Mechanism of porphobilinogen synthase. Requirement of Zn2+ for enzyme activity

Heme biosynthesis depends on previously unrecognized acquisition of iron-sulfur cofactors in human amino-levulinic acid dehydratase

Heme biosynthesis and iron-sulfur cluster (ISC) biogenesis are two major mammalian metabolic pathways that require iron. It has long been known that these two pathways interconnect, but the previously described interactions do not fully explain why heme biosynthesis depends on intact ISC biogenesis. Herein we identify a previously unrecognized connection between these two pathways through our discovery that human aminolevulinic acid dehydratase (ALAD), which catalyzes the second step of heme biosynthesis, is an Fe-S protein. We find that several highly conserved cysteines and an Ala306-Phe307-Arg308 motif of human ALAD are important for [Fe₄S₄] cluster acquisition and coordination. The enzymatic activity of human ALAD is greatly reduced upon loss of its Fe-S cluster, which results in reduced heme biosynthesis in human cells. As ALAD provides an early Fe-S-dependent checkpoint in the heme biosynthetic pathway, our findings help explain why heme biosynthesis depends on intact ISC biogenesis.

Heme biosynthesis depends on iron-sulfur (Fe-S) cluster biogenesis but the molecular connection between these pathways is not fully understood. Here, the authors show that the heme biosynthesis enzyme ALAD contains an Fe-S cluster, disruption of which reduces ALAD activity and heme production in human cells.

Reversing the adverse biochemical effects in lead-intoxicated rats by N,N`- bis[(1,2-didehydro-1-hydroxy-2-thioxopyrid-4-yl)-carbonyl]- L-lysine

N,N`-Bis[(1,2-didehydro-1-hydroxy-2-thioxopyrid-4-yl)-carbonyl]- L-lysine (HTPL) is a novel newly synthesized compound intended to be used for the chelation of lead in intoxicated animals. Subchronic lead intoxication experiments were carried out on Wistar male rats; these rats were intoxicated with lead and then treated with HTPL. Results were compared with those obtained with known compounds used for lead chelation therapy, such as disodium ethylnediaminetetraacetic acid (CaNa₂EDTA) and meso-2,3-dimercaptosuccininc acid (DMSA), using different routes of administration. Biological samples of whole blood and urine were collected and analyzed for urinary proporphyrins, δ-aminolevulinic acid dehydratase, and zinc protoporphyrin. Results revealed that HTPL can remarkably reverse the toxic effects of lead intoxication at biochemical levels. Additionally, results showed that this agent is as good or even more potent than calcium disodium ethylnediaminetetraacetic acid (CaNa₂EDTA) and meso-2,3-dimercaptosuccininc acid (DMSA) in reversing the toxic effect of lead. More importantly, HTPL was found effective when administrated intraperitoneally and orally.

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.

The Remarkable Character of Porphobilinogen Synthase

Porphobilinogen synthase (PBGS), also known as 5-aminolevulinate dehydratase, is an essential enzyme in the biosynthesis of all tetrapyrroles, which function in respiration, photosynthesis, and methanogenesis. Throughout evolution, PBGS adapted to a diversity of cellular niches and evolved to use an unusual variety of metal ions both for catalytic function and to control protein multimerization. With regard to the active site, some PBGSs require Zn²⁺; a subset of those, including human PBGS, contain a constellation of cysteine residues that acts as a sink for the environmental toxin Pb²⁺. PBGSs that do not require the soft metal ion Zn²⁺ at the active site instead are suspected of using the hard metal Mg²⁺. The most unexpected property of the PBGS family of enzymes is a dissociative allosteric mechanism that utilizes an equilibrium of architecturally and functionally distinct protein assemblies. The high-activity assembly is an octamer in which intersubunit interactions modulate active-site lid motion. This octamer can dissociate to dimer, the dimer can undergo a hinge twist, and the twisted dimer can assemble to a low-activity hexamer. The hexamer does not have the intersubunit interactions required to stabilize a closed conformation of the active site lid. PBGS active site chemistry benefits from a closed lid because porphobilinogen biosynthesis includes Schiff base formation, which requires deprotonated lysine amino groups. N-terminal and C-terminal sequence extensions dictate whether a specific species of PBGS can sample the hexameric assembly. The bulk of species (nearly all except animals and yeasts) use Mg²⁺ as an allosteric activator. Mg²⁺ functions allosterically by binding to an intersubunit interface that is present in the octamer but absent in the hexamer. This conformational selection allosteric mechanism is purported to be essential to avoid the untimely accumulation of phototoxic chlorophyll precursors in plants. For those PBGSs that do not use the allosteric Mg²⁺, there is a spatially equivalent arginine-derived guanidium group. Deprotonation of this residue promotes formation of the hexamer and accounts for the basic arm of the bell-shaped pH vs activity profile of human PBGS. A human inborn error of metabolism known as ALAD porphyria is attributed to PBGS variants that favor the hexameric assembly. The existence of one such variant, F12L, which dramatically stabilizes the human PBGS hexamer, allowed crystal structure determination for the hexamer. Without this crystal structure and octameric PBGS structures containing the allosteric Mg²⁺, it would have been difficult to decipher the structural basis for PBGS allostery. The requirement for multimer dissociation as an intermediate step in PBGS allostery was established by monitoring subunit disproportionation during the turnover-dependent transition of heteromeric PBGS (comprised of human wild type and F12L) from hexamer to octamer. One outcome of these studies was the definition of the dissociative morpheein model of protein allostery. The phylogenetically variable time scales for PBGS multimer interconversion result in atypical kinetic and biophysical behaviors. These behaviors can serve to identify other proteins that use the morpheein model of protein allostery.

Anaemia, Serum Iron Concentrations and δ-Aminolevulinate Dehydratase Activity in Laying Hens Infected Naturally by Salmonella Gallinarum

The aim of this study was to evaluate anaemia, serum iron concentrations and δ-aminolevulinate dehydratase (ALA-D) activity in laying hens infected naturally by Salmonella Gallinarum and having severe hepatic lesions. Liver and serum samples were collected from 27 laying hens (20 infected and seven uninfected). The δ-ALA-D activity, haematocrit and serum iron concentrations were evaluated. There were significant decreases in δ-ALA-D activity, haematocrit and serum iron concentrations (P <0.01) in birds infected by S. Gallinarum when compared with uninfected birds. There was a positive correlation (P <0.001) between serum iron concentration, haematocrit (r(2) = 0.82) and δ-ALA-D activity (r(2) = 0.75). A positive correlation was also observed between δ-ALA-D activity and haematocrit (r(2) = 0.78; P <0.01). Liver samples showed moderate focal coagulative necrosis associated with infiltration of lymphoplasmacytic cells, macrophages and heterophils. The anaemia in the infected hens may be related to reduction in δ-ALA-D activity and serum iron concentrations, since both are important for haemopoiesis.

