Purification and properties of 5-aminolaevulinate dehydratase from human erythrocytes

Mechanistic studies on Pyrobaculum calidifontis porphobilinogen synthase (5-aminolevulinic acid dehydratase)

Porphobilinogen synthase (PBG synthase) gene from Pyrobaculum calidifontis was cloned and expressed in E. coli. The recombinant enzyme was purified as an octamer and was found by mass spectrometry to have a subunit M_r of 37676.59 (calculated, 37676.3). The enzyme showed high thermal stability and retained almost all of its activity after incubation at 70 °C for 16 h in the presence of β-mercaptoethanol (β-ME) and zinc chloride. However, in the absence of the latter the enzyme was inactivated after 16 h although it regained full activity in the presence of β-ME and zinc chloride. The protein contained 4 mol of tightly bound zinc per octamer. Further, 4 mol of low affinity zinc could be incorporated following incubation with exogenous zinc salts. The enzyme was inactivated by incubation with levulinic acid followed by treatment with sodium borohydride. Tryptic digest of the modified enzyme and mass spectrometric analysis showed that Lys²⁵⁷ was the site of modification, which has previously been shown to be the site for the binding of 5-aminolevulinic acid giving rise to the propionate-half of porphobilinogen. P. calidifontis PBG synthase was inactivated by 5-chlorolevulinic acid and the residue modified was shown to be the central cysteine (Cys¹²⁷) of the zinc-binding cysteine-triad, comprising Cys^{125, 127, 135}. The present results in conjunction with earlier findings on zinc containing PBG synthases, are discussed which advocate that the catalytic role of zinc in the activation of the 5-aminolevulinic acid molecule forming the acetate-half of PBG is possible.

Structural studies of substrate and product complexes of 5-aminolaevulinic acid dehydratase from humans,Escherichia coliand the hyperthermophilePyrobaculum calidifontis

A number of X-ray analyses of an enzyme involved in a key early stage of tetrapyrrole biosynthesis are reported. Two structures of human 5-aminolaevulinate dehydratase (ALAD), native and recombinant, have been determined at 2.8 Å resolution, showing that the enzyme adopts an octameric quaternary structure in accord with previously published analyses of the enzyme from a range of other species. However, this is in contrast to the finding that a disease-related F12L mutant of the human enzyme uniquely forms hexamers [Breiniget al.(2003),Nature Struct. Biol.10, 757–763]. Monomers of all ALADs adopt the TIM-barrel fold; the subunit conformation that assembles into the octamer includes the N-terminal tail of one monomer curled around the (α/β)8barrel of a neighbouring monomer. Both crystal forms of the human enzyme possess two monomers per asymmetric unit, termedAandB. In the native enzyme there are a number of distinct structural differences between theAandBmonomers, with the latter exhibiting greater disorder in a number of loop regions and in the active site. In contrast, the second monomer of the recombinant enzyme appears to be better defined and the active site of both monomers clearly possesses a zinc ion which is bound by three conserved cysteine residues. In native human ALAD, theAmonomer also has a ligand resembling the substrate ALA which is covalently bound by a Schiff base to one of the active-site lysines (Lys252) and is held in place by an ordered active-site loop. In contrast, these features of the active-site structure are disordered or absent in theBsubunit of the native human enzyme. The octameric structure of the zinc-dependent ALAD from the hyperthermophilePyrobaculum calidifontisis also reported at a somewhat lower resolution of 3.5 Å. Finally, the details are presented of a high-resolution structure of theEscherichia coliALAD enzyme co-crystallized with a noncovalently bound moiety of the product, porphobilinogen (PBG). This structure reveals that the pyrrole side-chain amino group is datively bound to the active-site zinc ion and that the PBG carboxylates interact with the enzymeviahydrogen bonds and salt bridges with invariant residues. A number of hydrogen-bond interactions that were previously observed in the structure of yeast ALAD with a cyclic intermediate resembling the product PBG appear to be weaker in the new structure, suggesting that these interactions are only optimal in the transition state.

