Investigation of the effect of metal ions on the reactivity of thiol groups in human 5-aminolaevulinate dehydratase

The antioxidant properties of zinc

The ability of zinc to retard oxidative processes has been recognized for many years. In general, the mechanism of antioxidation can be divided into acute and chronic effects. Chronic effects involve exposure of an organism to zinc on a long-term basis, resulting in induction of some other substance that is the ultimate antioxidant, such as the metallothioneins. Chronic zinc deprivation generally results in increased sensitivity to some oxidative stress. The acute effects involve two mechanisms: protection of protein sulfhydryls or reduction of (*)OH formation from H(2)O(2) through the antagonism of redox-active transition metals, such as iron and copper. Protection of protein sulfhydryl groups is thought to involve reduction of sulfhydryl reactivity through one of three mechanisms: (1) direct binding of zinc to the sulfhydryl, (2) steric hindrance as a result of binding to some other protein site in close proximity to the sulfhydryl group or (3) a conformational change from binding to some other site on the protein. Antagonism of redox-active, transition metal-catalyzed, site-specific reactions has led to the theory that zinc may be capable of reducing cellular injury that might have a component of site-specific oxidative damage, such as postischemic tissue damage. Zinc is capable of reducing postischemic injury to a variety of tissues and organs through a mechanism that might involve the antagonism of copper reactivity. Although the evidence for the antioxidant properties of zinc is compelling, the mechanisms are still unclear. Future research that probes these mechanisms could potentially develop new antioxidant functions and uses for zinc.

Erythrocyte delta-aminolevulinic acid dehydratase genotype and other mechanisms affecting workers' susceptibility to lead

In this study, the role of delta-aminolevulinic acid dehydratase (ALAD) variants in lead susceptibility was examined. The study subjects comprised 223 male workers, and the relationship between their blood lead level and erythrocyte ALAD activity or plasma/urine delta-aminolevulinic acid level was studied. Leukocyte specimens from 11 workers, whose erythrocyte ALAD activities were as low as one-fifth that of the other normal workers, were subjected to analyses of their ALAD and ALAD alleles. Further, the entire exon fragment of the ALAD gene was analyzed by polymerase chain reaction, and the reaction product was used as a target for direct DNA sequencing. Genomic DNA analysis revealed that all 11 workers had the ALAD allele, whereas the entire ALAD gene analysis failed to indicate other variants, except for the Rsa I site. The depletion in erythrocyte ALAD activity was not found to be caused by the ALAD allele.

N-acetyl-L-cysteine protects against delta-aminolevulinic acid-induced 8-hydroxydeoxyguanosine formation

5-Aminolevulinic acid (ALA) is a heme precursor that accumulates in acute intermittent porphyria and lead poisoning. It has been shown that ALA induces free radical generation and may cause damage to proteins and DNA. In the present study, the effects of ALA on DNA damage and its prevention by N-acetyl-L-cysteine (NAC) and the antioxidant enzymes catalase (CAT) and superoxide dismutase (SOD) are investigated. Oxidative damage to DNA was quantitated by measuring the increase in 8-hydroxy-2'-deoxyguanosine (oh8dG) formation. The time-course study demonstrated that ALA causes a linear increase in oh8dG levels in Chinese hamster ovary (CHO) cells. However, direct lead exposure did not cause any measurable increase in oh8dG levels. In the presence of either NAC (1 mM) or antioxidant enzymes (10 u/ml SOD and 10 u/ml CAT), oh8dG levels returned to the corresponding control levels. This suggests a protective role for NAC and the antioxidant enzymes. To determine the effect of ALA on cell proliferation, cell numbers were counted at the end of 24 h of incubation in the presence and absence of ALA at different concentrations. Results showed that levels of ALA up to 5 mM do not inhibit cell proliferation.

delta-Aminolevulinate dehydratase inhibition by 2,3-dimercaptopropanol is mediated by chelation of zinc from a site involved in maintaining cysteinyl residues in a reduced state

