The effect of therapeutic drugs and other pharmacologic agents on activity of porphobilinogen deaminase, the enzyme that is deficient in intermittent acute porphyria.

Drugs and toxins precipitate life-threatening acute attacks in patients with intermittent acute porphyria. These materials may act by directly inhibiting enzyme activity, thus further reducing porphobilinogen (PBG) deaminase activity below the ca. 50% level that results from the gene defect. To test this, we studied the effects of drugs that precipitate acute attacks (lead, phenobarbital, griseofulvin, phenytoin, sulfanilamide, sulfisoxazole, 17alpha-ethinyl estradiol, 5beta-pregnan-3alpha-ol-20-one), drugs that are safe (lithium, magnesium, chlorpromazine, promethazine), and those with uncertain effects (ethyl alcohol, imipramine, diazepam, haloperidol) on activity of PBG deaminase in vitro and in vivo. In the in vitro studies, of PBG deaminase from human erythrocytes from normals and individuals with IAP, only lead (> or = .01 mM) inhibited enzyme activity. Chlorpromazine (> or = .01 mM), promethazine (> or = .01 mM) and imipramine (1 mM) seemed to increase enzyme activity. In most in vivo experiments, male rats were injected intraperitoneally with test material twice daily for 3 days and once on day four; and erythrocyte and hepatic PBG deaminase activity was assayed thereafter. Effects on enzyme activity were observed only with 17alpha-ethinyl estradiol (0.05 microg/kg/day; reduction of 11% in erythrocyte enzyme [NS], and of 20% in liver enzyme [P=.02]), and imipramine (12.5 mg/kg/day; reduction in erythrocyte enzyme activity of 13% [P<.001]). Rats given lead acetate in their drinking water (10 mg/ml) for the first 60 days of life, resulting in high blood and liver lead levels, had increased erythrocyte PBG deaminase (167% of control; P=.004). Thus, enzyme inhibition by lead in vitro was not reflected in a similar in vivo inhibition. The only inhibitory effects in vivo, with ethinyl estradiol and imipramine, appear to be mild and biologically inconsequential. We conclude that inhibition of PBG deaminase activity by materials that precipitate acute attacks is an unlikely mechanism by which these materials exert their harmful effects in patients with IAP.     

Site-directed mutagenesis and high-resolution NMR spectroscopy of the active site of porphobilinogen deaminase

The active site of porphobilinogen (PBG)1 deaminase (EC 4.3.1.8) from Escherichia coli has been found to contain an unusual dipyrromethane derived from four molecules of 5-aminolevulinic acid (ALA) covalently linked to Cys-224, one of the two cysteine residues conserved in E. coli and human deaminase. By use of a hemA- strain of E. coli the enzyme was enriched from [5-13C]ALA and examined by 1H-detected multiple quantum coherence spectroscopy, which revealed all of the salient features of a dipyrromethane composed of two PBG units linked head to tail and terminating in a CH2-S bond to a cysteine residue. Site-specific mutagenesis of Cys-99 and Cys-242, respectively, has shown that substitution of Ser for Cys-99 does not affect the enzymatic activity, whereas substitution of Ser for Cys-242 removes essentially all of the catalytic activity as measured by the conversion of the substrate PBG to uro'gen I. The NMR spectrum of the covalent complex of deaminase with the suicide inhibitor 2-bromo-[2,11-13C2]PBG reveals that the aninomethyl terminus of the inhibitor reacts with the enzyme's cofactor at the alpha-free pyrrole. NMR spectroscopy of the ES2 complex confirmed a PBG-derived head-to-tail dipyrromethane attached to the alpha-free pyrrole position of the enzyme. A mechanistic rationale for deaminase is presented.     

Biosynthesis of porphyrins and corrins. 2. Isolation, purification, and NMR investigations of the porphobilinogen-deaminase covalent complex

The procedures for the generation of enzyme-substrate complexes from labeled porphobilinogens [(2,11-13C]PBG and [2,6,11-3H]PBG) with deaminase and the methods employed for their purification are described. Use of 13C NMR failed to detect the substrate bound to the enzyme, suggesting that the line width must be inordinately large. The complex was found to disproportionate with time when stored at 25 degrees C. However, enzyme-bound uroporphyrinogen I (uro'gen I) was detected, both in the intact protein and in the oligopeptides from tryptic digestion and peptide mapping. The first detection of an enzyme-substrate complex by 3H NMR is described for [3H]PBG and deaminase. The line widths of the observed resonances were found to be extremely large and dependent upon temperature, giving chemical shifts that suggest the involvement of a sulfhydryl group as the nucleophilic enzyme group that binds the substrate. The catalytic competence of this complex was also demonstrated by displacing bound [3H]PBG with unlabeled PBG. During the resultant formation of [3H]uro'gen I, a transient low-intensity signal was detected that has been tentatively assigned to the highly reactive azafulvene species, proposed in several mechanistic schemes for porphyrin biosynthesis.     

