Purification of delta-aminolevulinate dehydratase from genetically engineered yeast

Genetic susceptibility of δ-ALAD associated with lead (Pb) intoxication: sources of exposure, preventive measures, and treatment interventions

Delta-aminolevulinic acid dehydratase (δ-ALAD) is involved in the synthesis of haem and exhibits a polymorphic nature. δ-ALAD polymorphism produces two alleles, namely δ-ALAD-1 and δ-ALAD-2, which in turn produce three different phenotypes, namely δ-ALAD1-1, δ-ALAD1-2, and δ-ALAD2-2. δ-ALAD gene is more susceptible to lead (Pb) toxicity than any other genes. Its genotype and phenotype frequencies change with respect to different geographical areas and extent of Pb exposure. The δ-ALAD-2 allele dominancy is linked with high concentration of lead in the body. It has also been thought that the δ-ALAD-2 allele can provoke Pb toxicity by producing a protein that binds more tightly with Pb than δ-ALAD-1 protein. However, few evidences suggest that δ-ALAD-2 may reduce harmful effects by increasing excretion of Pb from the body, thus producing its unavailability towards pathophysiologic alterations. However, the recent evidences have supported that the individuals who are heterozygote for the δ-ALAD-1 allele may be associated with a higher risk of long-term Pb toxicity. In this regard, the individuals who are exposed at occupational levels are among the most frequent study population. The main objective of our study was to explore the gene susceptibility associated with Pb poisoning. Moreover, this study also summarizes various sources of Pb exposure and thereafter outlined multiple strategies to minimize the Pb toxicity in order to save the exposed residential communities.

5-Aminolaevulinic acid dehydratase: characterization of the alpha and beta metal-binding sites of the Escherichia coli enzyme

The alpha and beta metal-binding sites of 5-aminolaevulinic acid dehydratase (ALAD) (porphobilinogen synthase, EC 4.2.1.24) from Escherichia coli were investigated to determine the function of each metal ion and the role of the reactive cysteines in metal binding. Occupancy of the alpha site by Zn2+ restored virtually all catalytic activity to the inactive metal-depleted ALAD (apoALAD). Occupancy of the alpha site by Co2+ also yielded an active enzyme and resulted in a charge-transfer band indicative of a single cysteine amongst the metal ligands. Subsequent labelling of this cysteine residue with 14C-labelled N-ethylmaleimide, followed by peptide analysis, indicated the involvement of Cys-130. The metal ion at the alpha site is thought to be essential for binding of the second molecule of substrate at the A substrate-binding site that forms the acetic acid side of the product, porphobilinogen. Binding of Zn2+ to the beta site restored little activity if the alpha site was unfilled. Metal ion binding to the beta site could be monitored by following the change in protein fluorescence with Zn2+ titration of apoALAD at pH 6. A conformational change upon beta site occupancy may explain why binding of Mg2+ at the alpha site can occur only if Zn2+ is bound at the beta site. The binding of Co2+ at the beta site produced an inactive enzyme that exhibited a charge-transfer band indicative of at least three cysteine ligands.

Identification of rate-limiting steps in yeast heme biosynthesis

The heme biosynthesis pathway in the yeast Saccharomyces cerevisiae is a highly regulated system, but the mechanisms accounting for this regulation remain unknown. In an attempt to identify rate-limiting steps in heme synthesis, which may constitute potential regulatory points, we constructed yeast strains overproducing two enzymes of the pathway: the porphobilinogen synthase (PBG-S) and deaminase (PBG-D). Biochemical analysis of the enzyme-overproducing strains revealed intracellular porphobilinogen and porphyrin accumulation. These results indicate that both enzymes play a rate-limiting role in yeast heme biosynthesis.

Porphyrin biosynthesis intermediates are not regulating delta-aminolevulinic acid transport in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, as in all eukaryotic organisms, delta-aminolevulinic acid (ALA) is a precursor of porphyrin biosynthesis, a very finely regulated pathway. ALA enters yeast cells through the gamma-aminobutyric acid (GABA) permease Uga4. The incorporation of a metabolite into the cells may be a limiting step for its intracellular metabolization. To determine the relationship between ALA transport and ALA metabolization, ALA incorporation was measured in yeast mutant strains deficient in the delta-aminolevulinic acid-synthase, uroporphyrinogen III decarboxylase, and ferrochelatase, three enzymes involved in porphyrin biosynthesis. Results presented here showed that neither intracellular ALA nor uroporphyrin or protoporphyrin regulates ALA incorporation, indicating that ALA uptake and its subsequent metabolization are not related to each other. Thus a key metabolite as it is, ALA does not have a transport system regulated according to its role.

Lead poisoning, haem synthesis and 5-aminolaevulinic acid dehydratase

In mammals and yeast, 5-aminolaevulinic acid dehydratase is a zinc-dependent enzyme that catalyses the synthesis of porphobilinogen-the pyrrole building block that is incorporated into all modified tetrapyrroles, including haem, chlorophyll and vitamin B12. The X-ray structure of this enzyme reveals how substitution of the catalytically important zinc ion by lead inactivates the enzyme and causes a form of pseudo-porphyria.

