The uptake and effects of lead in small mammals and frogs at a trap and skeet range

This study was performed to evaluate the bioavailability and effects of lead in wildlife at a trap and skeet range. The total lead concentration in a composite soil sample (pellets removed) was 75,000 micrograms/g dry weight. Elevated tissue lead concentrations and depressed ALAD activities in small mammals and frogs indicate that some of the lead deposited at the site is bioavailable. Mean tissue lead concentrations (micrograms/g dry wt.) in white-footed mice (Peromyscus leucopus) at the range liver = 4.98, kidney = 34.9, femur = 245) were elevated (P < 0.01) 5- to 64-fold relative to concentrations in mice from a control area. Tissue lead concentrations in the only shorttail shrew (Blarina brevicauda) captured at the range (liver = 34.1, kidney = 1506, femur = 437) were elevated 35- to 1038-fold. Femur lead concentrations in green frogs (Rana clamitans) at the range (1,728 micrograms/g) were elevated nearly 1000-fold, and the lead concentration in a pooled kidney sample (96.2 micrograms/g) was elevated 67-fold. There was significant depression of blood ALAD activity in mice (P = 0.0384) and depression of blood and liver ALAD activity in frogs (P < 0.001). Hematological and histopathological lesions associated with lead toxicosis were observed in some animals. Hemoglobin concentrations were reduced 6.7% in mice (P = 0.0249), but hematocrit was not significantly affected in mice or frogs. Intranuclear inclusions were present in the renal proximal tubular epithelium of two of the mice and the shrew that were captured at the range, and necrosis of the tubular epithelium was also evident in one of the mice. Kidney:body weight ratios were similar in range and control mice. Soil ingestion may be a significant route of lead uptake in small mammals at the range. However, the tendency of lead to concentrate in the bones rather than in more digestible soft tissues may minimize food chain uptake of lead by predators, especially raptors that regurgitate undigestible material.

Biosynthesis of vitamin B12: the preparative multi-enzyme synthesis of precorrin-3A and 20-methylsirohydrochlorin (a 2,7,20-trimethylisobacteriochlorin)

The Bacillus subtilis genes hemB, hemC and hemD, encoding respectively the enzymes porphobilinogen synthase, hydroxymethylbilane synthase and uroporphyrinogen III synthase, have been expressed in Escherichia coli using a single plasmid construct. An enzyme preparation from this source converts 5-aminolaevulinic acid (ALA) preparatively and in high yield into uroporphyrinogen III. The Pseudomonas denitrificans genes cobA and cobI, encoding respectively the enzymes S-adenosyl-L-methionine:uroporphyrinogen III methyltransferase (SUMT) and S-adenosyl-L-methionine:precorrin-2 methyltransferase (SP2MT), were also expressed in E. coli. When SUMT was combined with the coupled-enzyme system that produces uroporphyrinogen III, precorrin-2 was synthesized from ALA, and when SP2MT was also added the product from the coupling of five enzymes was precorrin-3A. Both of these products are precursors of vitamin B12, and they can be used directly for biosynthetic experiments or isolated as their didehydro octamethyl esters in > 40% overall yield. The enzyme system which produces precorrin-3A is sufficiently stable to allow long incubations on a large scale, affording substantial quantities (15-20 mg) of product.

An evaluation of unleaded petrol as a harm reduction strategy for petrol sniffers in an aboriginal community

OBJECTIVE

In mid 1989, leaded petrol was replaced by unleaded petrol to reduce lead toxicity in petrol sniffers in Maningrida, a remote Aboriginal community in Northern Australia. RETROSPECTIVE REVIEWS: Hospital admissions between 1987 and 1992 due to petrol sniffing were compared for Maningrida and a community using only leaded petrol.