Biochemical characterization of porphobilinogen deaminase-deficient mice during phenobarbital induction of heme synthesis and the effect of enzyme replacement

Acute intermittent porphyria (AIP) is a genetic disorder caused by a deficiency of porphobilinogen deaminase (PBGD), the 3rd enzyme in heme synthesis. It is clinically characterized by acute attacks of neuropsychiatric symptoms and biochemically by increased urinary excretion of the porphyrin precursors porphobilinogen (PBG) and 5-aminolevulinic acid (ALA). A mouse model that is partially deficient in PBGD and biochemically mimics AIP after induction of the hepatic ALA synthase by phenobarbital was used in this study to identify the site of formation of the presumably toxic porphyrin precursors and study the effect of enzyme-replacement therapy by using recombinant human PBGD (rhPBGD). After 4 d of phenobarbital administration, high levels of PBG and ALA were found in liver, kidney, plasma, and urine of the PBGD-deficient mice. The administration of rhPBGD intravenously or subcutaneously after a 4-d phenobarbital induction was shown to lower the PBG level in plasma in a dose-dependent manner with maximal effect seen after 30 min and 2 h, respectively. Injection of rhPBGD subcutaneously twice daily during a 4-d phenobarbital induction reduced urinary PBG excretion to 25% of the levels found in PBGD-deficient mice administered with only phenobarbital. This study points to the liver as the main producer of PBG and ALA in the phenobarbital-induced PBGD-deficient mice and demonstrates efficient removal of accumulated PBG in plasma and urine by enzyme-replacement therapy.     

Molecular Epidemiology and Diagnosis of PBG Deaminase Gene Defects in Acute Intermittent Porphyria

Acute intermittent porphyria (AIP) is the major autosomal dominant form of acute hepatic porphyrias. The disease is due to mutations in the gene encoding for porphobilinogen (PBG) deaminase and is characterized by life-threatening neurovisceral attacks, often precipitated by drugs, fasting, cyclical hormonal changes, or infectious diseases. This report describes a prospective study on the molecular epidemiology of PBG deaminase gene defects in AIP. It uses a sensitive, reliable, and easy-to-handle method for routine AIP molecular diagnosis and family study based on an exon-by-exon denaturing gradient gel electrophoresis (DGGE) strategy followed by direct sequencing. Fifteen genomic DNA fragments, including all the coding sequence and covering 3.35 kb of the PBG deaminase gene, were investigated in 405 subjects from 121 unrelated French Caucasian AIP families who had not been screened previously at the DNA level. PBG deaminase gene mutations were identified in 109 families, but only 78 were of different type, and each of them had a prevalence rate < 5%. Among these mutations, 33 had not been published previously. Sixty percent of these 78 mutations were located in only three exons (exons 10, 12, and 14), 44% were missense, 18% were splice defect, 19% were frameshift, and 16% were nonsense. In addition, two de novo mutational events were characterized. The evaluation of the efficiency of the standard PBG deaminase enzymatic screening method for gene-carrier detection indicated 95% of concordancy with the molecular-based diagnosis.     

Structure and expression of chloroplast-localized porphobilinogen deaminase from pea (Pisum sativum L.) isolated by redundant polymerase chain reaction.

