Regulation of 5-aminolevulinate synthase in mouse erythroleukemic cells is different from that in liver

Cell biology of heme

Heme is a complex of iron with protoporphyrin IX that is essential for the function of all aerobic cells. Heme serves as the prosthetic group of numerous hemoproteins (eg, hemoglobin, myoglobin, cytochromes, guanylate cyclase, and nitric oxide synthase) and plays an important role in controlling protein synthesis and cell differentiation. Cellular heme levels are tightly controlled; this is achieved by a fine balance between heme biosynthesis and catabolism by the enzyme heme oxygenase. On a per-cell basis, the rate of heme synthesis in the developing erythroid cells is at least 1 order of magnitude higher than in the liver, which is in turn the second most active heme producer in the organism. Differences in iron metabolism and in genes for 5-aminolevulinic acid synthase (ALA-S, the first enzyme in heme biosynthesis) are responsible for the differences in regulation and rates of heme synthesis in erythroid and nonerythroid cells. There are 2 different genes for ALA-S, one of which is expressed ubiquitously (ALA-S1), whereas the expression of the other (ALA-S2) is specific to erythroid cells. Because the 5'-untranslated region of the erythroid-specific ALA-S2 mRNA contains the iron-responsive element, a cis-acting sequence responsible for translational induction of erythroid ALA-S2 by iron, the availability of iron controls protoporphyrin IX levels in hemoglobin-synthesizing cells. In nonerythroid cells, the rate-limiting step of heme production is catalyzed by ALA-S1, whose synthesis is feedback-inhibited by heme. On the other hand, in erythroid cells, heme does not inhibit either the activity or the synthesis of ALA-S but does inhibit cellular iron acquisition from transferrin without affecting its utilization for heme synthesis. This negative feedback is likely to explain the mechanism by which the availability of transferrin iron limits heme synthesis rate. Moreover, in erythroid cells heme seems to enhance globin gene transcription, is essential for globin translation, and supplies the prosthetic group for hemoglobin assembly. Heme may also be involved in the expression of other erythroid-specific proteins. Furthermore, heme seems to play a role in regulating either transcription, translation, processing, assembly, or stability of hemoproteins in nonerythroid cells. Heme oxygenase, which catalyzes heme degradation, seems to be an important enzymatic antioxidant system, probably by providing biliverdin, which is an antioxidant agent.

Characterization of the rhodobacter sphaeroides 5-aminolaevulinic acid synthase isoenzymes, HemA and HemT, isolated from recombinant Escherichia coli

The hemA and hemT genes encoding 5-aminolaevulinic acid synthase (ALAS) from the photosynthetic bacterium Rhodobacter sphaeroides, were cloned to allow high expression in Escherichia coli. Both HemA and HemT appeared to be active in vivo as plasmids carrying the respective genes complemented an E. coli hemA strain (glutamyl-tRNA reductase deficient). The over-expressed isoenzymes were isolated and purified to homogeneity. Isolated HemA was soluble and catalytically active whereas HemT was largely insoluble and failed to show any activity ex vivo. Pure HemA was recovered in yields of 5-7 mg x L-1 of starting bacterial culture and pure HemT at 10 mg x L-1 x HemA has a final specific activity of 13 U x mg-1 with 1 unit defined as 1 micromol of 5-aminolaevulinic acid formed per hour at 37 degrees C. The Km values for HemA are 1.9 mM for glycine and 17 microM for succinyl-CoA, with the enzyme showing a turnover number of 430 h-1. In common with other ALASs the recombinant R. sphaeroides HemA requires pyridoxal 5'-phosphate (PLP) as a cofactor for catalysis. Removal of this cofactor resulted in inactive apo-ALAS. Similarly, reduction of the HemA-PLP complex using sodium borohydride led to > 90% inactivation of the enzyme. Ultraviolet-visible spectroscopy with HemA suggested the presence of an aldimine linkage between the enzyme and pyridoxal 5'-phosphate that was not observed when HemT was incubated with the cofactor. HemA was found to be sensitive to reagents that modify histidine, arginine and cysteine amino acid residues and the enzyme was also highly sensitive to tryptic cleavage between Arg151 and Ser152 in the presence or absence of PLP and substrates. Antibodies were raised to both HemA and HemT but the respective antisera were not only found to bind both enzymes but also to cross-react with mouse ALAS, indicating that all of the proteins have conserved epitopes.