RESULTS

Admissions from Maningrida decreased significantly following introduction of unleaded petrol (chi 2 on 2df = 22.25, p < 0.001). Lead and hydrocarbon exposures were also compared for three groups from Maningrida (27 sniffers using only unleaded petrol; 15 exsniffers and 13 nonsniffers) and 24 individuals admitted to hospital for petrol sniffing related illness from other communities using only leaded petrol. Median blood lead levels for hospitalized sniffers (using only leaded petrol). Maningrida sniffers (using only unleaded petrol), exsniffers and nonsniffers were 5.06, 1.87, 1.24 and 0.17 microM/L respectively. There were significant differences between blood lead level, delta-aminolevulinic acid dehydratase activity, and free erythrocyte protoporphyrin for sniffers of leaded and unleaded petrol, whereas these indices were not significantly different for current sniffers and exsniffers in Maningrida. Hydrocarbons were only detectable in the blood of active sniffers (toluene < or = 0.5 micrograms/mL; benzene < or = 0.17 micrograms/mL blood; n-hexane not detected).

CONCLUSIONS

The elimination of tetraethyl lead from petrol resulted in a significant decrease in hospitalization of petrol sniffers.

delta-Aminolevulinic acid in plasma or whole blood as a sensitive indicator of lead effects, and its relation to the other heme-related parameters

To evaluate the subclinical effect of lead exposure, we determined delta-aminolevulinic acid (ALA) levels in plasma (ALA-P), blood (ALA-B), and urine (ALA-U) and the activity of delta-aminolevulinic acid dehydratase (ALAD) in lead workers. Almost all of the ALA molecules in blood were present in plasma and not in blood cells, irrespective of the blood lead concentration (Pb-B). ALA-P or ALA-B levels increased slowly at Pb-B levels below 40 micrograms/dl (slow phase) and rapidly at levels above 40 micrograms/dl (rapid phase). In both phases, ALA-P and ALA-B were well correlated with Pb-B and ALAD activity. The threshold value (no-effect level) of Pb-B for elevation of the ALA-P or ALA-B level was coincident with that for ALAD inhibition; the value was around 5 micrograms/dl. In the rapid phase, ALA-P increased continuously up to 100 micrograms/dl of Pb-B, while ALAD activity reached a plateau. Receiver operative characteristic (ROC) plot analyses indicated that ALA-P and ALAD activity [ALAD(u)] had a similar diagnostic value at Pb-B levels between 10 and 40 micrograms/dl, although ALAD(%), the remaining ALAD activity as a percentage of the whole activity restored by zinc and dithiothreitol, had the most powerful diagnostic efficiency at these Pb-B levels. By contrast, ALA-U and zinc protoporphyrin were less effective for the diagnosis of lead exposure than ALAD and ALA-P. These findings indicate that ALA-P is the best discriminators of lead exposure form baseline to high levels of exposure.

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].

Cloning and overexpression of the Rhodobacter capsulatus hemH gene

In photosynthetically grown Rhodobacter capsulatus, heme is a qualitatively minor end product of the common tetrapyrrole pathway, but it may play a significant regulatory role. Heme is synthesized from protoporphyrin by the product of the hemH gene, ferrochelatase. We have cloned the R. capsulatus hemH gene by complementation of an Escherichia coli hemH mutant. When a plasmid carrying the hemH gene is returned to R. capsulatus, ferrochelatase activity increases, aminolevulinate synthase activity decreases, and bacteriochlorophyll levels are dramatically lowered. This is the first in vivo evidence to suggest that heme feedback inhibits aminolevulinate synthase in R. capsulatus, thereby reducing porphyrin synthesis.

The phylogenetically conserved histidines of Escherichia coli porphobilinogen synthase are not required for catalysis