Porphobilinogen (PBG) deaminase catalyzes the polymerization of four PBG monopyrrole units into the linear tetrapyrrole hydroxymethylbilane necessary for the formation of chlorophyll and heme in plant cells. Degenerate oligonucleotide primers were designed based on amino acid sequence data (generated by mass spectrometry) for purified PBG deaminase from pea (Pisum sativum L.) chloroplasts. These primers were used in TaqI polymerase-catalyzed polymerase chain reaction (PCR) amplification to produce partial cDNA and nuclear genomic fragments encoding the enzyme. Subsequently, a 1.6-kb cDNA was isolated by screening a cDNA library constructed in lambda gt11 from leaf poly(A)+ RNA with the PCR products. The cDNA encodes an approximately 40-kD polypeptide containing a 46-amino acid NH2-terminal transit peptide and a mature protein of 323 amino acids. The deduced amino acid sequence of the mature pea enzyme is similar to PBG deaminases from other species and contains the conserved arginine and cysteine residues previously implicated in catalysis. Northern blot analysis indicates that the pea gene encoding PBG deaminase is expressed to varying levels in chlorophyll-containing tissues and is subject to light induction.     

High frequency of mutations in exon 10 of the porphobilinogen deaminase gene in patients with a CRIM-positive subtype of acute intermittent porphyria.

Acute intermittent porphyria (AIP) is an autosomal dominant disease characterized by a partial deficiency of porphobilinogen (PBG) deaminase. Different subtypes of the disease have been defined, and more than 10 different mutations have been described. We focused our study on exon 10, since we previously found that three different mutations were located in this exon and that two of them seemed to be relatively common. We used denaturing gradient gel electrophoresis (DGGE) after in vitro amplification to detect all possible mutations in exon 10 in 41 unrelated AIP patients. In about one-fourth of these patients we could distinguish three abnormal migration patterns, indicating the presence of various mutations. Additional sequencing demonstrated the presence of three different single-base substitutions. Two of these mutations had already been described. A third one consisted of a C-to-T transition located at position 499 of the PBG deaminase mRNA and resulted in an Arg-to-Trp substitution. All three mutations were found in patients with cross-reacting immunological material (CRIM)-positive forms of AIP. The high frequency of these mutations make DGGE analysis of exon 10 a useful approach allowing the direct direction of the DNA abnormality in most of the families with the CRIM-positive subtype of AIP.     

Biosynthesis of porphyrins and corrins. 1. 1H and 13C NMR spectra of (hydroxymethyl)bilane and uroporphyrinogens I and III

High-field NMR spectroscopic methods have been applied to study the reactions catalyzed by porphobilinogen (PBG) deaminase and uroporphyrinogen III (uro'gen III) cosynthase, which are the enzymes responsible for the formation of the porphyrin macrocycle. The action of these enzymes in the conversion of PBG, [2,11-13C]PBG, and [3,5-13C]PBG to uro'gens I and III has been followed by 1H and 13C NMR, and assignments are presented. The principal intermediate that accumulated was the correspondingly labeled (hydroxymethyl)bilane (HMB), the assignments for which are also presented.     

Selenium as a novel regulator of porphyrin biosynthesis in germinating seedlings of mung bean (Phaseolus vulgaris)

5-Aminolevulinic acid, porphyrin and chlorophyll contents as well the activities of 5-aminolevulinic acid dehydratase and PBG deaminase were studied in selenium treated mung bean seedlings. Selenium had no effect on 5-aminolevulinic acid synthetic ability but inhibited 5-aminolevulinic acid dehydratase and PBG deaminase activities. Further, it was observed that selenium induced accumulation of protoporphyrin-IX and Mg-protoporphyrin ester and decreased chlorophyll levels in both light and dark-grown seedlings. The results suggest the possible regulatory role of selenium on chlorophyll synthesis by interacting with sulfhydryl containing enzymes 5-aminolevulinic acid dehydratase and porphobilinogen deaminase.     

Human red cell porphobilinogen deaminase. A simpler method of purification and some unusual properties

A simpler method for purifying human red cell deaminase, using a mixture of n-butanol and chloroform, which denatures hemoglobin, followed by ammonium sulphate fractionation, heat treatment, Sephadex G-100 and DEAE-cellulose chromatography, yielding a 3400 fold purified enzyme is described. Some properties of purified deaminase were studied. The enzyme seems to have a strict requirement for oxygen, neither PBG consumption nor uroporphyrinogens formation were measured under anaerobiosis. Uroporphyrinogens formation was linear with both protein and time over a wide range of enzyme concentration and up to 2 h. The optimum pH was 7.4 and the mol. wt was 40,000 +/- 4000. The enzyme was heat-stable and increased its activity by heating. Ammonium and hydroxylamine ions inhibited the reaction. K+ and Na+ ions did not greatly affect activity, while most divalent cations tested significantly diminished uroporphyrinogen formation and to a lesser degree PBG consumption. Direct plots of velocity against PBG concentration were hyperbolic, however double-reciprocal plots were non-linear, Hill plots gave an n value of 2 and Eadie plots were bell-shaped, indicating the existence of weakly positive cooperative effect between 2 binding sites for PBG per molecule of deaminase.     