Comparison of the beluga whale (Delphinapterus leucas) expressed genes for 5-aminolevulinate synthase with those in other vertebrates

The cDNA and inferred amino acid sequences were determined for beluga whale (Delphinapterus leucas) erythroid (E) and housekeeping (H) forms of 5-aminolevulinate synthase (ALS), and they were compared with known sequences for five other vertebrates with particular attention to regulatory features. The cDNAs for whale ALS-E and -H encode, respectively, proteins of 582 and 640 amino acids. Sequence alignments suggest that the whale ALS-H, like those for rat and chicken, has an N-terminal mitochondrial targeting sequence of 56 amino acids. There is a high degree of amino acid conservation between the beluga whale proteins and those of other vertebrates, including regulatory elements and functional residues that have been defined in other ALSs. Both whale proteins contain three heme regulatory motifs suggesting that mitochondrial uptake may be regulated by heme. The ALS-E mRNA contains an iron responsive element in its 5'-untranslated region indicating that its expression may be post-transcriptionally regulated by cellular iron. This extensive structural similarity and the presence of the same regulatory elements found in other ALSs indicate that regulation of ALS in beluga whale is similar to that in other vertebrates.

Molecular mechanism of heme biosynthesis

Two of the major organs producing heme are bone marrow and the liver. delta-Aminolevulinate synthase (ALAS) plays the key role to regulate heme biosynthesis in hepatocytes as well as in erythroid cells. In the liver, nonspecific (or housekeeping) isozyme of ALAS (ALAS-N) is expressed to be regulated by its end product, heme, in a negative feedback manner. The way to regulate ALAS-N in the liver is suitable to supply a constant level of heme for a family of drug metabolizing enzymes, cytochrome P-450 (CYP). In erythroid tissues, not only erythroid-specific isozyme of ALAS (ALAS-E) but also ALAS-N are expressed, and regulated by distinctive manners. Although heme regulates ALAS-N in a negative feedback manner even in erythroid cells, ALAS-E is upregulated by induced heme concentration. ALAS-N in undifferentiated erythroid cells, therefore, is suggested to produce heme for CYP, whereas heme for accumulating hemoglobin (Hb) in cells undergoing differentiation is synthesized via ALAS-E. In this article, we describe the molecular mechanisms to regulate heme biosynthesis in non-erythroid as well as in erythroid tissues, and discuss the pathological significance of the mechanisms in patients with inherited disorders, porphyrias.

Heme binding by a bacterial repressor protein, the gene product of the ferric uptake regulation (fur) gene of Escherichia coli

The fur gene product, Fur, of Escherichia coli is a repressor when it binds Fe(II). Since heme and iron metabolism are closely linked and Fur is rich in histidine, a ligand for heme, the binding of heme to Fur was investigated. The oxidized Fur-heme complex is stable and low spin with a Soret maximum at 404 nm and no 620-nm band. CO coordinates with the reduced heme-Fur complex, causing a shift from 412 nm to 410 nm, and stabilizes it, increasing the half-life from 5 to 15 min. Circular dichroism (CD) spectra in the Soret region show heme bound in an asymmetric environment in Fur, both in the oxidized and reduced-CO forms. Quenching of tyrosine fluorescence by heme revealed rapid, tight binding (Kd < 1 microM) with an unusual stoichiometry of 1 heme:1 Fur dimer. Fur binds Mn(II), a model ligand for the endogenous Fe(II), much more weakly (Kd > 80 microM). Far-ultraviolet CD spectroscopy showed that the alpha-helix content of apo-Fur decreases slightly with heme binding, but increases with Mn(II) binding. Competition experiments indicated that heme interacts with Fur dimers at the same site as Mn(II) and can displace the metal. In contrast to Mn(II), Zn(II) did not quench the tyrosine fluoroescence of Fur, affected the CD spectrum less than Mn(II), but did bind in a manner which prevented heme from binding. In sum, Fur not only binds heme and Zn(II) with sufficient affinity to be biologically relevant, but the interactions that occur between these ligands and their effects on Mn(II) binding need to be taken into account when addressing the biological function of Fur.