Inhibition of 5-aminolevulinic acid dehydratase by mercury in excised greening maize leaf segments

Mercury (Hg), a potent metallic toxicant, is known for having inhibitory effect on chlorophyll biosynthesis. In vivo supply of HgCl(2) inhibited 5-aminolevulinic acid dehydratase (ALAD, EC 4.2.1.24) activity in excised greening maize (Zea mays) leaf segments. The inhibition caused by Hg was alleviated by addition of KNO(3). Amongst the nutrients and metabolites tested, NH(4)Cl and sucrose increased the inhibitory effect of Hg on enzyme activity, while glutamine and glutathione decreased it. The inhibitors, levulinic acid and 5,5' dithio bis 2-nitrobenzoic acid, also reduced the % inhibition of enzyme activity caused by Hg supply. In vitro inclusion of Hg during assay of the enzyme preparations obtained from the tissue treated without Hg (-Hg enzyme) and with Hg (+Hg enzyme) caused the inhibition of -Hg enzyme but activation of +Hg enzyme. Almost similar trend was observed for the in vitro inclusion of Hg in the presence of levulinic acid. It is suggested that two forms of enzyme exist in Hg-treated tissue, i.e. the usual Mg dependent form and an unusual Hg modified form. Kinetic studies for the two enzymes, -Hg enzyme and +Hg enzyme, involving the effect of varying concentrations of δ-aminolevulinic acid yielded distinct apparent K(m) and apparent V(max) values being 532 μM and 118 units g(-1) fr. wt., respectively, for -Hg enzyme and 347 μM and 52 units g(-1) fr. wt., respectively, for +Hg enzyme indicating that +Hg enzyme has higher affinity for δ-aminolevulinic acid but lower activity as compared to the -Hg enzyme.

Dexmedetomidine protects blood δ-aminolevulinate dehydratase from inactivation caused by hyperoxygenation in total intravenous anesthesia

Delta-aminolevulinate dehydratase (δ-ALA-D) enzyme is sensitive to pro-oxidant agents, including molecular oxygen. Here, we tested whether hyperoxygenation after total intravenous (i.v.) anesthesia could interact with the type of anesthesia (dexmedetomidine, continuous infusion; 0.5 μg/kg/h or remifentanil, continuous infusion; 0.3 μg/kg/min) plus propofol using blood δ-ALA-D activity and thiobarbituric acid reactive substances (TBARS) levels as ending points of toxicity. In absence or presence of dithiothreitol (DTT), δ-ALA-D activity was reduced after hyperoxygenation in the group treated with remifentanil and was not modified in dexmedetomidine group. TBARS increased considerably in the blood of both groups of patients after oxygenation. The results obtained here suggest that the hyperoxygenation was associated with a marked increase in TBARS production regardless of the type of anesthesia. δ-ALA-D activity was only inhibited in remifentanil group, which indicates a possible interaction between oxygenation and the type of anesthetic. This is the first demonstration that dexmedetomidine may protect blood δ-ALA-D from oxidation. However, further studies are necessary to establish a possible antioxidant role of dexmedetomidine against hyperoxygenation in human blood.

Lead and cognitive function in ALAD genotypes in the third National Health and Nutrition Examination Survey

The relationship between the blood lead concentration and cognitive function in children and adults with different ALAD genotypes who participated in the third National Health and Nutrition Examination Survey was investigated. The relationship between blood lead and serum homocysteine concentrations was also investigated. In children 12 to 16 years old, no difference in the relationship between cognitive function and blood lead concentration between genotypes was found. In adults 20 to 59 years old, mean reaction time decreased as the blood lead concentration increased in the ALAD rs1800435 CC/CG group. This represents an improvement in performance. In adults 60 years and older, no difference in the relationship between cognitive function and blood lead concentration between genotypes was found. The serum homocysteine concentration increased as the blood lead concentration increased in adults 20 to 59 years old and 60 years and older, but there were no differences between genotypes. The mean blood lead concentration of children with the ALAD rs1800435 CC/CG genotype was less than that of children with the GG genotype.