The mechanisms underlying mouse delta-aminolevulinate dehydratase (ALA-D) inhibition by a chelating agent used in the treatment of heavy metal poisoning, 2,3-dimercaptopropanol (British Anti-Lewisite), were investigated. ALA-D inhibition by 2,3-dimercaptopropanol was totally reversed by 25-100 microM Zn2+, indicating that inhibition was due to chelation of zinc by 2,3-dimercaptopropanol. Our data suggested that zinc bound to a labile site (displaced by 25-40 microM EDTA or 500 microM 2,3-dimercaptopropanol) is involved in maintaining the sulfhydryl groups of ALA-D in a reduced state (essential for enzyme activity), since inhibition by these compounds was reversed by 10 mM dithiotreitol (a reducing agent). On the other hand, 10 mM dithiotreitol did not reverse ALA-D inhibition by a higher concentration of EDTA (100 microM). Accordingly, 2,3-dimercaptopropanol appears to inhibit ALA-D through a mechanism similar to that of low EDTA concentrations. Neither oxidized 2,3-dimercaptopropanol nor reactive oxygen species appeared to contribute for ALA-D inhibition by reduced 2,3-dimercaptopropanol. Taken together, these results suggest that 2,3-dimercaptopropanol inhibits ALA-D by chelating Zn2+ from a labile site that is involved in maintaining enzyme sulfhydryl groups in a reduced state. This site is compatible with the ZnB or Zn beta previously described in mammalian and bacterial ALA-D.

Enhancement of aminolevulinic acid based photodynamic therapy by adriamycin

This paper reports on studies that evaluate the interaction between delta-aminolevulinic acid (ALA)-based photodynamic therapy (PDT) and adriamycin (ADM) in an animal model system. Two groups of mice bearing a transplantable mammary adenocarcinoma received ADM i.p. in a single dose of 5 mg (low dose) and 30 mg (high dose) per kg body weight. Sixteen or 40 h after administration of the drug, mice were sacrificed, tumours, livers and hearts were removed and porphyrins, enzyme activities and malondialdehyde content were determined. Tumour explants of ADM-treated mice were incubated with ALA and irradiated with an He-Ne laser. Re-implantation of these in vitro PDT-treated explants into test animals showed that inhibition of tumour growth was significantly enhanced by combined treatment when the low dose of ADM was used. There were no significant changes in porphyrin content, ALA dehydratase and porphobilinogenase activities in the tissues analyzed after ADM treatment as compared with control values. ADM toxicity is thought to be related to semiquinone free radical formation with subsequent generation of reactive oxygen species such as peroxide and hydroxyl radical. These species are considered to initiate lipid peroxidation (LPO) and cause DNA damage. In the case of low-dose treatment with ADM a significant increase in the LPO product, malondialdehyde, was observed after PDT whereas with the high-dose regimen no changes were observed. In the case of explants of (non-irradiated) cardiac tissue malondialdehyde production was also found to be dependent on the dose and time of administration of adriamycin. In our in vivo/in vitro model system we have shown that pre-treatment with ADM increased the cytotoxicity of ALA-PDT at a dosage level of ADM which did not raise LPO levels in heart tissue. The mechanism of this effect has not been clearly elucidated but our data suggest that the observed enhancement of PDT may be attributed in part to the weakening of cellular defence mechanisms by the pre-treatment involving free radical generation by ADM.

Lead binding to delta-aminolevulinic acid dehydratase (ALAD) in human erythrocytes