Porphobilinogen synthase (PBGS) is a metalloenzyme that catalyzes the first common step of tetrapyrrole biosynthesis, the asymmetric condensation of two molecules of 5-aminolevulinic acid (ALA) to form porphobilinogen. Chemical modification data implicate histidine as a catalytic residue of PBGS from both plants and mammals. Histidine may participate in the abstraction of two non-ionizable protons from each substrate molecule at the active site. Only one histidine is species-invariant among 17 known sequences of PBGS which have high overall sequence similarity. In Escherichia coli PBGS, this histidine is His128. We performed site-directed mutagenesis on His128, replacing it with alanine. The mutant protein H128A is catalytically active. His128 is part of a histidine- and cysteine-rich region of the sequence that is implicated in metal binding. The apparent Kd for Zn(II) binding to H128A is about an order of magnitude higher than for the wild type protein. E. coli PBGS also contains His126 which is conserved through the mammalian, fungal, and some bacterial PBGS. We mutated His126 to alanine, and both His126 and His128 simultaneously to alanine. All mutant proteins are catalytically competent; the Vmax values for H128A (44 units/mg), H126A (75 units/mg), and H126/128A (61 units/mg) were similar to wild type PBGS (50 units/mg) in the presence of saturating concentrations of metal ions. The apparent Kd for Zn(II) of H126A and H126/128A is not appreciably different from wild type. The activity of wild type and mutant proteins are all stimulated by an allosteric Mg(II); the mutant proteins all have a reduced affinity for Mg(II). We observe a pKa of approximately 7.5 in the wild type PBGS kcat/Km pH profile as well as in those of H128A and H126/128A, suggesting that this pKa is not the result of protonation/deprotonation of one of these histidines. H128A and H126/128A have a significantly increased Km value for the substrate ALA. This is consistent with a role for one or both of these histidines as a ligand to the required Zn(II) of E. coli PBGS, which is known to participate in substrate binding. Past chemical modification may have inactivated the PBGS by blocking Zn(II) and ALA binding. In addition, the decreased Km for E. coli PBGS at basic pH allows for the quantitation of active sites at four per octamer.

Effects of 3'-azido-3'-deoxythymidine on erythroid inducible gene expression in human K-562 leukemia cells

We previously demonstrated that 3'-azido-3'-deoxythymidine (AZT) down regulates hemoglobin (Hb) synthesis and globin gene expression. That inhibition may therefore result either from a direct effect on globin gene transcription or an indirect effect through inhibition of K-562 cell induction, thereby leading to inhibition of other inducible genes of heme biosynthesis. The present results demonstrate that inhibition of globin gene expression by AZT is a direct gene effect rather than a general inhibition of K-562 cell induction as demonstrated by the absence of AZT effects on expression of three other erythroid inducible genes [erythroid-specific aminolevulate synthase (ALAS-E), aminolevulinic acid dehydrogenase (ALAD), and erythroid-specific porphobilinogen deaminase (PBGD-E)].

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.

The role of the erythroid-specific delta-aminolevulinate synthase gene expression in erythroid heme synthesis

Using antisense technology, the effects of suppressed gene expression of the erythroid-specific delta-aminolevulinate (ALA) synthase (ALAS-E) on heme synthesis, expression of mRNAs encoding an erythroid-specific transcription factor NF-E2, other heme pathway enzymes, and beta-globin were examined in murine erythroleukemia (MEL) cells. In MEL cells in which an antisense ALAS-E RNA was expressed (AS clone), sense ALAS-E mRNA levels in both untreated and dimethylsulfoxide (DMSO)-treated cells were decreased compared with their respective controls. Heme synthesis in AS clones was decreased in proportion to the suppressed levels of ALAS-E mRNA. In addition, mRNAs for ALA dehydratase, porphobilinogen deaminase, ferrochelatase (FeC), and beta-globin were also decreased in AS clones. There was a strong correlation between the level of ALAS-E mRNA and most of the mRNAs of the heme pathway enzymes and beta-globin. There was a decrease in the mRNA level of p45, but not of mafK, which are the large and the small subunits of NF-E2, respectively, in AS clones. Treatment of AS cells with hemin and ALA in the presence of DMSO partially restored the suppressed mRNA levels for beta-globin and FeC and heme content, respectively. These findings thus indicate that heme formation, which is determined by the level of ALAS-E, plays an essential role on gene expression of many proteins necessary for erythroid development.