Co-synthesis of Human delta-Aminolevulinate Dehydratase (ALAD) Mutants with the Wild-type Enzyme in Cell-free System-Critical Importance of Conformation on Enzyme Activity-

Properties of mutant delta-aminolevulinate dehydratase (ALAD) found in patients with ALAD porphyria were studied by enzymological and immunological analyses after the synthesis of enzyme complexes using a cell-free system. Enzyme activities of homozygous G133R, K59N/G133R, V153M, and E89K mutants were 11%, 22%, 67%, and 75% of the wild-type ALAD, respectively, whereas that of K59N, a normal variant, was 112%. Enzyme activities of L273R, C132R and F12L were undetectable. Co-synthesis of F12L, L273R, G133R, K59N/G133R, or C132R mutants with the wild-type at various ratios showed that ALAD activity was proportionally decreased in the amount of the wild-type in the complex. In contrast, co-synthesis of V153M, K59N, and E89K with the wild-type did not influence enzyme activity of the wild-type. Surface charge changes in K59N, E89K, C132R and G133R predicted by mutations were also confirmed by native polyacrylamide gel electrophoresis. A compound E89K and C132R complex showed ALAD activity similar to that was found in erythrocytes of the patient. These findings indicate that cell-free synthesis of ALAD proteins reflects enzymatic activities found in patients, and suggest that, in addition to the direct effect of mutations on the catalytic activity, conformational effects play an important role in determining enzyme activity.

Some biochemical properties of delta-aminolevulinic acid dehydratase in Gammarus pulex

The determination of delta-aminolevulinic acid dehydratase (ALAD; porphobilinogen synthetase, EC 4.2.1.24) activity in aquatic organisms might be a useful marker for the identification of lead exposure. ALADs from the variety of different sources have been grouped into two classes based on some biochemical properties such as molecular weight, pH optimum, metal requirement and susceptibility to EDTA. The first group includes the enzymes from mammals and birds, while the second group ALADs are derived primarily from bacteria and yeasts. In this study, we have characterized Gammarus ALAD in some biochemical aspects. Gammarus pulex were collected from the Porsuk River at Eskişehir (Turkey). The effect of pH, incubation temperature of reaction mixture, incubation period, metal ions and EDTA on enzyme activity were investigated. Comparisons between groups were performed by analysis of a paired t-test. Gammarus ALAD was found biochemically distinct from the mammalian enzyme. It seems to be considered in Class II rather than Class I.

Porphobilinogen synthase from the butterfly, Pieris brassicae: purification and comparative characterization

Porphobilinogen represents a key building block of tetrapyrroles serving as functional ligands of many vitally important proteins. Here we report the first purification of porphobilinogen synthase (PBGS) from whole insects by sequentially employing two modes of native electrophoresis on polyacrylamide gels subsequent to more conventional procedures. Using adults of Pieris brassicae L. (Lepidoptera: Pieridae) we achieved approximately 10,000-fold purification with final yields of up to 25% of electrophoretically pure PBGS with a specific activity of approximately 160 micromol PBG h(-1) mg(-1) at 37 degrees C and an affinity of 0.36 mM to its substrate 5-aminolevulinic acid. Enzyme activity was inhibited by the substrate mimics, levulinic acid and succinylacetone, and by chelating agents. PBGS behaved as a relatively heat-stable octameric complex of 292.3 kDa composed of 36.5 kDa subunits. Most general features of this insect PBGS were comparable to those published for other animal PBGS enzymes, while remarkable differences were found to the reported recombinant Drosophila enzyme. Moreover, rabbit antiserum directed against purified Pieris PBGS revealed significant immunological differences among insect PBGS enzymes from a wide range of orders contrasting to the overall evolutionary conserved features of this enzyme.

Synthesis of porphobilinogen via a novel ozonide cleavage reaction

Porphobilinogen lactam methyl ester (3a) has been prepared in seven steps, and approximately 20-30% overall yield, beginning with furfurylamine (4a).(24) Hydrolysis of 3a following the literature procedure then gave porphobilinogen (1). A key intermediate in our synthesis of 3a is the 7-oxonorbornene derivative 7a, which was derived from 4a utilizing a tandem Johnson ortho ester Claisen rearrangement followed by intramolecular Diels-Alder cyclization (five steps, 55-65%).(24) Interesting steric accelerating effects were observed in this sequence. Conversion of 7a to 3a was then accomplished employing a novel ozonide cleavage/oxidation reaction, which generated tetrahydrofurans 16a, 32, and 33 in the proper oxidation state for direct aminolysis to pyrrole 3a. A mechanism is proposed for the ozonide cleavage/oxidation that accounts for the observed stereoselectivity of this step.