Over 99% of the lead present in blood is usually found in erythrocytes. To investigate the nature of this selective accumulation of lead in erythrocytes, the specific binding of lead to proteins in human erythrocytes was studied using liquid chromatography coupled to inductively coupled plasma mass spectrometry (LC-ICP-MS). The principal lead-binding protein had a mass of approximately 240 kDa, and adsorption to specific antibodies showed that protein was delta-aminolevulinic acid dehydratase (ALAD). Thus, the previous notion that lead in erythrocytes was bound primarily to haemoglobin has to be revised. Furthermore, in lead-exposed workers, the percentage of lead bound to ALAD was influenced by a common polymorphism in the ALAD gene. Specifically, in seven carriers of the ALAD2 allele, 84% of the protein-bound lead recovered was bound to ALAD compared to 81% in seven homozygotes for the ALAD1 allele whose erythrocytes were matched for blood-lead concentration. The small difference was statistically significant in Wilcoxon matched-pairs signed-rank test (P = 0.03). No ALAD allele-specific difference in ALAD-bound lead was found among 20 unexposed controls. Perhaps the difference in ALAD-bound lead can provide an explanation for the previously reported finding of higher blood-lead levels among carriers of the ALAD2 allele than among ALAD1 homozygotes in lead-exposed populations.

[Zinc--update of an essential trace element]

Since the recognition of zinc as an essential trace element in man and animals there has been a remarkable progress in our knowledge of the role of zinc in nutritional physiology, biology and medicine during the last few decades. Highlights in zinc research, mechanisms and homeostatic regulation of zinc absorption, sources of zinc intake, dietary factors and mineral interactions affecting zinc bioavailability are reviewed in the present paper. This is followed by an overview of the biochemical functions of zinc in enzymes, gene expression, endocrinology, immunology and oxidative stress. General signs and metabolic consequences of zinc deficiency as well as excessive intake and toxicity of zinc are summarized. Furthermore, national and international dietary zinc recommendations and different methods to determine the zinc status are discussed.

N-actylcysteine protects Chinese hamster ovary (CHO) cells from lead-induced oxidative stress

In vitro administration of lead acetate (PbA) to cultures of Chinese hamster ovary (CHO) cells had a concentration-dependent inhibitory effect on colony formation. Colony formation was returned to control levels in lead-treated cultures that were supplemented with 1 mM N-actylcysteine (NAC), a well-documented synthetic antioxidant. In order to investigate the nature of NAC's protective effect, we measured L-gamma-glutamyl-L-cysteinylglycine (GSH), oxidized glutathione (GSSG), malondialdehyde (MDA) and catalase activity both in the presence and absence of NAC in lead-exposed CHO cells. Increases in both MDA levels (p < 0.05) and catalase activity (P < 0.05) were observed in cultures that received only PbA, but supplementation with NAC returned these measures to pretreatment levels. The ratio of GSH to GSSG increased in lead-exposed cells incubated in NAC-enhanced media, but declined in cultures treated with PbA only. Our results suggest that NAC can confer protection against lead-induced oxidative stress to CHO cells, possibly through the enhancement of the cell's own antioxidant defense mechanisms.

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].

The effect of zinc supplementation on plasma lipids and low-density lipoprotein oxidation in males

Recent studies in animals and in vitro support the hypothesis that zinc, an essential micronutrient, possesses antioxidant properties. The aims of this study were to determine whether zinc provides antioxidant protection in humans by decreasing low-density lipoprotein (LDL) oxidizability and to determine the effect of zinc supplementation on plasma lipid and its distribution among lipoproteins. Ten healthy male volunteers were recruited to participate in a randomized crossover trial. Subjects were asked to consume 50 mg zinc as 220 mg zinc sulphate (equivalent to 4 x recommended dietary intake, or RDI) daily for 4 weeks, followed by placebo, and vice versa. Venous blood samples were collected at 2-week intervals for the determination of plasma lipids and the in vitro oxidizability of LDL in the presence of copper ions. No changes in the oxidizability of LDL or the plasma concentrations of total cholesterol, high-density lipoprotein (HDL) cholesterol and its subfractions, LDL cholesterol, or triacylglycerol were observed following zinc supplementation compared to placebo. Plasma zinc concentrations were increased significantly, indicating that the lack of effect was not due to poor compliance. The antioxidant effect of zinc in relation to LDL was not demonstrated in humans at this dose, and higher doses are unlikely to be effective given the adverse interaction with copper metabolism and the potential decrease in the activity of superoxide dismutase, a free radical quenching enzyme. The findings of this study suggest that zinc provides little, if any, antioxidant protection against LDL oxidation in humans.