Effect of treatment with mercury chloride and lead acetate during the second stage of rapid postnatal brain growth on delta-aminolevulinic acid dehydratase (ALA-D) activity in brain, liver, kidney and blood of suckling rats

The sensitivity of developing rodents to toxic metals differs considerably from that of adults. In the present study, we investigated the in vivo and in vitro effects of inorganic mercury and lead on delta-aminolevulinic acid dehydratase (ALA-D) from brain, liver, kidney and blood of young rats. Eight day-old rats were injected with one or five doses of lead acetate (0, 3.5, or 7.0 mg/kg) or HgCl2 (0, 2.5, or 5.0 mg/kg). In vitro, the IC50 for mercury inhibition of cerebral, renal and hepatic ALA-D was in the 124 to 160 microM range, while values for lead acetate was in the 7 to 12 microM range. The IC50 of blood enzyme for lead (0.8 microM) and mercury (6.5 microM) was significantly lower than that observed for the other tissues. A single dose of lead did not affect the enzyme activity, but a single dose of HgCl2 (5 mg/kg) caused a significant inhibition of ALA-D from kidney (40%, P < 0.01) and liver (25%, P < 0.05). Five doses of lead acetate (3.5 or 7 mg/kg) caused an inhibition of about 25 and 40%, respectively (P < 0.01), of hepatic ALA-D, and an increase of 1.4-fold (P < 0.05) and 2.6-fold (P < 0.01) of blood enzyme, respectively. Treatment with five doses of HgCl2 (5 mg/kg) caused an inhibition of about 25, 60, 50, and 80% of ALA-D from brain, blood, liver and kidney, respectively (all P < 0.05). Five doses of 2.5 mg/kg HgCl2 caused an inhibition of ALA-D from liver (40%, P < 0.01) and kidney (45%, P < 0.01). These results demonstrate that ALA-D from young rat tissues show different sensitivities to mercury and lead. The enzyme was more affected by mercury than by lead in vivo, while in vitro lead was more potent that mercury as an ALA-D inhibitor.

Alterations in the heme biosynthetic pathway from the III-V semiconductor metal, indium arsenide (InAs)

The effects of indium and arsenic on the heme biosynthetic pathway have been well documented but the effects of indium arsenide (InAs), the next possible generation of the III-V semiconductors, are unknown. Male Syrian golden hamsters were given s.c. injections of sodium arsenite (As3+), indium chloride (In3+) or indium arsenide (InAs). Erythrocyte delta-aminolevulinic acid dehydratase (ALAD) activity was inhibited in all exposure groups, while hepatic ALAD activity was not significantly changed. In contrast, the activity of renal ALAD was found to be statistically decreased by As3+ at 10 days, but increased at 30 days, while In3+ and InAs inhibited this enzyme activity at all time points. In vitro studies showed that hepatic ALAD activity was more sensitive to In3+ than As3+, suggesting that the effects of InAs in vivo on this enzyme are due primarily to the In rather than the As moiety. Studies of urinary porphyrin excretion patterns in animals treated with InAs showed marked, early 2-4-fold increase in the excretion of the penta-, hexa- and heptacarboxyl porphyrin at 1-5 days which continued through day 30 of the study. In contrast, there was a slow and steady rise in the excretion of coproporphyrin I and III which reached a maximum at day 30. The results of these studies indicate that both the In and As moieties of InAs are biologically active following InAs exposure and that the enzymes in the heme pathway, such as ALAD, may have great utility as markers of exposure/toxicity for these agents.

STZ-induced diabetes in mice and heme pathway enzymes. Effect of allylisopropylacetamide and alpha-tocopherol

A frequent coexistence of diabetes and porphyria disease has been reported. Under normal conditions, porphyrin biosynthesis is well regulated to only form the amount of heme required for the synthesis of the various hemoproteins. The activity of some heme enzymes and rhodanese in streptozotocin (STZ) induced diabetic mice and in allylisopropylacetamide (AIA) induced experimental acute porphyria mice has been examined. The role of alpha-tocopherol (alpha-T), reported to prevent protein glycation in vitro, has also been investigated. AIA induced hepatic delta-aminolevulinic acid synthetase (ALA-S) activity in control animals but was ineffective in the diabetic group. alpha-Tocopherol did not modify ALA-S activity in either group. delta-Aminolevulinic acid dehydratase (ALA-D) and deaminase activities were significantly diminished both in liver and blood of diabetic animals. alpha-Tocopherol prevented inhibition of ALA-D, deaminase and blood rhodanese activities in diabetic animals but alpha-tocopherol by itself did not affect the basal levels of the enzymes studied. The potential use of alpha-tocopherol to prevent late complications of diabetes, including the onset of a porphyria like syndrome is considered.