Distinct response of lactating and nonlactating rats exposed to inorganic mercury on hepatic δ-aminolevulinic acid dehydratase activity

This study investigated if lactating and nonlactating rats presented differences in relation to hepatic sensitivity to HgCl2 and the potential preventive role of ZnCl2. Lactating (days 3-12 of lactation) and nonlactating rats received 27 mg/kg ZnCl2 for five consecutive days and 5 mg/kg HgCl2 for five subsequent days. Lactating and nonlactating rats exposed to HgCl2 presented a decrease in food intake, a decrease in plasma alanine aminotransferase (ALT), and an increase in hepatic Hg levels when compared to the control group. Only lactating rats exposed to HgCl2 presented an increase in hepatic δ-aminolevulinic acid dehydratase activity. On the other hand, only nonlactating rats exposed to HgCl2 presented an increase in plasma aspartate aminotransferase (AST). ZnCl2 pre-exposure partially protected the increase in plasma AST activity presented by nonlactating rats and potentiated the liver Hg accumulation in lactating rats. Pups from the Sal-Hg and Zn-Hg groups showed a decrease in absolute liver weight and an increase in liver Hg levels. Summarizing, this study demonstrated that lactating rats presented distinct biochemical responses compared to nonlactating rats exposed to HgCl2 when hepatic parameters were evaluated.

Syzygium jambos and Solanum guaraniticum show similar antioxidant properties but induce different enzymatic activities in the brain of rats

Syzygium jambos and Solanum guaraniticum are both employed in Brazil as medicinal plants, even though their potential toxicity is not well established and they are frequently misused. The aim of this study was investigate the effect of the aqueous leaf extracts of both plants on δ-aminolevulinate dehydratase (δ-ALA-D) and acetylcholinesterase (AChE) activities and the antioxidant action against oxidative damage induced by sodium nitroprusside in rats, using in vitro assays. In addition, the presence of gallic, caffeic and chlorogenic acids, as well as rutin, quercetin and kaempferol as bioactive compounds in the extracts was identified by HPLC and their levels quantified. The antioxidant activities of both extracts were assessed by their capabilities to scavenge nitric oxide and to inhibit lipid peroxidation. Only Syzygium jambos presented thiol-peroxidase-like activity. Although neither extract affected the AChE activity, the aqueous extract of Solanum guaraniticum inhibited brain δ-ALA-D activity, suggesting a possible impairment effect on the central nervous system. Our results showed that both extracts exhibited efficient free radical scavenger activity and are an interesting source of bioactive compounds, justifying their use in folk medicine, although Solanum guaraniticum extract could have neurotoxicity properties and we therefore suggest that its use should be restricted to ensure the health of the population.

Catalytic mechanism of porphobilinogen synthase: the chemical step revisited by QM/MM calculations

Porphobilinogen synthase (PBGS) catalyzes the asymmetric condensation and cyclization of two 5-aminolevulinic acid (5-ALA) substrate molecules to give porphobilinogen (PBG). The chemical step of PBGS is herein revisited using QM/MM (ONIOM) calculations. Two different protonation states and several different mechanisms are considered. Previous mechanisms based on DFT-only calculations are shown unlikely to occur. According to these new calculations, the deprotonation step rather than ring closure is rate-limiting. Both the C-C bond formation first mechanism and the C-N bond formation first mechanism are possible, depending on how the A-site ALA binds to the enzyme. We furthermore propose that future work should focus on the substrate binding step rather than the enzymatic mechanism.

Allostery and the dynamic oligomerization of porphobilinogen synthase

The structural basis for allosteric regulation of porphobilinogen synthase (PBGS) is modulation of a quaternary structure equilibrium between octamer and hexamer (via dimers), which is represented schematically as 8mer ⇔ 2mer ⇔ 2mer∗⇔ 6mer∗. The "∗" represents a reorientation between two domains of each subunit that occurs in the dissociated state because it is sterically forbidden in the larger multimers. Allosteric effectors of PBGS are both intrinsic and extrinsic and are phylogenetically variable. In some species this equilibrium is modulated intrinsically by magnesium which binds at a site specific to the 8mer. In other species this equilibrium is modulated intrinsically by pH with the guanidinium group of an arginine being spatially equivalent to the allosteric magnesium ion. In humans, disease associated variants all shift the equilibrium toward the 6mer∗ relative to wild type. The 6mer∗ has a surface cavity that is not present in the 8mer and is proposed as a small molecule allosteric binding site. In silico and in vitro approaches have revealed species-specific allosteric PBGS inhibitors that stabilize the 6mer∗. Some of these inhibitors are drugs in clinical use leading to the hypothesis that extrinsic allosteric inhibition of human PBGS could be a mechanism for drug side effects.

Delta-aminolevulinate dehydratase activity in red blood cells of rats infected with Trypanosoma evansi

The aim of this study was to evaluate the activity of delta-aminolevulinate dehydratase (δ-ALA-D) in red blood cells of rats infected with Trypanosoma evansi and establish its association with haematocrit, serum levels of iron and zinc and lipid peroxidation. Thirty-six male rats (Wistar) were divided into 2 groups with 18 animals each. Group A was non-infected while Group B was intraperitoneally infected, receiving 7·5×106 trypomastigotes per animal. Each group was divided into 3 subgroups of 6 rats and blood was collected during different periods post-infection (p.i.) as follows: day 5 (A1 and B1), day 15 (A2 and B2) and day 30 PI (A3 and B3). Blood samples were collected by cardiac puncture to estimate red blood cell parameters (RBC), δ-ALA-D activity and serum levels of iron, zinc and thiobarbituric acid reactive substances (TBARS). Rats in group B showed a significant (P<0·05) reduction of RBC count, haemoglobin concentration and haematocrit at days 5 and 15 p.i. The activity of δ-ALA-D in blood was significantly (P<0·001) increased at days 15 and 30 p.i. δ-ALA-D activity in blood had a significant (P<0·05) negative correlation with haematocrit (r=−0·61) and haemoglobin (r=−0·70) at day 15 p.i. There was a significant (P<0·05) decrease in serum iron and zinc levels and an increase in TBARS levels (P<0·05) during infection. The δ-ALA-D activity in blood was negatively correlated with the levels of iron (r=−0·68) and zinc (r=−0·57) on day 30 p.i. It was concluded that the increased activity of δ-ALA-D in blood might have occurred in response to the anaemia in remission as heme synthesis was enhanced.