Characterization of the two 5-aminolaevulinic acid binding sites, the A- and P-sites, of 5-aminolaevulinic acid dehydratase from Escherichia coli

Experiments are described in which the individual properties of the two 5-aminolaevulinic acid (ALA) binding sites, the A-site and the P-site, of 5-aminolaevulinic acid dehydratase (ALAD) have been investigated. The ALA binding affinity at the A-site is greatly enhanced (at least 10-fold) on the binding of the catalytic metal ion (bound at the alpha-site). The nature of the catalytic metal ion, Mg2+ or Zn2+, also gave major variations in the substrate Km, P-site affinity for ALA, the effect of potassium and phosphate ions and the pH-dependence of substrate binding. Modification of the P-site by reaction of the enzyme-substrate Schiff base with NaBH4 and analysis of the reduced adduct by electro-spray mass spectrometry indicated a maximum of 1 mol of substrate incorporated/mol of subunit, correlating with a linear loss of enzyme activity. The reduced Schiff-base adduct was used to investigate substrate binding at the A-site by using rate-of-dialysis analysis. The affinity for ALA at the A-site of Mg alpha Zn beta ALAD was found to determine the Km for the reaction and was pH-dependent, with its affinity increasing from 1 mM at pH 6 to 70 microM at pH 8.5. The affinity of ALA at the P-site of Zn alpha An beta ALAD is proposed to limit the Km at pH values above 7, since the measured Kd for ALA at the A-site in 45 microM Tris, pH 8, was well below the observed Km (600 microM) under the same conditions. The amino group of the ALA molecule bound at the P-site was identified as a critical binding component for the A-site, explaining why ALA binding to ALAD is ordered, with the P-site ALA binding first. Structural requirements for ALA binding at the A- and P-sites have been identified: the P-site requires the carbonyl and carboxylate groups, whereas the A-site requires the amino, carbonyl and carboxylate groups of the substrate.

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.

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.

Are free radicals involved in lead poisoning?

1. The enolamine form of 5-aminolaevulinic acid (ALA), a haem precursor that accumulates in lead poisoning and in acute intermittent porphyria (AIP), undergoes fast autoxidation at slightly alkaline pH with concomitant generation of reactive oxygen species. 2. The transmembrane potential, Ca2+ ion fluxes and state-4 respiratory rate, of isolated rat liver mitochondria are severely affected by mM addition of ALA; the toxic role of ALA-produced oxygen radicals was demonstrated by use of appropriate scavengers. 3. Induction of superoxide dismutase biosynthesis in lead-exposed workers, in AIP carriers and in ALA-treated rats, is viewed as a protective response against oxygen radical toxicity. 4. 5-Aminolaevulinic acid-generated oxygen radicals, together with Pb-stimulated Fe-dependent lipid peroxidation, might be involved in the aetiology of the neuropsychiatric manifestations of both plumbism and acute intermittent porphyria.

delta-Aminolevulinate dehydratase deficient porphyria: identification of the molecular lesions in a severely affected homozygote

delta-Aminolevulinate dehydratase deficient porphyria, a recently recognized inborn error of heme biosynthesis, results from the markedly deficient activity of the heme biosynthetic enzyme, delta-aminolevulinate dehydratase (ALA-D). The four homozygotes described to date with this disorder have remarkably distinct phenotypes, ranging from a severely affected infant with failure to thrive to an essentially asymptomatic 68-year-old male. To investigate the molecular nature of the lesions causing the severe infantile-onset form, total RNA was isolated from cultured lymphoblasts of the affected homozygote, RNA was reverse-transcribed to cDNA, and the 990-bp ALA-D-coding region was amplified by the PCR. Heterozygosity for an RsaI RFLP within the ALA-dehydratase-coding region permitted identification of the paternal and maternal mutant alleles prior to sequencing. The maternal mutation (designated G133R), a G-to-A transition of nucleotide 397, predicted a glycine-to-arginine substitution at residue 133 at the carboxyl end of the highly conserved zinc-binding site in the enzyme subunit. The G133R mutation created a PstI site and permitted the confirmation and rapid detection of this lesion in amplified genomic DNA from maternal relatives. The paternal mutation, a G-to-A transition of nucleotide 823, predicted a valine-to-methionine substitution of residue 275 (designated V275M). This mutation was confirmed in genomic DNA from family members by the competitive PCR technique. Both missense mutations, which occurred at CpG dinucleotides, resulted in the synthesis of enzyme subunits such that the activity of the homooctameric enzyme was markedly reduced, thereby causing the severe infantile-onset phenotype in the affected homozygote.