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.

Renal toxicity after oral administration of lead acetate during pre- and post-implantation periods: effects on trace metal composition, metallo-enzymes and glutathione

The present study was undertaken to compare the effects of 10-50 mg/kg b.wt. of Pb acetate after chronic treatment through oral gavage on: (a) the distribution of trace elements such as Fe, Cu, Zn, and Mn, (b) enzyme activity of delta-amino levulinic acid dehydratase (delta-ALAD) and alkaline phosphatase, and (c) glutathione (GSH) in kidney and (d) delta-ALAD in blood of pregnant and non-pregnant mice. Treatment with Pb acetate was given on every alternate day for 4 weeks prior to mating and for 3-4 weeks until pregnancy became apparent and confirmed by laporatomy. Lead administration reduced the rate of reproduction as assessed by number of living viable embryos. During normal pregnancy renal Cu, Fe and GSH tended to decline although non-significantly and continued to do so after lead administration. Mn was considerably and significantly elevated, whereas activity of delta-ALAD (non-activated) was quite low in pregnant mice. Following administration of Pb acetate, kidneys of pregnant and non-pregnant dams accumulated Pb in a dose-dependent manner, but as compared to non-pregnant mice, Pb increase in pregnant dams was significantly lower. Pb toxicity was associated with the loss of delta-ALAD in blood and kidney, but unlike the non-activated form of delta-ALAD, the dithiothreitol-activated form of delta-ALAD declined in a significant amount. The residual activity showed a high degree of negative correlation with endogenous Pb as well as with Zn/Pb ratio. Pb toxicity did not modify renal Fe, Cu, and Zn in the pregnant state, but reduced renal Fe during pregnancy.(ABSTRACT TRUNCATED AT 250 WORDS)

Structure and expression of the Chlamydomonas reinhardtii alad gene encoding the chlorophyll biosynthetic enzyme, delta-aminolevulinic acid dehydratase (porphobilinogen synthase)

cDNA clones for the alad gene encoding the chlorophyll biosynthetic enzyme ALA dehydratase (ALAD) from Chlamydomonas reinhardtii were isolated by complementation of an Escherichia coli ALAD mutant (hemB). The C. reinhardtii alad gene encodes a protein that has 50 to 60% sequence identity with higher plant ALADs, and includes a putative Mg(2+)-binding domain characteristic of plant ALADs. Multiple classes of ALAD cDNAs were identified which varied in the length of their 3'-untranslated region. Genomic Southern analysis, using an ALAD cDNA as a probe, indicates that it is a single-copy gene. This suggests that the differently sized ALAD cDNAS are not the products of separate genes, but that a primary ALAD transcript is polyadenylated at multiple sites. A time course determination of ALAD mRNA levels in 12-h light:12-h dark synchronized cultures shows a 7-fold increase in ALAD mRNA at 2 h into the light phase. The ALAD mRNA level gradually declines but continues to be detectable up to the beginning of the dark phase. ALAD enzyme activity increases 3-fold by 6 h into the light phase and remains high through 10 h. Thus, there is an increase in both ALAD mRNA level and ALAD enzyme activity during the light phase, corresponding to the previously observed increase in the rate of chlorophyll accumulation.