Delayed biochemical changes induced by mercury intoxication are prevented by zinc pre-exposure

This work evaluated the delayed effects of mercury and the effectiveness of zinc in preventing such effects. Pups were pre-treated with 1 daily dose of ZnCl(2) (27 mg/kg/day, by subcutaneous injections) from 3rd to 7th postnatal day and received 1 daily dose of 5 mg/kg of HgCl(2), for 5 subsequent days (8-12 days old). Animals were euthanized 21 days after the end of Hg-exposure. Porphobilinogen-synthase activity as well as zinc and mercury contents was determined in the liver and kidneys. Alanine aminotransferase, aspartate aminotransferase and lactic dehydrogenase activities as well as urea, creatinine and glucose levels were analyzed in plasma or serum. Some animals were considered more sensitive to mercury, since they did not recover the body weight gain and presented an increase of renal and hepatic mercury content, urea and creatinine levels; a decrease in renal porphobilinogen-synthase and alanine aminotransferase activities, as well as a decrease in the liver and an increase in kidney weights. Some animals were considered less sensitive to mercury because they recovered the body weight and presented no biochemical alterations in spite of mercury in the tissues. Zinc prevents partially or totally the alterations caused by mercury even those that persisted for a long time after the end of exposure. These findings suggest that there is difference among the animals regarding the sensitivity to mercury.

Effects of co-expression of molecular chaperones on heterologous soluble expression of the cold-active lipase Lip-948

The cold-active lipase gene Lip-948, cloned from Antarctic psychrotrophic bacterium Psychrobacter sp. G, was ligated into plasmid pColdI. The recombinant plasmid pColdI+Lip-948 was then transformed into Escherichia coli BL21. SDS-PAGE analysis showed that there was substantive expression of lipase LIP-948 in E. coli with a yield of about 39% of total protein, most of which was present in the inclusion body. The soluble protein LIP-948 only consisted of 1.7% of total LIP-948 with a specific activity of 66.51U/mg. Co-expression of molecular chaperones with the pColdI+Lip-948 were also carried out. The results showed that co-expression of different chaperones led to an increase or decrease in the formation of soluble LIP-948 in varying degrees. Co-expression of pColdI+Lip-948 with chaperone pTf16 and pGro7 decreased the amount of soluble LIP-948, while the soluble expression was enhanced when pColdI+Lip-948 was co-expressed with "chaperone team" plasmids (pKJE7, pG-Tf2, pG-KJE8), respectively. LIP-948 was most efficiently expressed in soluble form when it was co-expressed with pG-KJE8, which was up to 19.8% of intracellular soluble proteins and with a specific activity of 108.77U/mg. The soluble LIP-948 was purified with amylase affinity chromatography and its enzymatic characters were studied. The optimal temperature and pH of LIP-948 was 35°C and 8, respectively. The activity of LIP-948 dropped dramatically after incubation at 50°C for 15min and was enhanced by Sr(2+), Ca(2+). It preferentially hydrolyzed 4-nitrophenyl esters with the shorter carbon chain.

Redox and metal-regulated oligomeric state for human porphobilinogen synthase activation

The oligomeric state of human porphobilinogen synthase (PBGS) [EC.4.2.1.24] is homooctamer, which consists of conformationally heterogenous subunits in the tertiary structure under air-saturated conditions. When PBGS is activated by reducing agent with zinc ion, a reservoir zinc ion coordinated by Cys(223) is transferred in the active center to be coordinated by Cys(122), Cys(124), and Cys(132) (Sawada et al. in J Biol Inorg Chem 10:199-207, 2005). The latter zinc ion serves as an electrophilic catalysis. In this study, we investigated a conformational change associated with the PBGS activation by reducing agent and zinc ion using analytical ultracentrifugation, negative staining electron microscopy, native PAGE, and enzyme activity staining. The results are in good agreement with our notion that the main component of PBGS is octamer with a few percent of hexamer and that the octamer changes spatial subunit arrangement upon reduction and further addition of zinc ion, accompanying decrease in f/f (0). It is concluded that redox-regulated PBGS activation via cleavage of disulfide bonds among Cys(122), Cys(124), and Cys(132) and coordination with zinc ion is closely linked to change in the oligomeric state.

Comparison of ALAD activities of Citrobacter and Pseudomonas strains and their usage as biomarker for Pb contamination

Delta-aminolevulinate dehydratase (ALAD) activity has been used in prokaryotes and eukaryotes as a biomarker for environmental lead (Pb) exposure and toxicity. Microorganisms are sensitive indicators of toxicity at the fundamental level of ecological organization, but bacterial biomarker studies are focused on the Pseudomonas strains in Group I and E coli. The objectives of the present work were to determine if Burkholderia gladioli belonging to group II, due to its 16SrRNA similarity, can be used as biomarker in metal contamination and compare its possible usage with Pseudomonas aeruginosa and Citrobacter freundii (previously known as Esherichia freundii) and Bacterium freundii which are classified in Group I. In this study, ALAD activity in an environmental strains of Burkholderia gladioli, Pseudomonas aeruginosa, Citrobacter freundii were investigated to evaluate potential inhibition by Pb and other toxic metals. When the ALAD activity of Burkholderia gladioli was tested, Co and Pb decreased activity by 27 and 71%, respectively. In addition to these findings, Zn increased the activity up to 26%. These effects were found to be statistically meaningful (p < 0.05). It was determined that the increase of lead concentration inhibites the ALAD activity at each of the three strains. There was a statistically significant dose-response relationship between ALAD activity in cells of Burkholderia gladioli and Pb (Pearson correlation coefficent = -0.665; r(2) = 0.665, and p < 0.001). The strongest ALAD inhibition which was measured was 90% at Burkholderia gladioli when protein extracts were incubated with 750 muM of Pb. The relationship between Pb and ALAD activity was statistically described by [ALAD Activity] = 0.476-0.000597 x [Pb]. According to the obtained results, we suggest that the ALAD of Burkholderia gladioli can be used as a biomarker for lead contamination in the environment.

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.

Biosynthesis of heme in mammals

Most iron in mammalian systems is routed to mitochondria to serve as a substrate for ferrochelatase. Ferrochelatase inserts iron into protoporphyrin IX to form heme which is incorporated into hemoglobin and cytochromes, the dominant hemoproteins in mammals. Tissue-specific regulatory features characterize the heme biosynthetic pathway. In erythroid cells, regulation is mediated by erythroid-specific transcription factors and the availability of iron as Fe/S clusters. In non-erythroid cells the pathway is regulated by heme-mediated feedback inhibition. All of the enzymes in the heme biosynthetic pathway have been crystallized and the crystal structures have permitted detailed analyses of enzyme mechanisms. All of the genes encoding the heme biosynthetic enzymes have been cloned and mutations of these genes are responsible for a group of human disorders designated the porphyrias and for X-linked sideroblastic anemia. The biochemistry, structural biology and the mechanisms of tissue-specific regulation are presented in this review along with the key features of the porphyric disorders.