An artificial gene for human porphobilinogen synthase allows comparison of an allelic variation implicated in susceptibility to lead poisoning

Porphobilinogen synthase (PBGS) is an ancient enzyme essential to tetrapyrrole biosynthesis (e.g. heme, chlorophyll, and vitamin B(12)). Two common alleles encoding human PBGS, K59 and N59, have been correlated with differential susceptibility of humans to lead poisoning. However, a model for human PBGS based on homologous crystal structures shows the location of the allelic variation to be distant from the active site with its two Zn(II). Previous microbial expression systems for human PBGS have resulted in a poor yield. Here, an artificial gene encoding human PBGS was constructed by recursive polymerase chain reaction from synthetic oligonucleotides to rectify this problem. The artificial gene was made to resemble the highly expressed homologous Escherichia coli hemB gene and to remove rare codons that can confound heterologous protein expression in E. coli. We have expressed and purified recombinant human PBGS variants K59 and N59 in 100-mg quantities. Both human PBGS proteins purified with eight Zn(II)/octamer; Zn(II) binding was shown to be pH-dependent; and Pb(II) could displace some of the Zn(II). However, there was no differential displacement of Zn(II) by Pb(II) between K59 and N59, and simple Pb(II) inhibition studies revealed no allelic difference.

The affinity of human erythrocyte porphobilinogen synthase for Zn2+ and Pb2+

Porphobilinogen synthase activity has been measured in human erythrocyte lysates supplemented with metal-ion buffers to control free Zn2+ and Pb2+ concentrations. The enzyme is activated by Zn2+ with a Km of 1.6 pM and inhibited by Pb2+ with a Ki of 0.07 pM. Pb2+ and Zn2+ appear to compete for a single metal-binding site. The half-time for loss of Zn2+ from the active site, or replacement of Pb2+ by Zn2+, were in the 10-20-min range at 37 degrees C. Zn2+ did not affect the affinity for the substrate 5-aminolevulinate, but Pb2+ reduced it non-competitively. All the experiments were conducted with a blood sample of the common 1-1 phenotype [Astrin, K. H., Bishop, D. F., Wetmur, J. G., Kaul, B., Davidow, B. & Desnick, R. J. (1987) Ann. NY Acad. Sci. 514, 23-29].

Investigation of the nature of the two metal-binding sites in 5-aminolaevulinic acid dehydratase from Escherichia coli

Two distinct metal-binding sites, termed alpha and beta, have been characterized in 5-aminolaevulinic acid dehydratase from Escherichia coli. The alpha-site binds a Zn2+ ion that is essential for catalytic activity. This site can also utilize other metal ions able to function as a Lewis acid in the reaction mechanism, such as Mg2+ or Co2+. The beta-site is exclusively a transition-metal-ion-binding site thought to be involved in protein conformation, although a metal bound at this site only appears to be essential for activity if Mg2+ is to be bound at the alpha-site. The alpha- and beta-sites may be distinguished from one another by their different abilities to bind divalent-metal ions at different pH values. The occupancy of the beta-site with Zn2+ results in a decrease of protein fluorescence at pH 6. Occupancy of the alpha- and beta-sites with Co2+ results in u.v.-visible spectral changes. Spectroscopic studies with Co2+ have tentatively identified three cysteine residues at the beta-site and one at the alpha-site. Reaction with N-ethyl[14C]maleimide preferentially labels cysteine-130 at the alpha-site when Co2+ occupies the beta-site.

Studies of hybrid hematopoietic growth factor receptors

To gain further insight into the mechanisms by which both granulocyte-macrophage-colony stimulating factor (GM-CSF) and erythropoietin receptors function, we have utilized a GM-CSF erythropoietin hybrid receptor with GM-CSF as the external domain and erythropoietin as the intracellular domain. Results show that the beta common GM-CSF receptor both enhances the affinity binding of GM-CSF to the receptor and plays an important role in signaling through the receptor. A truncated form of the beta common receptor actually acts as a dominant negative regulatory factor.

