxylase utilized the substrate at random and a mixture of isomers was produced; whereas with uroporphyrinogen III generated enzymically from porphobilinogen as substrate a clockwise decarboxylation sequence was observed, resulting in the formation of intermediates mainly with the ring-D, rings-AD and rings-ABD acetate groups decarboxylated. Using [14C]uroporphyrinogen III as substrate at low concentrations (0.01-0.5 microM) also led to preferential decarboxylation of the ring-D acetate group. It was concluded that the order of uroporphyrinogen III decarboxylation is substrate-concentration-dependent, and under normal physiological conditions enzymic decarboxylation is most probably orderly and clockwise, starting at the ring-D acetate group.

Random decarboxylation of uroporphyrinogen III by human hepatic uroporphyrinogen decarboxylase

The type III heptacarboxylic porphyrinogens derived from enzymic decarboxylation of an acetic acid substituent on uroporphyrinogen III to a methyl group by human hepatic uroporphyrinogen decarboxylase has been analysed by reversed-phase high-performance liquid chromatography with electrochemical detection. The results showed that all four possible heptacarboxylic acid porphyrinogen isomers, with the methyl group attached to rings A, B, C and D of the tetrapyrrole macrocycle, respectively, were formed in almost equal proportions. It was concluded that the normal pathway of uroporphyrinogen III decarboxylation in human liver follows a random mechanism.

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.

Decarboxylation of uroporphyrinogen III by erythrocyte uroporphyrinogen decarboxylase. Evidence for a random decarboxylation mechanism

The isomeric composition of type-III heptacarboxylic porphyrinogens derived from decarbosylation of uroporphyrinogen III by erythrocyte uroporphyringogen decarboxylase was analysed by h.p.l.c. with electrochemical detection. All four possible isomers were identified, and there were little differences in the proportion of isomers formed by erythrocytes from normal subjects and from patients with sporadic porphyria cutanea tarda. The results provide conclusive evidence that the normal decarboxylation pathway is random in nature, and the fourth isomer only increases when enzyme abnormality is found.

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.

Chlorophyll-free chromoplasts from daffodil contain most of the enzymes for chlorophyll synthesis in a highly active form

Chromoplasts isolated from chlorophyll-free daffodil flowers utilize in vitro delta-aminolevulinic acid (ALA) as precursor for the synthesis of large amounts of at least nine different products. Their identification as intermediates of the chlorophyll biosynthetic pathway demonstrates the presence of the majority of the respective enzymes in this nongreen plastid preparation. Porphobilinogen synthase was investigated more closely and found to be similar in its properties to the corresponding enzyme from other plastid sources. Protoporphyrin IX was also accepted as a substrate by chromoplast homogenate; here, as in the case of ALA as a substrate, Mg-protoporphyrin IX monomethyl ester was the last product formed. Formation of the isocyclic chlorophyll ring was not observed.

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.

Inhibitory effect of tetrachloro-p-hydroquinone and other metabolites of hexachlorobenzene on hepatic uroporphyrinogen decarboxylase activity with reference to the role of glutathione

Exposure of rats to HCB caused a dose-dependent depletion of GSH. Chlorophenolic and sulfur-containing metabolites of HCB incubated with GSH-free rat liver cytosolic protein drastically diminished the UROD activity. In addition, HCB also exhibited inhibitory potency. The most effective compounds studied were TCH and its oxidation product, chloranil. Incubation of liver cytosolic protein and of GSH with HCB and its metabolites yielded results that suggested interaction between the compounds and cell constituents--an interaction that may cause inhibition of the hepatic UROD activity in the HCB-exposed organism.

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.