The activation mechanism of human porphobilinogen synthase by 2-mercaptoethanol: intrasubunit transfer of a reserve zinc ion and coordination with three cysteines in the active center

Human porphobilinogen synthase [EC.4.2.1.24] is a homo-octamer enzyme. In the active center of each subunit, four cysteines are titrated with 5,5'-dithiobis(2-nitrobenzoic acid). Cys(122), Cys(124) and Cys(132) are placed near two catalytic sites, Lys(199) and Lys(252), and coordinate a zinc ion, referred to as "a proximal zinc ion", and Cys(223) is placed at the orifice of the catalytic cavity and coordinates a zinc ion, referred to as "a distal zinc ion", with His(131) . When the wild-type enzymes C122A (Cys(122)-->Ala), C124A (Cys(124)-->Ala), C132A (Cys(132)-->Ala) and C223A (Cys(223)-->Ala) were oxidized by hydrogen peroxide, the levels of activity were decreased. Two cysteines were titrated with 5,5'-dithiobis(2-nitrobenzoic acid) in the wild-type enzyme, while on the other hand, one cysteine was titrated in the mutant enzymes. When wild-type and mutant enzymes were reduced by 2-mercaptoethanol, the levels of activity were increased: four and three cysteines were titrated, respectively, suggesting that a disulfide bond was formed among Cys(122), Cys(124) and Cys(132) under oxidizing conditions. We analyzed the enzyme-bound zinc ion of these enzymes using inductively coupled plasma mass spectrometry with gel-filtration chromatography. The results for C223A showed that the number of proximal zinc ions correlated to the level of enzymatic activity. Furthermore, zinc-ion-free 2-mercaptoethanol increased the activity of the wild-type enzyme without a change in the total number of zinc ions, but C223A was not activated. These findings suggest that a distal zinc ion moved to the proximal binding site when a disulfide bond among Cys(122), Cys(124) and Cys(132) was reduced by reductants. Thus, in the catalytic functioning of the enzyme, the distal zinc ion does not directly contribute but serves rather as a reserve as the next proximal one that catalyzes the enzyme reaction. A redox change of the three cysteines in the active center accommodates the catch and release of the reserve distal zinc ion placed at the orifice of the catalytic cavity.

The porphobilinogen synthase catalyzed reaction mechanism

Porphobilinogen synthase (PBGS) catalyzes the first common reaction in the biosynthesis of the tetrapyrroles, the asymmetric condensation of two molecules of delta-aminolevulinic acid to form porphobilinogen. There is a variable requirement for an essential active site zinc that necessitates consideration of PBGS as an enzyme that may exhibit phylogenetic diversity in its chemical reaction mechanism. Recent crystal structures suggest reaction mechanisms that involve two covalent Schiff base linkages between adjacent active site lysine residues and each of the two substrate molecules. The reaction appears to stall at a covalently bound almost-product intermediate that is poised for breakdown to product upon binding of a substrate molecule to an adjacent active site and a subsequent conformational change.

Delta-aminolevulinic acid dehydratase from Plasmodium falciparum: indigenous versus imported

The heme biosynthetic pathway of the malaria parasite is a drug target and the import of host delta-aminolevulinate dehydratase (ALAD), the second enzyme of the pathway, from the red cell cytoplasm by the intra erythrocytic malaria parasite has been demonstrated earlier in this laboratory. In this study, ALAD encoded by the Plasmodium falciparum genome (PfALAD) has been cloned, the protein overexpressed in Escherichia coli, and then characterized. The mature recombinant enzyme (rPfALAD) is enzymatically active and behaves as an octamer with a subunit Mr of 46,000. The enzyme has an alkaline pH optimum of 8.0 to 9.0. rPfALAD does not require any metal ion for activity, although it is stimulated by 20-30% upon addition of Mg2+. The enzyme is inhibited by Zn2+ and succinylacetone. The presence of PfALAD in P. falciparum can be demonstrated by Western blot analysis and immunoelectron microscopy. The enzyme has been localized to the apicoplast of the malaria parasite. Homology modeling studies reveal that PfALAD is very similar to the enzyme species from Pseudomonas aeruginosa, but manifests features that are unique and different from plant ALADs as well as from those of the bacterium. It is concluded that PfALAD, while resembling plant ALADs in terms of its alkaline pH optimum and apicoplast localization, differs in its Mg2+ independence for catalytic activity or octamer stabilization. Expression levels of PfALAD in P. falciparum, based on Western blot analysis, immunoelectron microscopy, and EDTA-resistant enzyme activity assay reveals that it may account for about 10% of the total ALAD activity in the parasite, the rest being accounted for by the host enzyme imported by the parasite. It is proposed that the role of PfALAD may be confined to heme synthesis in the apicoplast that may not account for the total de novo heme biosynthesis in the parasite.

A structural basis for half-of-the-sites metal binding revealed in Drosophila melanogaster porphobilinogen synthase

Porphobilinogen synthase (PBGS) proteins fall into several distinct groups with different metal ion requirements. Drosophila melanogaster porphobilinogen synthase (DmPBGS) is the first non-mammalian metazoan PBGS to be characterized. The sequence shows the determinants for two zinc binding sites known to be present in both mammalian and yeast PBGS, proteins that differ in the exhibition of half-of-the-sites metal binding. The pH-dependent activity of DmPBGS is uniquely affected by zinc. A tight binding catalytic zinc binds at 0.5/subunit with a Kd well below microm. A second inhibitory zinc exhibits a Kd of approximately 5 microm and appears to bind at a stoichiometry of 1/subunit. A molecular model of DmPBGS suggests that the inhibitory zinc is located at a subunit interface using Cys-219 and His-10 as ligands. Zinc binding to this previously unknown inhibitory site is proposed to inhibit opening of the active site lid. As predicted, the DmPBGS mutant H10F is active but is not inhibited by zinc. H10F binds a catalytic zinc at 0.5/subunit and binds a second nonessential and noninhibitory zinc at 0.5/subunit. This result reveals a structural basis for half-of-the-sites metal binding that is consistent with a reciprocating motion model for function of oligomeric PBGS.