The physiological role of zinc as an antioxidant

The purpose of this review is to consider whether an essential biochemical function of zinc (Zn) is to serve as an antioxidant. Zn has been shown to have an antioxidant role(s) in defined chemical systems. Two mechanisms have been elucidated; the protection of sulfhydryl groups against oxidation and the inhibition of the production of reactive oxygens by transition metals. Supraphysiological concentrations of Zn have antioxidant-like effects in organelle-based systems and isolated cell-based systems in vitro. Administration of pharmacological doses of Zn in vivo has a protective effect against general and liver-specific prooxidants. Dietary Zn deficiency causes increased susceptibility to oxidative damage in membrane fractions from some tissues suggesting that increased oxidative stress may be a small but significant component of the pathology observed in dietary Zn deficiency. However, the biochemical basis for Zn deficiency pathology remains unelucidated; critical antioxidant functions for Zn may still be uncovered.

Effect of some metal ions on blood and liver delta-aminolevulinate dehydratase of Pimelodus maculatus (Pisces, Pimelodidae)

1. delta-ALA-D from Pimelodus maculatus was inhibited in vitro by Cd2+ greater than Pb2+ greater than Hg2+ greater than Cu2+ greater than Zn2+ in blood and by Pb2+ greater than Cd2+ greater than Hg2+ greater than Cu2+ = Zn2+ in the liver. 2. Kinetic analysis of the inhibition by the metal ions showed that Cd2+ and Hg2+ act as non-competitive inhibitors on both sources. 3. Pb2+ showed a mixed type of inhibition in blood and a non-competitive type in the liver. 4. Zn2+ acted as a competitive or mixed inhibitor, on both sources, depending on concentration.

Mucopolysaccharidosis type I subtypes. Presence of immunologically cross-reactive material and in vitro enhancement of the residual alpha-L-iduronidase activities

The enzymatic and immunologic properties of the defective residual alpha-L-iduronidase activities were investigated in fibroblast extracts from the three subtypes of mucopolysaccharidosis type I, Hurler (MPS IH), Scheie (MPS IS), and Hurler-Scheie (MPS IH-S) diseases. Using 4-methylumbelliferyl-alpha-L-iduronide (4MU-alpha-Id), the activities in fibroblast extracts from all three subtypes were less than 0.1% of normal. Rocket immunoelectrophoresis with monospecific rabbit anti-human alpha-L-iduronidase polyclonal antibodies, as well as immunoblots using a monoclonal antibody, revealed the presence of cross-reactive immunologic material (CRIM) in extracts prepared from each subtype. When the samples were equalized for beta-hexosaminidase A activity, 38-105% of normal enzyme protein was detected. The sequential addition of cystamine, MgCl2 and pyridoxal phosphate increased the residual 4MU-alpha-Id activities in subtype extracts up to about 35% of normal mean fibroblast activity. Cystamine, MgCl2 or pyridoxal phosphate alone enhanced the residual activities two- to fourfold, whereas the sequential addition of all three compounds was required for maximal effect. Of the six B6 vitamers evaluated, only the negatively charged forms, pyridoxamine (PLN), pyridoxamine phosphate (PNP), and pyridoxal phosphate (PLP), stimulated the residual activities. The addition of dermatan sulfate or heparan sulfate to the subtype extracts, followed by treatment with the effector compounds, similarly inhibited both the normal and enhanced MPS I activities. Heat inactivation experiments confirmed the fact that the mutant iduronidase activity was reconstituted and that the observed increase in enzymatic activity was not an artifact of the fluorogenic assay. These results suggest that the presence of certain thiol reducing agents, divalent cations and negatively charged B6 vitamers can alter the conformation of the mutant alpha-L-iduronidase in vitro such that the hydrolysis of 4MU-alpha-Id is enhanced into the heterozygote range.