Genetic heterogeneity of the porphobilinogen deaminase gene in Swedish families with acute intermittent porphyria

Summary Acute intermittent porphyria (AIP) is an autosomal dominant metabolic disorder affecting the enzyme porphobilinogen (PBG) deaminase in the heme biosynthetic pathway. The highest prevalence of the disorder has been observed in Scandinavia, especially in northern Sweden (Lappland) where it occurs with a prevalence of 1 in 1500. Biochemical assays of the activity and concentration of PBG deaminase in red blood cells, haplotyping with 4 intragenic restriction fragment length polymorphisms (RFLPs) (MspI, PstI, BstNI, ApaLI) using the polymerase chain reaction (PCR) and screening for known base substitutions by oligonucleotide probes was performed in 28 Swedish AIP families. There was no close relationship between haplotype, biochemical findings (PBG deaminase activity, enzyme-linked immuno-sorbent assay [ELISA], and excess urinary excretion of delta-aminolevulinic acid or PBG), and a specific mutation. Three different haplotypes were identified. The haplotype 2/1/1/2 (MspI/PstI/BstNI/ApaLI; +/-/-/+) was found to be the most frequent among gene carriers (P < 0.001). The disease segregated with the haplotype 2/1/1/2 in the 10 families originating from northern Sweden. All 28 families were screened for three known point mutations. Only one was found to carry one of these mutations. Thus, the genetic background of AIP is heterogeneous in Sweden.     

Nucleotide sequence of the hemC locus encoding porphobilinogen deaminase of Escherichia coli K12.

Porphobilinogen deaminase, the product of the hemC locus in Escherichia coli K12, catalyses the tetrapolymerisation of porphobilinogen (PBG) into the hydroxymethylbilane, preuroporphyrinogen. The hemC locus has been subcloned from the Clarke and Carbon plasmid pLC41-4. The sequence of the hemC structural gene and flanking DNA was determined by the dideoxy chain termination method of Sanger. The structural gene for hemC is located within a 942bp sequence encoding the monomeric PBG deaminase, molecular weight 33,857. The extent of the coding region was confirmed by sequencing the N-terminus of the purified enzyme and by determination of the molecular weight. The hemC locus is closely linked to the cyaA locus, the genes being transcribed in a divergent manner. Upstream of the hemC coding region, a possible promoter and three repeated GGATG sequences were identified. This is the first report of a complete DNA sequence for a structural gene specifying an enzyme of the heme biosynthetic pathway in prokaryotes.     

Spectroscopic evidence for a porphobilinogen deaminase-tetrapyrrole complex that is an intermediate in the biosynthesis of uroporphyrinogen III

Incubation of porphobilinogen (PBG) with PBG deaminase from Rhodopseudomonas sphaeroides in carbonate buffer (pH 9.2) to total PBG consumption resulted in low yields of uroporphyrinogen I (uro'gen I). In the reaction mixture a pyrrylmethane accumulated, which at longer incubation periods was transformed into uro'gen I. The accumulated pyrrylmethane gave an Ehrlich reaction which was different from that of a 2-(aminomethyl)dipyrrylmethane or 2-(aminomethyl)tripyrrane. It resembled that of a bilane (tetrapyrrylmethane) but was different from that of a 2-(hydroxymethyl)bilane. The 13C NMR spectra of incubations carried out with [11-13C]PBG indicated that the pyrrylmethane was a tetrapyrrole with methylene resonances at 22.35-22.50 ppm. It was loosely bound to the deaminase, and when separated from the enzyme by gel filtration or gel electrophoresis, it immediately cyclized to uro'gen I. No enzyme-bound methylene could be detected by its chemical shift, suggesting that its line width must be very broad. When uro'gen III-cosynthase was added to the deaminase-tetrapyrrole complex, uro'gen III was formed at the expense of the latter in about 75% yield. The tetrapyrrole could only be partially displaced from the enzyme by ammonium ions, although a small amount of 2-(aminomethyl)bilane was always formed together with the tetrapyrrole intermediate. A protonated uro'gen I structure for this intermediate was ruled out by incubations using [2,11-13C]PBG. Uro'gen III formation from 2-(hydroxymethyl)bilane (HMB) and from the deaminase-tetrapyrrole intermediate was compared by using deaminase-cosynthase and cosynthase from several sources. It was found that while the HMB inhibited uro'gen III formation at higher concentrations and longer incubation times, uro'gen III formation from the complex did not decrease with time.     