Reaction of diphenyl diselenide with hydrogen peroxide and inhibition of delta-aminolevulinate dehydratase from rat liver and cucumber leaves

The interaction of the product of H2O2 and (PhSe)2 with delta-aminolevulinate dehydratase (delta-ALA-D) from mammals and plants was investigated. (PhSe)2 inhibited rat hepatic delta-ALA-D with an IC50 of 10 microM but not the enzyme from cucumber leaves. The reaction of (PhSe)2 with H2O2 for 1 h increased the inhibitory potency of the original compound and the IC50 for animal delta-ALA-D inhibition was decreased from 10 to 2 microM. Delta-ALA-D from cucumber leaves was also inhibited by the products of reaction of (PhSe)2 with H2O2 with an IC50 of 4 microM. The major product of reaction of (PhSe)2 with H2O2 was identified as seleninic acid and produced an intermediate with a (lambda)max at 265 nm after reaction with t-BuSH. These results suggest that the interaction of (PhSe)2 with mammal delta-ALA-D requires the presence of cysteinyl residues in close proximity. Two cysteine residues in spatial proximity have been recently described for the mammalian enzyme. Analysis of the primary structure of plant delta-ALA-D did not reveal an analogous site. In contrast to (PhSe)2, seleninic acid, as a result of the higher electrophilic nature of its selenium atom, may react with additional cysteinyl residue(s) in mammalian delta-ALA-D and also with cysteinyl residues from cucumber leaves located at a site distinct from that found at the B and A sites in mammals. Although the interaction of organochalcogens with H2O2 may have some antioxidant properties, the formation of seleninic acid as a product of this reaction may increase the toxicity of organic chalcogens such as (PhSe)2.

5-Chlorolevulinate modification of porphobilinogen synthase identifies a potential role for the catalytic zinc

Porphobilinogen synthase (PBGS) is a Zn(II) metalloenzyme which catalyzes the asymmetric condensation of two molecules of 5-aminolevulinate (ALA). The nitrogen of the first substrate ends up in the pyrrole ring of product (P-side ALA); by contrast, the nitrogen of the second substrate molecule remains an amino group (A-side ALA). A reactive mimic of the substrate molecules, 5-chlorolevulinate (5-CLA), has been prepared and used as an active site directed irreversible inhibitor of PBGS. Native octameric PBGS binds eight substrate molecules and eight Zn(II) ions, with two types of sites for each ligand. As originally demonstrated by Seehra and Jordan [(1981) Eur. J. Biochem. 113, 435-446], 5-CLA inactivates the enzyme at the site where one of the two substrate molecules binds, and modification at four sites per octamer (one per active site) affords near-total inactivation. Here we report that 5-CLA-modified PBGS (5-CLA-PBGS) can bind up to four substrate molecules and four Zn(II) ions. Contrary to the conclusion of Seehra and Jordan, we find that the preferential site of 5-CLA inactivation is the A-side ALA binding site. On the basis of the dissociation constants, the metal ion binding sites lost upon 5-CLA modification are assigned to the four catalytic Zn(II) sites. 5-CLA-PBGS is shown to be modified at cysteine-223 on half of the subunits. We conclude that cysteine-223 is near the amino group of A-side ALA and propose that this cysteine is a ligand to the catalytic Zn(II). The vacant substrate binding site on 5-CLA-PBGS is that of P-side ALA. We have used 13C and 15N NMR to view [4-13C]ALA and [15N]ALA bound to 5-CLA-PBGS. The NMR results are nearly identical to those obtained previously for the enzyme-bound P-side Schiff base intermediate [Jaffe et al. (1990) Biochemistry 29, 8345-8350]. It appears that, in the absence of the catalytic Zn(II), 5-CLA-PBGS does not catalyze the condensation of the amino group of the P-side Schiff base intermediate with the C4 carbonyl derived from 5-CLA. On this basis we propose that Zn(II) plays an essential role in formation of the first bond between the two substrate molecules.

The molecular mechanism of lead inhibition of human porphobilinogen synthase

Human porphobilinogen synthase (PBGS) is a main target in lead poisoning. Human PBGS purifies with eight Zn(II) per homo-octamer; four ZnA have predominantly nonsulfur ligands, and four ZnB have predominantly sulfur ligands. Only four Zn(II) are required for activity. To better elucidate the roles of Zn(II) and Pb(II), we produced human PBGS mutants that are designed to lack either the ZnA or ZnB sites. These proteins, MinusZnA (H131A, C223A) and MinusZnB (C122A, C124A, C132A), each become purified with four Zn(II) per octamer, thus confirming an asymmetry in the human PBGS structure. MinusZnA is fully active, whereas MinusZnB is far less active, verifying an important catalytic role for ZnB and the removed cysteine residues. Kinetic properties of the mutants and wild type proteins are described. Comparison of Pb(II) inhibition of the mutants shows that ligands to both ZnA and ZnB interact with Pb(II). The ZnB ligands preferentially interact with Pb(II). At least one ZnA ligand is responsible for the slow tight binding behavior of Pb(II). The data support a novel model where a high affinity lead site is a hybrid of the ZnA and ZnB sites. We propose that the lone electron pair of Pb(II) precludes Pb(II) to function in PBGS catalysis.

Mechanistic implications of mutations to the active site lysine of porphobilinogen synthase

Porphobilinogen synthase (PBGS) is a homo-octameric protein that catalyzes the complex asymmetric condensation of two molecules of 5-aminolevulinic acid (ALA). The only characterized intermediate in the PBGS-catalyzed reaction is a Schiff base that forms between the first ALA that binds and a conserved lysine, which in Escherichia coli PBGS is Lys-246 and in human PBGS is Lys-252. In this study, E. coli PBGS mutants K246H, K246M, K246W, K246N, and K246G and human PBGS mutant K252G were characterized. Alterations to this lysine result in a disabled but not totally inactive protein suggesting an alternate mechanism in which proximity and orientation are major catalytic devices. (13)C NMR studies of [3,5-(13)C]porphobilinogen bound at the active sites of the E. coli PBGS and the mutants show only minor chemical shift differences, i.e. environmental alterations. Mammalian PBGS is established to have four functional active sites, whereas the crystal structure of E. coli PBGS shows eight spatially distinct and structurally equivalent subunits. Biochemical data for E. coli PBGS have been interpreted to support both four and eight active sites. A unifying hypothesis is that formation of the Schiff base between this lysine and ALA triggers a conformational change that results in asymmetry. Product binding studies with wild-type E. coli PBGS and K246G demonstrate that both bind porphobilinogen at four per octamer although the latter cannot form the Schiff base from substrate. Thus, formation of the lysine to ALA Schiff base is not required to initiate the asymmetry that results in half-site reactivity.