Molecular regulation of iron proteins

Cellular iron metabolism comprises pathways of iron-protein synthesis and degradation, iron uptake via transferrin receptor (TfR) or release to the extracellular space, as well as iron deposition into ferritin and remobilization from such stores. Different cell types, depending on their rate of proliferation and/or specific functions, show strong variations in these pathways and have to control their iron metabolism to cope with individual functions. Studies with cultured cells have revealed a specific cytoplasmic protein, called 'iron regulatory protein' (IRP) (previously known as IRE-BP or IRF), that plays a key role in iron homoeostasis by regulating coordinately the synthesis of TfR, ferritin, and erythroid 5-aminolevulinate synthase (eALAS). Present in all tissues analysed, IRP is identical with the [4Fe-4S] cluster containing cytoplasmic aconitase. Under conditions of iron chelation, IRP is an apo-protein which binds with high affinity to specific RNA stem-loop elements (IREs) located 5' of the initiation codon in ferritin and eALAS mRNA, and 3' in the untranslated region of TfR mRNA. At 5' sites IRF blocks mRNA translation, whereas 3' it inhibits TfR mRNA degradation. Both effects compensate for low intracellular iron concentrations. Under high iron conditions, IRP is converted to the holo-protein and dissociates from mRNA. This reverses the control towards less iron uptake and more iron storage. Iron can therefore be considered as a feedback regulator of its own metabolism. It has recently become evident that nitric oxide, produced by macrophages and other cell types in response to interferon-gamma, induces the IRE-binding activity of IRF. Moreover measurements of the RNA-binding activity of IRP in tissue extracts may provide valuable information on iron availability.

Hereditary tyrosinaemia type I: a long-term study of the relationship between the urinary excretions of succinylacetone and delta-aminolevulinic acid

Patients with hereditary tyrosinaemia type I (HT) excrete large amounts of succinylacetone (SA) in urine. Owing to structural resemblance of SA to delta-aminolevulinic acid (ALA), SA inhibits the second enzyme in the pathway for haeme biosynthesis, porphobilinogen synthase, resulting in increased urinary ALA excretion. We investigated the relationship between urinary SA and ALA excretions of two patients with different forms of HT (late-infantile and juvenile). In both patients the urinary SA and ALA excretions showed a more or less inverse correlation. The patient with the early-infantile form of HT had a relatively greater increase in urinary SA and ALA excretions in comparison to the patient with the juvenile form of HT. A possible explanation for this unexpected inverse correlation between the urinary excretion of SA and ALA might be a lack of intramitochondrial glycine, a substrate for delta-aminolevulinic acid synthesis. It has been reported previously that high concentrations of SA reversibly and competitively inhibit the transport of glycine through membranes.

Sequential activation of genes for heme pathway enzymes during erythroid differentiation of mouse Friend virus-transformed erythroleukemia cells

Changes in the level of transcripts encoding enzymes of the heme biosynthetic pathway as well as those encoding ubiquitous proteins were examined in murine Friend virus-transformed erythroleukemia cells during erythroid cell differentiation induced by chemicals including dimethyl sulfoxide (DMSO). Early changes following DMSO treatment were marked decreases in mRNAs for three ubiquitous proteins, i.e., a 70 kDa heat shock protein (less than 6 h), heme oxygenase and nonspecific delta-aminolevulinate synthase (ALAS) (less than 12 h). These changes were followed by sequential increases in mRNAs for enzymes in the heme biosynthetic pathway. Namely, mRNAs for the erythroid-specific ALAS, delta-aminolevulinate dehydratase, porphobilinogen deaminase and uroporphyrinogen decarboxylase started to increase at 12, 18, 18-24 and 24 h, respectively. Nuclear runoff studies revealed that these changes are largely transcriptional. Treatments with other inducers of erythroid differentiation, e.g., hexamethylene bisacetamide, n-butyric acid and N'-methylnicotinamide, also showed similar effects on mRNAs as those following DMSO. These findings suggest that both suppression of ubiquitous genes and activation of heme pathway enzyme genes are associated with erythroid differentiation, and the former occurs preceding changes in the latter.