delta-Aminolevulinic acid dehydratase isozymes and lead toxicity

ALAD is a zinc metalloenzyme whose inhibition by lead is the first and most sensitive indicator of lead exposure and whose decreased activity has been implicated in the pathogenesis of lead poisoning. This heme biosynthetic enzyme is encoded by a gene located at chromosome 9q34, which has two codominant alleles, ALAD1 and ALAD2. The occurrence of two frequent alleles for ALAD stimulated an investigation into the possible pharmacogenetic role of the enzyme polymorphism in lead poisoning. In a New York City population at high risk for lead exposure, individuals heterozygous or homozygous for the less common allele, ALAD2, had blood lead levels greater than or equal to 30 micrograms/dl more frequently than expected. These findings suggest a potential genetic susceptibility to lead poisoning in individuals with the ALAD 1-2 and 2-2 phenotypes.

Human delta-aminolevulinate dehydratase: nucleotide sequence of a full-length cDNA clone

Two cDNAs encoding human delta-aminolevulinate dehydratase (ALA-D; porphobilinogen synthase; EC 4.2.1.24), the second enzyme in the heme biosynthetic pathway, were identified, recloned into bacteriophage M13, and sequenced by primer extension. The first clone with an 827-base-pair (bp) pEX-ALA-D cDNA insert, shown to contain DNA sequences that were colinear with four bovine ALA-D peptide sequences, was used to screen a pKT218 human liver library. A second clone containing a 1200-bp insert was identified that contained an open reading frame of 990 bp as well as 5' (66 bp)- and 3' (94 bp)-untranslated regions, the latter terminating in poly(dA). The predicted N-terminal amino acid sequence was colinear with the first 13 residues of microsequenced ALA-D purified from human erythrocytes. The ATG initiation codon was preceded by ACGCC, a functional initiation sequence, while an upstream (position -32), in-phase AACTG ATG sequence was entirely nonhomologous with the initiation consensus sequence and, therefore, presumed to be nonfunctional. The unusual polyadenylylation signal, AGTAAA, has been reported only in the human HRAS1 gene. The nucleotide sequences of the two cDNA clones differed at position 730 or 733 and encoded two differently charged amino acids. This nucleotide difference may be the basis for the polymorphic charge isozymes of human ALA-D. The sequence encoding this zinc metalloenzyme contained a cysteine- and histidine-rich binding site for zinc and an unusual region of charge complementarity surrounding the active lysine residue in the catalytic site.

Reduced synthesis of 5-aminolevulinate dehydratase in styrene-treated rats

5-Aminolevulinate dehydratase (porphobilinogen synthase; 5-aminolevulinate hydro-lyase, EC 4.2.1.24) preparations purified from rat liver and erythrocytes are indistinguishable in terms of molecular weight, subunit size, immunoreactivity, amino-acid composition and kinetic properties, suggesting that the enzyme from liver and erythrocytes are identical. Intraperitoneal injection of styrene to rats decreased 5-aminolevulinate dehydratase activity in both erythrocyte (to 8% of the control) and the liver (to 40% of the control). Studies utilizing polysome-directed cell-free translation indicated that hepatic synthesis of the enzyme was inhibited by styrene at the transcriptional level. In vitro addition of styrene 7,8-oxide, a major intermediate of styrene, to purified 5-aminolevulinate dehydratase resulted in a loss of immunoassayable enzyme protein to less than 1% of the untreated control. These findings suggest that the decrease in 5-aminolevulinate dehydratase caused by in vivo treatment of styrene is partially due to a transcription-dependent decrease in the enzyme synthesis, and partially to post-translational alteration of the structure of the enzyme protein.