Prophylactic effect of melatonin on lead-induced inhibition of heme biosynthesis and deterioration of antioxidant systems in male rats

We studied the protective role of the pineal hormone melatonin on lead-induced suppression of the heme synthesis pathway as a consequence of reduced antioxidant systems in rat. We injected rats intramuscularly with lead acetate (10 mg/kg body weight) daily for 7 days, which significantly abolished heme synthesis as evidenced by decreased blood hemoglobin, liver delta-aminolevulinic acid synthetase, erythrocytic delta-aminolevulinic acid dehydratase, and hepatic iron content. These effects were accompanied with marked elevation of hepatic lipid peroxidation and decreased enzymatic antioxidants such as glutathione reductase, glutathione-S-transferase, superoxide dismutase, and catalase, as well as nonenzymatic antioxidants such as total sulfhydryl groups and glutathione. Furthermore, lead treatment caused hepatic deficiency in copper and zinc accompanied by a significant elevation of lead concentration in both plasma and liver. Daily pretreatment with melatonin (30 mg/kg body weight) intragastrically prevented the suppressive effects of lead on heme-synthesizing enzymes and iron deficiency. In addition, preadministration of melatonin reduced the inhibitory effect of lead on both enzymatic and nonenzymatic antioxidants. This was accompanied by marked normalization of lipid peroxidation and modulation of copper and zinc levels in liver. The action of melatonin on lead-induced changes was attributed to protection of the antioxidant capacity in cells in addition to the ability of melatonin to scavenge free radicals.

Porphobilinogen synthase from pea: expression from an artificial gene, kinetic characterization, and novel implications for subunit interactions

Porphobilinogen synthase (PBGS) is present in all organisms that synthesize tetrapyrroles such as heme, chlorophyll, and vitamin B(12). The homooctameric metalloenzyme catalyzes the condensation of two 5-aminolevulinic acid molecules to form the tetrapyrrole precursor porphobilinogen. An artificial gene encoding PBGS of pea (Pisum sativum L.) was designed to overcome previous problems during bacterial expression caused by suboptimal codon usage and was constructed by recursive polymerase chain reaction from synthetic oligonucleotides. The recombinant 330 residue enzyme without a putative chloroplast transit peptide was expressed in Escherichia coli and purified in 100-mg quantities. The specific activity is protein concentration dependent, which indicates that a maximally active octamer can dissociate into less active smaller units. The enzyme is most active at slightly alkaline pH; it shows two pK(a) values of 7.4 and 9.7. Atomic absorption spectroscopy shows maximal binding of three Mg(II) per subunit; kinetic data support two functionally distinct types of Mg(II) and the third appears to be nonphysiologic and inhibitory. Analysis of the protein concentration dependence of the specific activity suggests that the minimal functional unit is a tetramer. A model of octameric pea PBGS was built to predict the location of intermolecular disulfide linkages that were revealed by nonreducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis. As verified by site-specific mutagenesis, disulfide linkages can form between four cysteines per octamer, each located five amino acids from the C-terminus. These data are consistent with the protein undergoing conformational changes and the idea that whole-body motion can occur between subunits.

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.

Pseudomonas aeruginosa contains a novel type V porphobilinogen synthase with no required catalytic metal ions

Porphobilinogen synthases (PBGS) are metalloenzymes that catalyze the first common step in tetrapyrrole biosynthesis. The PBGS enzymes have previously been categorized into four types (I-IV) by the number of Zn(2+) and/or Mg(2+) utilized at three different metal binding sites termed A, B, and C. In this study Pseudomonas aeruginosa PBGS is found to bind only four Mg(2+) per octamer as determined by atomic absorption spectroscopy, in the presence or absence of substrate/product. This is the lowest number of bound metal ions yet found for PBGS where other enzymes bind 8-16 divalent ions. These four Mg(2+) allosterically stimulate a metal ion independent catalytic activity, in a fashion dependent upon both pH and K(+). The allosteric Mg(2+) of PBGS is located in metal binding site C, which is outside the active site. No evidence is found for metal binding to the potential high-affinity active site metal binding sites A and/or B. P. aeruginosa PBGS was investigated using Mn(2+) as an EPR probe for Mg(2+), and the active site was investigated using [3,5-(13)C]porphobilinogen as an NMR probe. The magnetic resonance data exclude the direct involvement of Mg(2+) in substrate binding and product formation. The combined data suggest that P. aeruginosa PBGS represents a new type V enzyme. Type V PBGS has the remarkable ability to synthesize porphobilinogen in a metal ion independent fashion. The total metal ion stoichiometry of only 4 per octamer suggests half-sites reactivity.

Production, purification, and characterization of a Mg2+-responsive porphobilinogen synthase from Pseudomonas aeruginosa

During tetrapyrrole biosynthesis the metalloenzyme porphobilinogen synthase (PBGS) catalyzes the condensation of two molecules of 5-aminolevulinic acid to form the pyrrole porphobilinogen. Pseudomonas aeruginosa PBGS was synthesized in Escherichia coli, and the enzyme was purified as a fusion protein with glutathione S-transferase (GST). After removal of GST, a molecular mass of 280 000 +/- 10 000 with a Stokes radius of 57 A was determined for native PBGS, indicating a homooctameric structure of the enzyme. Mg2+ stabilized the oligomeric state but was not essential for octamer formation. Alteration of N-terminal amino acids changed the oligomeric state and reduced the activity of the enzyme, revealing the importance of this region for oligomerization and activity. EDTA treatment severely inhibited enzymatic activity which could be completely restored by the addition of Mg2+ or Mn2+. At concentrations in the micromolar range Co2+, Zn2+, and Ni2+ partially restored EDTA-inhibited enzymatic activity while higher concentrations of Zn2+ inhibited the enzyme. Pb2+, Cd2+, and Hg2+ did not restore activity. A stimulatory effect of monovalent ions was observed. A Km of 0.33 mM for ALA and a maximal specific activity of 60 micromol h-1 mg-1 at the pH optimum of 8.6 in the presence of Mg2+ and K+ were found. pH-dependent kinetic studies were combined with protein modifications to determine the structural basis of two observed pKa values of approximately 7.9 (pKa1) and 9.5 (pKa2). These are postulated respectively as ionization of an active site lysine residue and of free substrate during catalysis. Some PBGS inhibitors were characterized. Finally, we succeeded in obtaining well-ordered crystals of P. aeruginosa PBGS complexed with the substrate analogue levulinic acid.

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.

Environmental lead exposure and activity of delta-aminolevulinic acid dehydratase (ALA-D) in maternal and cord blood

The hypothesis that environmental lead exposure measured from blood (Pb-B) inhibits delta-aminolevulinic acid dehydratase activity (ALA-D) from whole blood was tested in 241 urban mothers and their newborns. Geometric means and (5th and 95th Percentiles) for maternal and cord Pb-B were 6.4 microg dl(-1) (3.4-11.9) and 4.6 microg dl(-1) (2.8-9.2). Spearman correlations between mother and cord Pb-B and ALA-D were all negative but statistically significant only for cord Pb-B and mother ALA-D. A potential lead threshold, was identified between 3.2 and 4.8 microg dl(-1), above which ALA-D may be inhibited by lead, and below which ALA-D may be insensitive or even activated. In conclusion, low environmental exposure to lead is responsible for a demonstrable biochemical effect. This potential ALA-D inhibition may lead to neurotoxic effects, especially in newborns who have high level of neurogenesis.