Cloning and characterisation of genes for tetrapyrrole biosynthesis from the cyanobacterium Anacystis nidulans R2

The genes for 5-aminolevulinic acid dehydratase (ALAD) and uroporphyrinogen III synthase (UROS), two enzymes in the biosynthetic pathway for tetrapyrroles, were independently isolated from a plasmid-based genomic library of Anacystis nidulans R2 (also called Synechococcus sp. PCC7942), by their ability to complement Escherichia coli strains carrying mutations in the equivalent genes (hemB and hemD respectively). The identity of the genes was confirmed by comparing the appropriate enzyme activities in complemented and mutant strains. Subclones of the original plasmids that were also capable of complementing the mutants were sequenced. The inferred amino acid sequence of the cyanobacterial HemB protein indicates a significant difference in the metal cofactor requirement from the higher-plant enzymes, which was confirmed by overexpression and biochemical analysis. The organisation of the cyanobacterial hemD locus differs markedly from other prokaryotes. Two open reading frames were found immediately upstream of hemD. The product of one shows considerable similarity to published sequences from other organisms for uroporphyrinogen III methylase (UROM), an enzyme involved in the production of sirohaem and cobalamins (including vitamin B-12). The product of the other shows motifs which are similar to those found in proteins responsible for metabolic regulation in yeast and indicates that this family of transcription control proteins, which has previously been reported only from eukaryotes, is also represented in prokaryotes.

Purification and characterization of 5-aminolaevulinic acid dehydratase from Escherichia coli and a study of the reactive thiols at the metal-binding domain

5-Aminolaevulinic acid dehydratase (ALAD) from a recombinant strain of Escherichia coli was purified to homogeneity. The enzyme is a homo-octamer of subunit M(r) 36554 +/- 17. Enzyme activity was dependent on the presence of Zn2+ ions and an exogenous thiol. Two molar equivalents of Zn2+ are bound/mol of subunit under reducing conditions. On exposure to the metal chelator EDTA, the two Zn2+ ions are removed, giving an inactive metal-depleted apo-ALAD. On oxidation of holo-ALAD, two disulphide bonds are formed with the loss of 1 mol of Zn2+/mol of subunit. The formation of the first disulphide led to the loss of catalytic activity. Replacement of the two bound Zn2+ ions with Co2+ resulted in the formation of a green protein with a spectrum indicative of the presence of charge-transfer bands from one or more cysteine-Co2+ ligands. While Mg2+ could not activate apo-ALAD alone, it was able to substitute for the second molar equivalent of bound Zn2+, leading to a further 4-fold stimulation in activity. The four cysteine residues involved in the formation of the two disulphide bonds were identified by protein-chemistry studies and were all located in a region of the protein extending from amino acid residues 120-134. Protein sequence data obtained in the present study has permitted the resolution of several differences between the published gene-derived protein sequences for ALAD from E. coli.

Reevaluation of a sensitive indicator of early lead exposure. Measurement of porphobilinogen synthase in blood

A principal target for the environmental toxin lead (Pb) is porphobilinogen synthase (PBGS), a Zn-metalloenzyme necessary for heme biosynthesis. Measurement of blood Pb inhibited PBGS is the most sensitive indicator of subclinical Pb intoxication, but problems with the assay have diminished its use. This report identifies Pb as a slow acting inhibitor of PBGS. The activity of PBGS could change up to sixfold during an hourlong clinical assay of Pb contaminated blood, and activity is profoundly effected by the presence of serum proteins, such as albumin. When PBGS catalyzed PBG production is allowed to reach a steady state rate, kinetic data on purified PBGS support the hypothesis that Pb inhibition of PBGS results from direct substitution for Zn.