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.

Molecular cloning of a cDNA for human delta-aminolevulinate dehydratase

A cDNA encoding human delta-aminolevulinic acid dehydratase (ALA-D; EC 4.2.1.24), the second enzyme in the heme biosynthetic pathway, was isolated from a human liver cDNA expression library. Of the original 17 clones selected with anti-ALA-D antibody, only four expressed anti-ALA-D epitopes as assessed by rescreening with antibody preabsorbed with purified antigen. Subsequent screening of the antibody-positive clones with mixed oligodeoxynucleotide (oligo) probes, synthesized to correspond to human N-terminal and bovine active-site peptide sequences, identified three clones which hybridized only with the oligo probes for the bovine amino acid (aa) sequences. Restriction endonucleases analysis revealed that these three clones contained the same 800-bp cDNA insert. This insert was recloned into bacteriophage M13mp18 and mp19 and sequenced by primer extension. The aa sequence predicted from the partial nucleotide sequence was found to be essentially colinear with the sequences of four bovine ALA-D peptides, totaling 35 non-overlapping aa residues.

Purification and properties of 5-aminolaevulinate dehydratase from human erythrocytes

A new procedure for the isolation of homogeneous human 5-aminolaevulinate dehydratase (porphobilinogen synthase, EC 4.2.1.24) is described in which the enzyme is purified 35000-fold and in 65-74% yield. The specific activity of the purified enzyme, 24 units/mg, is the highest yet reported. An efficient stage for the removal of haemoglobin is incorporated in the method, which has general application to the purification of other erythrocyte enzymes. The erythrocyte dehydratase (Mr 285 000) is made up of eight apparently identical subunits of Mr 35 000. The enzyme is sensitive to oxygen, and its activity is maintained by the presence of thiols such as dithioerythritol. Zn2+ is obligatory for enzyme activity, the apoenzyme being essentially inactive (approximately equal to 12% of control) when assayed in buffers devoid of Zn2+. Addition of Zn2+ to the apoenzyme restores activity as long as the sensitive thiol groups are fully reduced; optimal stimulation occurs between 100 and 300 microM-Zn2+. The human enzyme is inhibited by Pb2+ in a non-competitive fashion [KiI (dissociation constant for E X S X Pb2+ complex) = 25.3 +/- 3.0 microM; KiS (dissociation constant for E X Pb2+ complex) = 9.0 +/- 2.0 microM]. Modification of thiol groups, inactivation by oxidation, alkylation or reaction with thiophilic reagents demonstrates the importance of sensitive thiol groups for full enzymic activity.

Mechanism of action of 5-aminolaevulinate dehydratase from human erythrocytes

Purified 5-aminolaevulinate dehydratase (porphobilinogen synthase, EC 4.2.1.24) from human erythrocytes was incubated initially with limiting amounts of 5-amino [5-14C]laevulinate in a rapid-mixing apparatus. The single-turnover reaction with respect to the bound labelled 5-aminolaevulinate was completed by the addition of unlabelled 5-aminolaevulinate and the resulting radioactive porphobilinogen was isolated and degraded. The 14C label was found to be located predominantly at C-2 of the product, demonstrating that, of the two substrate molecules participating in the reaction, the 5-aminolaevulinate molecule initially bound to the enzyme provides the propionic acid 'side' of the porphobilinogen. The same enzyme-[14C]substrate species that yields regiospecific porphobilinogen may be trapped by reaction with NaBH4, showing that the substrate molecule initially bound to the enzyme does so in the form of a Schiff base. A conventional incubation with 5-amino[5-14C]laevulinate yielded porphobilinogen with an equal distribution of the label between C-2 and C-11. The reaction mechanism of the human erythrocyte 5-aminolaevulinate dehydratase thus follows the same course as that of other dehydratases studied in our laboratory by using single-turnover techniques.