Biosynthesis of porphyrins in Rhodopseudomonas palustris--VI. The effect of metals, thiols and other reagents on the activity of uroporphyrinogen decarboxylase

The effect of several metals and reagents on the decarboxylation rate of uroporphyrinogen I by using a 16-fold purified preparation of Uroporphyrinogen Decarboxylase from Rhodopseudomonas palustris, was studied. 1 mM Hg2+ and Cu2+ were strong inhibitors, 1 mM Zn2+ and Fe2+ under certain conditions and 1 mM Fe3+ and Cr3+ also inactivated the enzyme, but Pb2+, Cd2+ and Al3+ did not. Metals inhibition was reversed by 1 mM GSH or CySH. 0.1 mM DTNB and PCMB, 1 mM pyridoxal phosphate and 100 mM chloral hydrate, as well as 1 mM 2-methoxy-5-nitrotropone and 0.2 mM diethylpyrocarbonate inhibited Uroporphyrinogen Decarboxylase; while GSH, CySH, N-ethylmaleimide, sodium thioglycolate, 1,4-dithioerythritol, EDTA and O-phenantroline did not modify activity. Data obtained would indicate that one cysteine, one or two histidine residues and probably a lysine group are required for enzyme activity.

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.

Mechanism of hexachlorobenzene-induced porphyria in rats. Effect of phenobarbitone pretreatment

The effect of a pretreatment with phenobarbitone (PB) on the porphyrinogenic action exerted by hexachlorobenzene (HCB) was examined in female rats. Kinetic studies of enzyme function after HCB poisoning showed that porphyrinogen carboxy-lyase was the only enzyme of haem biosynthesis that markedly lowered its activity. Both stages of uroporphyrinogen (UPG) III decarboxylation were decreased. This enzyme, together with UPG I synthase (increased levels) were the first enzymes altered. Subsequently, an increase in delta-aminolaevulinate (AmLev) synthase and ferrochelatase was detected; AmLev dehydratase was the last to increase. On long-term exposure, PB alone did not modify the basal values of haem intermediates; only the content of cytochrome P-450 increased. All the enzyme activities studied showed no significant changes, except ferrochelatase, which increased. With both drugs the metabolic impairment promoted by HCB was accelerated and enhanced by prior PB treatment leading to the onset of an earlier and stronger porphyria. A more noticeable accumulation and excretion of higher carboxylated porphyrins and precursors was more promptly observed as a consequence of the early porphyrinogen carboxy-lyase blockade and the concomitant induction of AmLev synthase. Although the enzymic activities of both AmLev dehydratase and ferrochelatase were enhanced, this response differed in time. For UPG I synthase this pretreatment elicited lower values than those found in the HCB group. Cytochrome P-450 contents were immediately and slightly enhanced by all the drugs, but the values for the combined treatment were the lowest. Of the several hypotheses that could explain the action of HCB on the haem pathway, our results would suggest that the porphyrinogenic action of HCB is mediated by some of its metabolic products.

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.

Erythrocyte porphyrinogen carboxy-lyase activity in porphyria cutanea tarda and certain other human porphyrias

Red cell porphyrinogen carboxy-lyase activity was measured using uroporphyrinogen III as substrate in 18 normal persons, 7 male patients with porphyria cutanea tarda, 3 female patients with erythropoietic protoporphyria and 2 female patients with variegate porphyria. The mean values obtained in normal subjects were 0.151 nmol of uroporphyrinogen disappeared in 30 min per mg of protein, and 0.038 nmol of coproporphyrinogen formed in 30 min per mg of protein. We have not been able to detect significant differences between males and females. In porphyria cutanea tarda the enzyme activity was the same as in normal subjects considering either substrate disappearance or end product formation. The differences were not significant at the p less than 0.05 level. Patients with variegata porphyria also exhibited normal erythrocyte porphyrinogen carboxy-lyase activity. The enzyme activity of erythrocytes from patients with erythropoietic protoporphyria was higher than in normals; mean values for specific activities being 0.204 nmol of uroporphyrinogen disappeared, and 0.071 nmol of coproporphyrinogen formed. The significance of the results with respect to the chemical picture of different porphyrias is discussed.