Purification of delta-aminolevulinate dehydratase from genetically engineered yeast

Saccharomyces cerevisiae transformed with a multicopy plasmid carrying the yeast structural gene HEM2, which codes for delta-aminolevulinate dehydratase, was enriched 20-fold in the enzyme. Beginning with cell-free extracts of transformed cells, the dehydratase was purified 193-fold to near-homogeneity. This represents a 3900-fold purification relative to the enzyme activity in normal, untransformed yeast cells. The specific activity of the purified enzyme was 16.2 mumol h-1 per mg protein at pH 9.4 and 37.5 degrees C. In most respects the yeast enzyme resembles mammalian enzymes. It is a homo-octamer with an apparent Mr of 275,000, as determined by centrifugation in glycerol density gradients, and under denaturing conditions behaved as a single subunit of Mr congruent to 37,000. The enzyme requires reduced thiol compounds to maintain full activity, and maximum activity was obtained in the presence of 1.0 mM-Zn2+. It is sensitive to inhibition by the heavy metal ions Pb2+ and Cu2+. The enzyme exhibits Michaelis-Menten kinetics and has an apparent Km of 0.359 mM. Like dehydratases from animal tissues, the yeast enzyme is rather thermostable. During the purification process an enhancement in total delta-aminolevulinate dehydratase activity suggested the possibility that removal of an inhibitor of the enzyme could be occurring.

Heterologous expression of human 5-aminolevulinate dehydratase in Saccharomyces cerevisiae

A cDNA coding for human 5-amino-levulinate dehydratase was placed in a yeast expression vector under the control of the GAL10 promoter. The resulting multicopy plasmid was then used to transform a yeast mutant which contains a defective hem2 gene coding for 5-aminolevulinate dehydratase. Expression of the human cDNA was shown in four ways: (1) restoration of normal growth on glycerol/galactose as primary carbon source, (2) decrease in intracellular 5-aminolevulinic acid concentration, (3) restoration of cytochrome biosynthesis and (4) direct, in situ assay of 5-aminolevulinic acid dehydratase. Curing transformed cells of plasmid restored the hem2 mutant phenotype. This heterologous system could be used to produce large quantities of human 5-aminolevulinic acid dehydratase for physical and biochemical studies.

Isolation of a rat liver delta-aminolevulinate dehydrase (ALAD) cDNA clone: evidence for unequal ALAD gene dosage among inbred mouse strains

We have isolated several cDNA clones encoding delta-aminolevulinate dehydrase [ALAD; porphobilinogen synthase; 5-aminolevulinate hydro-lyase (adding 5-aminolevulinate and cyclizing), EC 4.2.1.24], the second enzyme in the heme biosynthetic pathway. We used a rabbit polyclonal antibody developed against the purified 35-kDa subunit of rat liver ALAD to screen a lambda gt11 cDNA expression library constructed from rat liver mRNA. A prototype clone (ALAD-1) contained a 680-base-pair insert and expressed a 140-kDa beta-galactosidase gene cDNA insert fusion protein immunoreactive with both polyclonal and monoclonal anti-ALAD. Identity of ALAD-1 was verified by hydridization to ALAD mRNA that had been enriched via immunopurification of rat liver polysomes with anti-ALAD. Intensity of such hybridization to a 1500-nucleotide-long mRNA was approximately equal to 200-fold greater than that realized with whole liver mRNA, a result consistent with the degree of immunoenrichment of ALAD mRNA found independently by analysis of cell-free translation products. A second ALAD cDNA clone (ALAD-3) was isolated when the rat liver cDNA expression library was rescreened with ALAD-1. The identity of both ALAD cDNA clones was established by correspondence between their nucleotide sequence and the reported amino-terminal protein sequence of bovine ALAD. Hybridization of ALAD cDNA to mouse genomic DNA indicates that the previously unexplained incremental differences in ALAD enzymatic activity among inbred mouse strains has arisen through alterations in ALAD gene dose.

Identification of lysine at the active site of human 5-aminolaevulinate dehydratase

Reduction of human 5-aminolaevulinate dehydratase with NaBH4 in the presence of 14C-labelled substrate led to complete loss of catalytic activity and to incorporation of label into the enzyme protein. By comparison with authentic lysyl-aminolaevulinic acid, prepared chemically, the modified active-site amino acid obtained by acid hydrolysis was shown to be lysine. Sequencing of a CNBr-cleavage peptide isolated from the inactivated 14C-labelled enzyme revealed that the lysine was present within the sequence M-V-K-P-G-M.