Characterization of the role of the stimulatory magnesium of Escherichia coli porphobilinogen synthase

The synthesis of tetrapyrroles is essential to all phyla. Porphobilinogen synthase (PBGS) is a zinc metalloenzyme that catalyzes the formation of porphobilinogen, the monopyrrole precursor of all biological tetrapyrroles. The enzyme from various organisms shows considerable sequence conservation, suggesting a common fold, quaternary structure, and catalytic mechanism. Escherichia coli and plant PBGS are activated by magnesium, a property that is absent from mammalian PBGS. This stimulatory Mg(II) is called Mgc. Mgc is not required for activity and is distinct from the two zinc ions (ZnA and ZnB) common to mammalian and E. coli PBGS (PBGSE.coli). For PBGSE.coli, both the Km for the substrate 5-aminolevulinic acid (ALA) and the Vmax are altered by the presence of Mgc; Mg(II) causes the Km to drop from approximately 3 to 0.30 mM and the maximum specific activity to increase from 23 to 50 mumol h-1 mg-1. Mgc also causes the saturating concentration of the required Zn(II) to decrease from 0.1 mM to 10 microM. Maximal activation by Mg(II) occurs at 0.5 mM; thus, in E. coli the Mgc site is probably saturated under physiological conditions. Mn(II) is a good substitute for Mgc, giving a comparable increase in catalytic activity. Consequently, Mn(II) has been used as an EPR active probe of the Mgc binding site. Mn(II) binds at a stoichiometry of eight ions per enzyme octamer. The X- and Q-band EPR spectra reflect a single type of binding site with rhombic symmetry and are consistent with oxygen and/or nitrogen ligands. The addition of unlabeled or 1-13C-labeled ALA does not significantly affect the Mn(II) EPR spectra.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Highlights in haem biosynthesis

The haem biosynthesis pathway continues to provide surprises, from the first enzyme, 5-aminolaevulinic acid synthase, the mRNA of which contains an iron-responsive element, to the last, ferrochelatase, that contains an iron sulphur cluster. 5-Aminolaevulinate dehydratase from animals are zinc-dependent enzymes while those from plants require magnesium. The first X-ray structure of a haem synthesis enzyme, porphobilinogen deaminase, has not only yielded clues about the mechanism of tetrapyrrole assembly but has also provided insight into the molecular basis of the human disease acute intermittent porphyria. Evidence is growing to suggest that a previously unsuspected alternative haem pathway may exist.

Differential reduction in soluble and membrane-bound c-type cytochrome contents in a Paracoccus denitrificans mutant partially deficient in 5-aminolevulinate synthase activity

A mutant of Paracoccus denitrificans, DP104, unable to grow anaerobically with nitrate as the terminal electron acceptor or aerobically with methanol as the electron donor and staining negatively in the dimethylphenylene diamine oxidation (Nadi) test, was isolated by transposon Tn5::phoA mutagenesis. P. denitrificans DP104 grown aerobically with succinate or choline had very low levels (2 to 3% of the wild-type levels) of spectroscopically detectable soluble c-type cytochromes. In contrast, membrane cytochromes of the a, b, and c types were present at 50% of the levels found in the wild type. The apo form of cytochrome c550, at an approximately 1:1 molar ratio with the holo form, was found in the periplasm of DP104. The TnphoA element was shown to be inserted immediately upstream of the translational start of hemA, the gene coding for 5-aminolevulinate synthase, which was sequenced. Low-level expression of this gene, driven off an incidental promoter provided by TnphoA-cointegrated suicide vector DNA, is the basis of the phenotype which could be complemented by the addition of 5-aminolevulinate to growth media. Disruption of the hemA gene generated a P. denitrificans strain auxotrophic for 5-aminolevulinate, establishing that there is no hemA-independent pathway of heme synthesis in this organism. The differential deficiency in periplasmic c-type cytochromes relative to membrane cytochromes in DP104 is suggested to arise from unequal competition for the restricted supply of heme which results from the effects of the transposon insertion.