Decreased plasma membrane thiol concentration is associated with increased osmotic fragility of erythrocytes in zinc-deficient rats

Zinc deficiency leads to pathological signs that are related to impaired function of plasma membrane proteins. The purpose of this study was to assess the effect of dietary zinc status on the sulfhydryl (SH) content of erythrocyte plasma membranes and erythrocyte function. Three experiments were performed. In the first, immature male rats were fed for 21 d either a low-zinc (<1.0 mg/kg) diet free choice (-ZnAL), an adequate-zinc (100 mg/kg) diet free choice (+ZnAL), or the adequate-zinc diet limited to the intake of -ZnAL pair-mates (+ZnPF). Tail blood was sampled to measure osmotic fragility and SH concentration of erythrocyte membrane proteins. The zinc-deficient rats were then repleted for 2 d and erythrocytes assayed for fragility and SH content. In the second experiment blood was sampled at 3-d intervals to determine the time course of change in fragility and SH concentration. In the third experiment the SH concentration of erythrocyte band 3 protein and the binding of zinc to isolated plasma membranes were measured. SH concentration decreased from approximately 75 nmol/mg protein to 68 nmol/mg protein during 21 d of depletion and returned to control level within 2 d of repletion. There was an inverse relationship between osmotic fragility and SH concentration of erythrocyte membrane proteins. Maximal decrease in SH occurred within 6 d of consuming the low-zinc diet. The SH content of band 3 protein isolated from deficient rats was also significantly lower than that of pair-fed controls (45 vs. 51 nmol/mg protein). The zinc-binding affinity of plasma membrane proteins tended to be decreased by zinc deficiency. In summary, low-zinc status lowers the plasma membrane SH concentration, and the decreased reducing potential is inversely related to osmotic fragility, and presumably, with impaired volume recovery of erythrocytes.

Delta aminolevulinate dehydratase (ALA-D) activity in human and experimental diabetes mellitus

The haem pathway is impaired in porphyrias and a frequent coexistence of diabetes mellitus and porphyria disease has been reported. We have therefore decided to investigate delta-aminolevulinate dehydratase, one of the more sensitive enzymes in the haem pathway, in both human diabetic patients and diabetic rats. We have studied 131 diabetes mellitus patients, 32 insulin dependent and 99 non-insulin dependent. The latter group was further subdivided according to treatment: diet alone (n = 24), diet plus oral hypoglycemic agents (n = 28) and diet plus insulin (n = 47). We have also performed similar studies in the rat model of diabetes mellitus, induced in 11 Wistar rats by streptozotocin. Control groups of both humans and animals were used. Erythrocytic aminolevulinate dehydratase activity was reduced in both insulin dependent and non-insulin dependent diabetic patients as compared to their controls (p < 0.001). This activity was only partially restored by addition of zinc and thiols to the incubation media. In insulin-dependent diabetes mellitus, reduction of enzyme activity was related to the glycosilated hemoglobin concentration (p < 0.05) and in non-insulin dependent diabetes mellitus to the glycemia (p < 0.01). In the diabetic rat, aminolevulinate dehydratase activity was diminished on both erythrocytes (p < 0.01) and hepatic tissue (p < 0.01) when compared to the control group. The decrease in activity of erythrocyte aminolevulinate dehydratase observed in diabetic patients, may represent an additional and useful parameter for the assessment of the severity of carbohydrate metabolism impairment.

Purification of a 38-kDa protein from rabbit reticulocyte lysate which promotes protein renaturation by heat shock protein 70 and its identification as delta-aminolevulinic acid dehydratase and as a putative DnaJ protein

We reported recently that a rabbit reticulocyte 66-kDa protein (termed RF-hsp 70 by our laboratory and p60 and hop by others) functions as a hsp 70 recycling protein and markedly enhances the renaturation of luciferase by hsp 70 (Gross, M., and Hessefort, S. (1996) J. Biol. Chem. 271, 16833-16841). In this report, we confirm that the ability of RF-hsp 70 to promote the conversion of hsp 70. ADP to hsp 70.ATP, thus enhancing the protein folding activity of hsp 70, is caused by the purified 66-kDa protein and not by a trace DnaJ/hsp 40 protein contaminant. To determine the relationship between RF-hsp 70 and the DnaJ/hsp 40 heat shock protein family, which also enhances protein renaturation by hsp 70, we purified a 38-kDa protein from rabbit reticulocyte lysate based upon its ability to stimulate renaturation of luciferase by hsp 70. Partial amino acid sequencing of this 38-kDa protein has indicated, unexpectedly, that it is the enzyme delta-aminolevulinic acid dehydratase (ALA-D) and that it does not contain detectable sequences corresponding to the DnaJ/hsp 40 protein family. In addition, immunoblot analysis with a polyclonal antibody made to HeLa cell hsp 40 (from StressGen) confirms that our purified ALA-D contains no hsp 40, although hsp 40 is present in relatively crude rabbit reticulocyte protein fractions. Rabbit reticulocyte ALA-D is about as active in converting delta-aminolevulinic acid to porphobilinogen and as Zn2+-dependent as ALA-D purified from other sources. Rabbit reticulocyte ALA-D stimulates the renaturation of luciferase by hsp 70 up to 10-fold at concentrations that are the same as or less than that of hsp 70, and it has no renaturation activity in the absence of hsp 70. The renaturation effect of ALA-D is additive with that of RF-hsp 70 at limiting or saturating concentrations of each, and, unlike RF-hsp 70, ALA-D does not promote the dissociation of hsp 70.ADP in the presence of ATP. The renaturation-enhancing effect of ALA-D may be caused by a region near its carboxyl terminus which has sequence homology to the highly conserved domain of the DnaJ protein family, which is similar to the sequence homology between this domain and a carboxyl-terminal region in auxilin, a DnaJ-like protein that requires this region for its hsp 70-dependent function (Ungewickell, E., Ungewickell, H., Holstein, S. E. H., Lindner, R., Prasad, K., Barouch, W., Martin, B., Greene, L. E., and Eisenberg, E. (1995) Nature 378, 632-635).

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.