Second derivative-high-performance liquid chromatographic-fluorometric detection of porphyrins in chick embryo liver cell culture medium

A high-performance liquid chromatographic system is described which is suitable for the separation and quantitative determination of a mixture of porphyrin esters in nanogram quantities from the culture medium used for maintenance of a monolayer culture of chick embryo liver cells. The desired sensitivity was obtained by coupling a high-performance liquid chromatograph with a second derivative-fluorometric detection system. The most effective method for preparation of the porphyrin methyl esters prior to chromatography was found to be lyophilization of the culture medium prior to esterification with 5% sulphuric acid in methanol.

Some kinetic properties of human red cell uroporphyrinogen decarboxylase

Several kinetic properties of uroporphyrinogen decarboxylase (uroporphyrinogen-III carboxy-lyase, EC 4.1.1.37) from human hemoglobin-free hemolysates were studied, using substrates of both isomeric series I and III (uroporphyrinogen, hepta and pentacarboxyl porphyrinogens). Enzyme affinity for series II isomers was always found to be higher than for corresponding series I isomers. Mixed substrate experiments using porphyrinogen (both labelled with 14C and unlabelled) showed: (a) a reciprocal inhibition of decarboxylation of series III porphyrinogens by series I porphyrinogens with the same number of carboxylic groups; (b) no inhibition of hepta- and pentacarboxylic series III porphyrinogens decarboxylation by uroporphyrinogen III. It is demonstrated that porphyrinogens of both isomeric series with the same number of carboxylic groups are decarboxylated at the same active center; in contrast, the sequential decarboxylation of uroporphyrinogen III to coproporphyrinogen III occurs at four different active centers. Relationship between the kinetic properties of uroporphyrinogen decarboxylase and biological data of porphyria cutanea are discussed.

Tetrapyrrole biosynthesis from 4,5-dioxovaleric acid in rhodopseudomonas spheroides

Prophyrin biosynthesis from 4,5-dioxovaleric acid was studied in cell suspensions of R. spheroides. The experiments show that 4,5-dioxovaleric acid is a far precursor of porphyrins through delta amino laevulinic acid formation in a transmination reaction involving also l-alanine. It differs radically from the classical delta aminolaevulinic acid synthesis using glycine and succinyl CoA as substrates.

Familial and sporadic porphyria cutanea: two different diseases

Uroporphyrinogen (URO) decarboxylase was measured in hemoglobin-free erythrocytes from subjects with familial porphyria cutanea: the mean activity was about 50% of that found in erythrocytes from normal subjects. Asymptomatic carriers were always found in the family. No enzyme deficiency was found in erythrocytes from subjects with sporadic porphyria cutanea. The measurement of URO decarboxylase in erythrocytes seems to allow an easy distinction between these two groups of porphyria cutanea.

Porphyrins and porphyrinogen carboxy-lase in hexachlorobenzene-induced porphyria

1. Qualitative and quantitative studies of the porphyrins and the porphyrinogen carboxylyase of the liver, spleen, kidney, harderian gland and erythrocytes from normal rats and from those hexachlorobenzene-induced porphyria were carried out. 2. Hexachlorobenzene has no effect on erythrocyte porphyrin content, but produces a decrease in that of Harderian gland and an increase in the porphyrin content of the kidney and spleen, and a marked increase in the liver (1 mumol/g of tissue). Octacarboxylic (isomer III) and heptacarboxylic porphyrins accumulated in kidney, spleen and liver, the former porphyrin being predominant. 3. Hexachlorobenzene has no effect on the activity of porphyrinogen carboxy-lase in erythrocytes; there is a slight decrease in enzyme activity in the Harderian gland, and a marked decrease in the liver and kidney enzyme activities. In the liver the removal of each carboxyl group from uroporphyrinogen III appears to be affected by this treatment. 4. The liver is the principal site of action of hexachlorobenzene, with the kidney next in decreasing order of effect, and erythropoietic tissue is unaffected. The marked decrease in porphyrinogen carboxy-lyase activities observed in liver and kidney could explain the high accumulation of octacarboxylic and heptacarboxylic porphyrins found in these tissues. 5. The results are discussed in relation to changes promoted by hexachlorobenzene in other enzymes of the haem pathway.