5-Chloro[1,4-13C]levulinic acid modification of mammalian and bacterial porphobilinogen synthase suggests an active site containing two Zn(II)

5-Chloro[1,4-13C]levulinic acid ([1,4-13C]CLA) is an active site-directed inactivator of porphobilinogen synthase (PBGS). PBGS asymmetrically condenses two molecules of 5-aminolevulinic acid (ALA) which are called A-side ALA and P-side ALA in reference to their fates as the acetyl and propionyl halves of the product. [1,4-13C]CLA modifies bovine PBGS at the A-side ALA binding site. The C4 chemical shift indicates an intact keto moiety; the C1 chemical shift indicates a deprotonated carboxyl group. In contrast, [1,4-13C]CLA modification of Escherichia coli PBGS is heterogeneous and occurs preferentially at the P-side ALA binding site. The C1 chemical shifts indicate substantially deprotonated carboxylic acid groups. For one of four observed forms of [1,4-13C]CLA-modified E. coli PBGS, an analog of the P-site Schiff base is found. Bovine and E. coli PBGS contain two different zincs, ZnA and ZnB. Past results placed ZnA near A-side ALA. [1,4-13C]CLA modifies E. coli PBGS at Cys119 or Cys129, which is part of a four-cysteine cluster implicated in binding ZnB. This result places ZnB near P-side ALA. E. coli PBGS binds a third type of divalent metal, MgC or MnC, which is found to have no significant effect on the 13C NMR spectrum of the [1,4-13C]CLA-modified protein.

Influence of protein calorie malnutrition and fasting on the activities of delta-aminolevulinic acid dehydratase and porphobilinogen deaminase in rats

Rats were fed a restricted standard diet (6-8 g daily) for 4 weeks. The erythrocyte activity of delta-aminolevulinic acid dehydratase showed a very expressed decrease (7.9 times) and its hepatic activity diminished more than twice. Porphobilinogen deaminase activity was reduced by 40% and 17% respectively. After a 3-day total food deprivation delta-aminolevulinic acid dehydratase activity remained unchanged in red blood cells, but an increase by 52% was found in the liver. The erythrocyte activity of porphobilinogen deaminase was reduced by 42% and its hepatic activity--by 16%.

Effect of Plasmodium berghei infection and antimalarial treatment on heme synthesis in mice

Plasmodium berghei infection impaired the hepatic heme synthesizing machinery of mice. Key enzymes, viz. S-aminolevulinic acid synthase, S-aminolevulinic acid dehydrase and ferrochelatase were found to be decreased. In contrast, tryptophane pyrrolase noticeably increased during parasitic infection. Oral feeding of chloroquine [16 mg (kg body weight)-1 x 4 days] cleared the parasitaemia from infected mice within 72 h and returned the altered levels of enzymes almost to normal a week after cessation of treatment, the exception being tryptophane pyrrolase, which remained unaffected. Chloroquine treatment did not cause any significant alteration in the above-mentioned enzymes of normal mice.

[Lack of association between alcoholism and alleles in the delta-aminolevulinic acid dehydratase (ALAD) gene]

delta-Aminolevulinic acid dehydratase (ALAD) is the second enzyme in the heme biosynthetic pathway and catalyzes two molecules of delta-aminolevulinate (ALA), which is a potent agonist for GABA autoreceptors. ALAD has two common alleles and thus consists of three distinct isozymes, designated 1-1, 1-2, and 2-2. It has been shown recently that ALAD1 allele is associated with alcoholic liver injury. This association was ascribed to possible differences among isozymes in sensitivity to oxidized glutathione (GSSG), and this sensitivity is increased in erythrocytes of alcoholic patients. In the present study we measured erythrocyte ALAD activity from subjects with different ALAD genotype and found ALAD-1 is most sensitive to GSSG. We then investigated allele frequencies of ALAD in alcoholics (n = 126) and healthy controls (n = 115). For the control group, the frequencies were 0.94 (ALAD1) and 0.06 (ALAD2) and for the overall alcoholic group, 0.91 (ALAD1) and 0.09 (ALAD2). There were no significant differences in allele frequencies at the ALAD locus between the two groups. Subtyping the alcoholics according to the presence or absence of delirium tremens, hallucinosis, withdrawal seizure or liver cirrhosis failed to show statistically significant differences in the allele frequencies. We conclude that our data do not support the evidence of an allelic association between the ALAD1 and alcoholism.

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

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