A role for HEM2 in cadmium tolerance

A Candida glabrata cadmium-sensitive mutant partially defective in glutathione production and exhibiting a complete absence of phytochelatins was used to clone a gene required for Cd tolerance. Transformation of the Cd-sensitive mutant with a genomic library from the wild-type C. glabrata led to the cloning of a gene that restored Cd tolerance and formation of Cd-glutathione and Cd-phytochelatin complexes. The cloned gene showed high levels of nucleic acid and protein sequence homology to the HEM2 genes, encoding porphobilinogen synthases, from several sources. It was shown that the C, glabrata Cd-sensitive mutant indeed exhibited a significant reduction in porphobilinogen synthase levels. The cloned C. glabrata gene complemented a hem2 mutant of Saccharomyces cerevisiae and restored porphobilinogen synthase activity in the mutant. The Cd sensitive mutant predictably showed decreased levels of sulfite reductase that requires siroheme, a metabolite produced in the heme biosynthetic pathway. The addition of cysteine, but not methionine, increased glutathione levels and Cd tolerance of both the wild-type and the mutant strain. However, addition of hemin chloride and methionine together restored Cd tolerance indicating that heme was required for transsulfuration of homocysteine to cysteine.

Purification, metal cofactor, N-terminal sequence and subunit composition of a 5-aminolevulinic acid dehydratase from the unicellular green alga Scenedesmus obliquus, mutant C-2A'

5-Aminolevulinic acid dehydratase was purified to apparent homogeneity from Scenedesmus obliquus, mutant C-2A', starting with serial affinity chromatography according to Wang et al., followed by separation on DEAE-Cellulose DE 52, TSKgel Toyopearl HW-55 and FPLC on Mono Q. The enzyme was purified 117-fold compared with the initial crude soluble enzyme preparation and showed a final specific activity of 9.17 microkat/kg protein at pH 8.2 at a total recovery of 7%. Mg2+ was determined to be the metal cofactor of the enzyme. It can, to a certain extent, be substituted by other divalent cations. From the purified enzyme the first 15 amino acids of the N-terminus could be determined, showing a moderate similarity to 5-aminolevulinic acid dehydratases from spinach, pea, Escherichia coli and yeast. The molecular mass of the native protein was determined by gel filtration to be 282+/-5 kDa. 42+/-1 kDa were ascertained for the subunit size by SDS/PAGE. These investigations, supported by electron microscopy, revealed that the enzyme from Scenedesmus consists of six subunits arranged in a six-membered ring. Additionally, there is some evidence that two of the rings form a sandwich-like complex.

A mutant Bradyrhizobium japonicum delta-aminolevulinic acid dehydratase with an altered metal requirement functions in situ for tetrapyrrole synthesis in soybean root nodules

The tetrapyrrole synthesis enzyme delta-aminolevulinic acid (ALA) dehydratase requires Mg2+ for catalytic activity in photosynthetic organisms and in Bradyrhizobium japonicum, a bacterium that can reside symbiotically within plant cells of soybean root nodules or as a free-living organism. ALA dehydratase from animals and other non-photosynthetic organisms is a Zn(2+)-dependent enzyme. A modified B. japonicum ALA dehydratase, ALAD*, was constructed by site-directed mutagenesis of hemB in which three proximal amino acids conserved in plant dehydratases were changed to cysteine residues as is found in the Zn(2+)-dependent enzyme of animals. These substitutions resulted in an enzyme that required Zn2+ rather than Mg2+ for catalytic activity, and therefore a region of the ALA dehydratase from B. japonicum, and probably from plants, was identified that is involved in Mg2+ dependence. In addition, the data show that a change in only a few residues is sufficient to change a Mg(2+)-dependent ALA dehydratase to a Zn(2+)-dependent one. B. japonicum strains were constructed that contained a single copy of either hemB or the altered gene hemB* integrated into the genome of a hemB- mutant. Cultures of the hemB* strain KPZn3 had Zn(2+)-dependent ALA dehydratase activity that functioned in vivo as discerned by its heme prototrophy and expression of wild type levels of cellular hemes. Strain KPZn3 elicited root nodules on soybean that contained viable bacteria and exhibited traits of normally developed nodules, and the symbiotic bacteria expressed nearly wild type levels of cellular hemes. We conclude that the Zn(2+)-dependent ALAD* can function and support bacterial tetrapyrrole synthesis within the plant milieu of root nodules.

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.

The role of ALA-S and ALA-D in regulating porphyrin biosynthesis in a normal and a HEM R+ mutant strain of Saccharomyces cerevisiae

Catabolite repression and derepression on delta-aminolevulinate synthase (ALA-S) and delta-aminolevulinate dehydratase (ALA-D) in a normal yeast strain, D27, and its derived D27/C6 (HEM R+) were investigated. ALA-S and ALA-D activities and intracellular ALA (I-ALA) at different physiological states of the cells were measured. In YPD medium, under conditions of repression and when glucose was exhausted, both strains behaved identically as if the mutation was not expressed. In YPEt medium, however, both ALA-S and ALA-D activities were higher than in YPD, but the I-ALA content and the enzymic activity profiles shown by the two strains were quite different. It appears, therefore, that the mutation causes a deregulation of ALA-S, so that its activity is kept at a high level throughout the cell cycle. This would explain the increased levels of cytochromes present in the mutant. This mutation may affect some regulatory aspect of ALA formation and renders an ALA-S of high activity; moreover, this enzyme species seems to be more stable than in the normal strain.