Metabolic pathways of delta-aminolaevulinic acid in Rhodopseudomonas spheroides

Incorporation of delta-aminolaevulinic acid 5 14C and 4 14C into the inosine monophosphate pool and into porphyrins, was studied in cell suspensions of R. spheroides. The results contradict a direct incorporation of delta-aminolaevulinic acid into the purine ring of nucleotides through gammadelta-dioxovaleric acid. It would suggest a nonspecific incorporation after degradation of delta-aminolaevulinic acid without a transamination as a first reaction.

Studies on porphyrin biosynthesis in lead-intoxicated rabbits

The present report deals with studies on porphyrins and porphyrinogen carboxy-lyase of red cells and urinary porphyrins from lead-intoxicated rabbits. It was shown that the free erythrocyte porphyrins are mixture of protoporphyrin 9, the main component, and minor proportions of Coproporphyrin, Uroporphyrin III and Phyriaporphyrin. Analysis of the urinary porphyrins deomonstrates the presence of Coproporphyrins III as the major component, together with 15-20% of other porphyrins: 10-14% 5-COOH, 1-2% 6-COOH, 2-3% 7-COOH porphyrin and 1-2% Uroporphyrin III. We have not been able to detect an increase of Uroporphyrin I. Assays of porphyrinogen carboxy-lyase activity in hemolysate supernatant using Uroporphyrinogen III and Phyriaporphyrinogen (Phyria'gen) III as substrates, showed the existence of a slight decrease of both decarboxylase activities, being more affected during the second stage, the Phyria'gen decarboxylation. A possible regulation mechanism responsible for the porphyrin picture is discussed.

Studies on protoporphyrin biosynthetic pathway in Saccharomyces cerevisiae ; characterization of the tetrapyrrole intermediates

An acellular extract of the yeast, Saccharomyces cerevisiae, incubated with ALA, is able to synthesize protoporphyrin from this precursor. Several tetrapyrrole intermediates were extracted from the medium and purified by silica gel chromatography. The chromatographic behaviour and the spectral properties of the isolated seven free carboxylic porphyrins (and of the corresponding esters), show that each product has a different carboxyle number, varying from eight (uroporphyrin) to two (protoporphyrin). The identification of five of them (octa- to tetracarboxymethyl-porphyrinester) is confirmed by mass spectrometry. The effect of physical factors (temperature, pH, time) on the protoporphyrin biosynthesis system indicates that the enzymes catalysing the first steps of the pathway (ALA leads to Coproporphyrin) are more stable than those catalysing the last steps (Coproporphyrin leads to Protoporphyrin). Results obtained with some enzymatic inhibitors (EDTA, OP, pCMB) show the sensitivity of the ALA dehydratase to OP and to pCMB (confirming therefore its nature as a metallo- and sulfhydryl enzyme) and also of the overall porphyrin synthesis system to these three agents.

Decreased red cell uroporphyrinogen I synthetase activity in intermittent acute porphyria

Intermittent acute porphyria has recently been distinguished biochemically from other genetic hepatic porphyrias by the observation of diminished hepatic uroporphyrinogen I synthetase activity and increased delta-aminolevulinic acid synthetase activity. Since deficient uroporphyrinogen I synthetase may be reflected in nonhepatic tissues, we have assayed this enzyme in red cell hemolysates from nonporphyric subjects and from patients with genetic hepatic porphyria. Only patients with intermittent acute porphyria had decreased erythrocyte uroporphyrinogen I synthetase activity which was approximately 50% of normal. The apparent K(m) of partially purified uroporphyrinogen I synthetase was 6 x 10(-6)m in both nonporphyrics and patients with intermittent acute porphyria. These data provide further evidence for a primary mutation affecting uroporphyrinogen I synthetase in intermittent acute porphyria. Further-more, results of assay of red cell uroporphyrinogen I synthetase activity in a large family with intermittent acute porphyria suggest that this test may be a reliable indicator of the heterozygous state.