Adenosine deaminase deficiency and severe combined immunodeficiency disease.

The nature of the association of adenosine deaminase deficiency and severe combined immunodeficiency disease is reviewed. The basis for the molecular heterogeneity exhibited by adenosine deaminase in human tissue and the mechanisms whereby a deficiency of this activity results in the extreme perturbation of the immune system as observed in severe combined immunodeficiency are critically discussed.     

Correction of the biochemical defect in porphobilinogen deaminase deficient cells by non-viral gene delivery

Porphobilinogen deaminase (PBGD), the third enzyme in the biosynthesis of heme, is deficient in acute intermittent porphyria (AIP). AIP is a genetic disease characterized by neurovisceral and psychiatric disturbances. Despite a palliative treatment, it may still be lethal. An initial step towards gene therapy was recently taken by showing that PBGD could be expressed to correct the enzyme deficiency in AIP fibroblasts. The aim of the present study was to investigate whether the biochemical defect can be corrected by using non-viral gene delivery. The biochemical defect in human and mouse PBGD deficient fibroblasts was demonstrated by analyzing synthesis of the heme precursor, protoporphyrin (PP), after addition of 5-aminolevulinic acid (ALA). Human AIP fibroblasts synthesized 21% and mouse PBGD deficient fibroblasts only 11% of the PP amount synthesized in respective control cells. Gene delivery increased the PBGD activity 88–200 fold in human AIP fibroblasts and synthesis of PP was increased from 21–152% of normal after ALA incubation. Similar results were obtained in mouse PBGD deficient cells, although the PP levels were several-fold lower as compared to human cells. HPLC analysis confirmed that PP was the main porphyrin intermediate that was formed. Addition of porphobilinogen (PBG) resulted in 3–7 fold lower synthesis of PP as compared to ALA addition. These results show that non-viral gene delivery of plasmids encoding PBGD results in a high expression of functional PBGD shown by induced synthesis of PP in PBGD deficient cells after supplementation of ALA and PBG.     

Immunologic distinction of human muscle adenylate deaminase from the isozyme(s) in human peripheral blood cells: Implications for myoadenylate deaminase deficiency

Rabbit antisera to human muscle adenylate deaminase efficiently precipitate this enzyme from crude homogenates of muscle from man, monkey, and horse, without affecting other enzymes and proteins in the sample. They fail to react, however, with the adenylate deaminase of purified human erythrocytes, granulocytes, lymphocytes, and platelets. The exclusive presence of an antigenic determinant in the muscle adenylate deaminase suggests that this isozyme is determined by a unique gene. Since patients with myoadenylate deaminase deficiency have normal levels of the enzyme in their erythrocytes and leukocytes, this constitutes the strongest evidence at present that the deficiency state is due to an inherited gene defect.     

Heme synthesis in Crithidia deanei: influence of the endosymbiote

The activity of the following enzymes involved in the biosynthesis of porphyrins was determined in endosymbiote-free and endosymbiote-containing Crithidia deanei grown in a chemically defined medium: succinyl Coenzyme A synthetase (Suc.CoA-S), 5-aminolevulinate synthetase (ALA-S), 4,5-dioxovaleric acid transaminase (DOVA-T), 5-aminolevulinate dehydratase (ALA-D), porphobilinogenase (PBGase), deaminase and heme synthetase (Heme-S). The amount of 5-aminolevulinic acid (ALA) and porphobilinogen, porphyrins and heme was also determined. ALA and PBG were detected in C. deanei. The levels of free porphyrins was low. Heme concentration was nil. The activity of ALA-D, deaminase and PBGase was not detected in C. deanei. The activity of Suc.CoA-S and ALA-S were twice higher in symbiote-containing than in aposymbiotic C. deanei. Aposymbiotic cells had a higher activity of DOVA-T than symbiote-containing cells. The level of Heme-S, measured using protoporphyrin as substrate, was twice as high in symbiote-containing than in symbiote-free cells.