Effect of organic forms of selenium on delta-aminolevulinate dehydratase from liver, kidney, and brain of adult rats

The inhibitory effect of various forms of organic selenium compounds and of diphenyl ditelluride (PhTe)2 on delta-aminolevulinate dehydratase (delta-ALA-D) from liver, kidney, and brain of rats was investigated because it has been reported that organocalcogens catalyze the oxidation of thiols. Diphenyl diselenide (PhSe)2, rho-chloro-diphenyl diselenide (rho ClPhSe)2, propyl-2-2-diphenyl diselenide, and propyl-2-methoxy-2-phenyl selenide inhibited delta-ALA-D and the IC50 ranged from 2 to 32 microM depending on the selenium compound and whether it was preincubated with the enzyme. (rho ClPhSe)2 was the most potent inhibitor of delta-ALA-D, and preincubation increased the inhibitory potency of all the tested compounds. Inorganic selenium compounds (sodium selenite, Na2SeO3 and selenium dioxide, SeO2) inhibited delta-ALA-D, and the potency of SeO2 was greater than that of (rho ClPhSe)2. Diphenyl ditelluride (PhTe)2 also inhibited delta-ALA-D but with relatively lower potency than that of organic and inorganic selenium compounds. The inhibitory effect of propyl-2-2-diphenyl diselenide and propyl-2-methoxy-2-phenyl selenide seems to be mediated by (PhSe)2 since the compounds decomposed rapidly to (PhSe)2 in aqueous medium. The inhibitory action of selenium forms on delta-ALA-D from liver, kidney, and brain was antagonized by sulfhydryl protecting agents (dithiotreitol and reduced glutathione). The effects of organic selenium compounds on delta-ALA-D were related to the stability of the Se-Se (or Se-C) bond because the compound methyl-diphenyl diselenide (which possesses the most stable Se-C-Se bond) did not inhibit the enzyme. The inhibitory action of (PhSe)2 was not related to the formation of oxyradicals in the medium since superoxide dismutase and catalase did not affect the inhibition of delta-ALA-D by (PhSe)2. delta-ALA-D from cucumber leaves was not inhibited by selenium or tellurium compounds which suggests that these compounds act directly on the B or beta-site of the animal enzyme. These results suggest that delta-ALA-D from liver, kidney, and brain is a potential molecular target for the toxic effect of organic forms of selenium and tellurium.

Kinetics of aluminum-induced inhibition of delta-aminolevulinic acid dehydratase in vitro

Anemia, one consequence of aluminum toxicity, may be due to inhibition of enzymes in the heme biosynthetic pathway. In this study, the in vitro effect of aluminum on rat liver and erythrocyte delta-aminolevulinic acid dehydratase (delta-ALA dehydratase), an enzyme that is sensitive to a number of metal ions, was investigated. The presence of 1-10 microM AlCl3 caused a concentration-dependent inhibition of liver delta-ALA dehydratase activity. The Ki for AlCl3-induced inhibition of delta-ALA dehydratase was 4.1 microM, and 10 microM AlCl3 virtually abolished delta-ALA dehydratase activity (99% inhibition). Erythrocyte delta-ALA dehydratase was also inhibited by similar concentrations of AlCl3 and displayed a Ki of 1.1 microM. AlCl3 (5 microM) decreased the Vmax by 50% but did not change the Km, suggestive of reversible, noncompetitive inhibition. Sodium citrate (50 microM) when added with AlCl3 completely restored delta-ALA dehydratase activity to basal levels. Thus, disruption of delta-ALA dehydratase occurred at low micromolar levels of AlCl3 in vitro, which may help to explain abnormalities in the heme pathway in cases of aluminum poisoning.

Bradyrhizobium japonicum porphobilinogen synthase uses two Mg(II) and monovalent cations

Bradyrhizobium japonicum porphobilinogen synthase (B. japonicum PBGS) has been purified and characterized from an overexpression system in an Escherichia coli host (Chauhan, S., and O'Brian, M. R. (1995) J. Biol. Chem. 270, 19823-19827). B. japonicum PBGS defines a new class of PBGS protein, type IV (classified by metal ion content), which utilizes a catalytic MgA present at a stoichiometry of 4/octamer, an allosteric MgC present at a stoichiometry of 8/octamer, and a monovalent metal ion, K+. However, the divalent MgB or ZnB present in some other PBGS is not present in B. japonicum PBGS. Under optimal conditions, the Kd for MgA is <0.2 microM, and the Kd for MgC is about 40 microM. The response of B. japonicum PBGS activity to monovalent and divalent cations is mutually dependent and varies dramatically with pH. B. japonicum PBGS is also found to undergo a dynamic equilibrium between active multimeric species and inactive monomers under assay conditions, a kinetic characteristic not reported for other PBGSs. B. japonicum PBGS is the first PBGS that has been rigorously demonstrated to lack a catalytic ZnA. However, consistent with prior predictions, B. japonicum PBGS can bind Zn(II) (presumably as ZnA) at a stoichiometry of 4/octamer with a Kd of 200 microM; but this high concentration is outside a physiologically significant range.

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].

Porphobilinogen synthase, the first source of heme's asymmetry

Porphobilinogen is the monopyrrole precursor of all biological tetrapyrroles. The biosynthesis of porphobilinogen involves the asymmetric condensation of two molecules of 5-aminolevulinate and is carried out by the enzyme porphobilinogen synthase (PBGS), also known as 5-aminolevulinate dehydratase. This review documents what is known about the mechanism of the PBGS-catalyzed reaction. The metal ion constituents of PBGS are of particular interest because PBGS is a primary target for the environmental toxin lead. Mammalian PBGS contains two zinc ions at each active site. Bacterial and plant PBGS use a third metal ion, magnesium, as an allosteric activator. In addition, some bacterial and plant PBGS may use magnesium in place of one or both of the zinc ions of mammalian PBGS. These phylogenetic variations in metal ion usage are described along with a proposed rationale for the evolutionary divergence in metal ion usage. Finally, I describe what is known about the structure of PBGS, an enzyme which has as yet eluded crystal structure determination.

Vanadium compounds. Their action on alkaline phosphatase activity

The direct effect of different vanadium compounds upon alkaline phosphatase (ALP) activity was investigated. Vanadate and vanadyl inhibited both the soluble and particulate ALP activity from UMR.106 cells and from bovine intestinal ALP. We have also shown the inhibition of ALP activity in the soluble fraction of osteoblasts by peroxo and hydroperoxo vanadium compounds. ALP activity in the particulate fraction was not inhibited by these species; nor was the bovine intestinal ALP. Using inhibitors of Tyr-phosphatase (PTPases), the soluble ALP was partially characterized as a PTPase. The major activity in the particulate fraction represents the bone-specific ALP-activity. This study demonstrates that different forms of vanadium are direct inhibitors of ALP activity. This effect is dependent on the enzymatic activity investigated and on the origin of the ALP.