Different responses of the hepatic and erythropoietic delta-aminolevulinic acid synthetase of mice

Clinical and biochemical characteristics and genotype-phenotype correlation in 143 Finnish and Russian patients with acute intermittent porphyria

Acute intermittent porphyria (AIP), resulting from a deficiency of porphobilinogen deaminase (PBGD) in heme biosynthesis, is genetically heterogeneous and manifests with variable penetrance. The clinical outcome, prognosis, and correlation between PBGD genotype and phenotype were investigated in 143 Finnish and Russian AIP patients with 10 mutations (33G-->T, 97delA, InsAlu333, R149X, R167W, R173W, R173Q, R225G, R225X, 1073delA). Thirty-eight percent of the patients had experienced 1 or more acute attacks during their lives. The proportion of symptomatic patients has decreased dramatically from 49% to 17% among patients diagnosed before and after 1980, respectively. Patients with the R167W and R225G mutations showed lower penetrance (19% and 11%, respectively) and recurrence rate (33% and 0%, respectively) than patients with other mutations (range, 36%-67% and 0%-66%, respectively). Moreover, urinary excretions of porphyrins and their precursors were significantly lower in these patients (porphobilinogen [PBG], 47 +/- 10 vs. 163 +/- 21 micromol/L, p < 0.001; uroporphyrin, 130 +/- 40 vs. 942 +/- 183 nmol/d, p < 0.001). Erythrocyte PBGD activity did not correlate with PBG excretion in remission or with the clinical severity of the disease. Mutations R167W and R225G resulted in milder biochemical abnormalities and clinical symptoms indicating a milder form of AIP in these patients. In all AIP patients, normal PBG excretion predicted freedom from acute attacks. The risk of symptoms was highest for female patients with markedly increased PBG excretion (>100 micromol/L). Proper counseling contributed to the prevention of subsequent attacks in 60% of previously symptomatic and in 95% of previously symptom-free patients.

International air travel: a risk factor for attacks in acute intermittent porphyria

Five patients are reported with acute intermittent porphyria in whom attacks were apparently precipitated by international air travel. In four subjects this was the initial presenting attack and in a fifth the cause of an acute relapse in a patient requiring regular haem arginate prophylaxis. Multifactorial precipitants implicated include, dehydration, missed meals, alcohol use, infection, chronic hypoxia, premenstrual syndrome and stress. Acute intermittent porphyria should be suspected in individuals presenting with unexplained acute abdominal pain following international air travel. Appropriate precautions may reduce the incidence of attacks in known porphyrics.

Evolutionary consideration on 5-aminolevulinate synthase in nature

5-Aminolevulinic acid (ALA), a universal precursor of tetrapyrrole compounds can be synthesized by two pathways: the C5 (glutamate) pathway and ALA synthase. From the phylogenetic distribution it is shown that distribution of ALA synthase is restricted to the alpha subclass of purple bacteria in prokaryotes, and further distributed to mitochondria of eukaryotes. The monophyletic origin of bacterial and eukaryotic ALA synthase is shown by sequence analysis of the enzyme. Evolution of ALA synthase in the alpha subclass of purple bacteria is discussed in relation to the energy-generating and biosynthetic devices in subclasses of this bacteria.

5-Aminolevulinate synthase and the first step of heme biosynthesis

5-Aminolevulinate synthase catalyzes the condensation of glycine and succinyl-CoA to yield 5-aminolevulinate. In animals, fungi, and some bacteria, 5-aminolevulinate synthase is the first enzyme of the heme biosynthetic pathway. Mutations on the human erythroid 5-aminolevulinate synthase, which is localized on the X-chromosome, have been associated with X-linked sideroblastic anemia. Recent biochemical and molecular biological developments provide important insights into the structure and function of this enzyme. In animals, two aminolevulinate synthase genes, one housekeeping and one erythroid-specific, have been identified. In addition, the isolation of 5-aminolevulinate synthase genomic and cDNA clones have permitted the development of expression systems, which have tremendously increased the yields of purified enzyme, facilitating structural and functional studies. A lysine residue has been identified as the residue involved in the Schiff base linkage of the pyridoxal 5'-phosphate cofactor, and the catalytic domain has been assigned to the C-terminus of the enzyme. A conserved glycine-rich motif, common to all aminolevulinate synthases, has been proposed to be at the pyridoxal 5'-phosphate-binding site. A heme-regulatory motif, present in the presequences of 5-aminolevulinate synthase precursors, has been shown to mediate the inhibition of the mitochondrial import of the precursor proteins in the presence of heme. Finally, the regulatory mechanisms, exerted by an iron-responsive element binding protein, during the translation of erythroid 5-aminolevulinate synthase mRNA, are discussed in relation to heme biosynthesis.

Chemistry and biology of heme. Effect of metal salts, organometals, and metalloporphyrins on heme synthesis and catabolism, with special reference to clinical implications and interactions with cytochrome P-450

Although free porphyrins occur in nature in small quantities, no known function has been assigned to them. In contrast, heme and cobalamin, which are Fe and Co chelates of porphyrins or porphyrin derivatives, respectively, carry out crucial biological functions. Heme is the prosthetic group for a number of hemoproteins. These include myoglobin and hemoglobin, which carry out oxygen binding or transport; mitochondrial cytochromes aa3, b, c, and c3, which are important in transferring electrons; microsomal cytochrome P-450, which catalyzes mixed-function oxidations; catalase, which decomposes H2O2; peroxidase, which activates H2O2; and tryptophan pyrrolase, which catalyzes the oxidation of tryptophan. Recently, heme has also been shown to be the prosthetic group of prostaglandin and peroxide synthetase and indoleamine dioxygenase. The elegant studies of the biochemical pathway for the formation of heme demonstrated the arrangement in the porphyrin macrocycle of the carbon and nitrogen atoms originating from the eight glycine and the succinic acid molecule that are the precursors of porphyrins. There are eight enzymes involved in the synthesis of heme. The first and last three of these enzymes are localized in mitochondria, while the intermediate enzymes are localized in cytosol. The catalytic site of HMOX recognizes metalloporphyrins with central metal atoms other than iron; it favors some of these metalloporphyrins over heme as a potential substrate, sometimes by a large factor, permitting the synthetic heme analogue to serve as a potent competitive inhibitor of HMOX reaction. Since these synthetic metalloporphyrins do not bind molecular oxygen, they are not metabolically degraded by ring rupture and do not add to the body pool of bile pigment. One possible consequence of this competitive inhibition of heme degradation is suppression of bile pigment formation to such a degree that excessive plasma levels of bilirubin may be diminished. The studies of Drummond and Kappas (1981) and later studies in rats, mice, monkeys, and man, and also our studies have proved the latter phenomenon. The compound does not appear to affect the metabolic disposition of preformed bilirubin but inhibits biliary bilirubin excretion derived from the metabolism of endogenous or exogenous heme. Whether some of the effect of Sn-PP on naturally occurring or experimentally induced jaundice in animals reflects diversion of heme to nonheme to oxygenase-dependent pathways of heme metabolism, or whether a pathway which is normally latent becomes activated concurrent with HMOX inhibition is not known.(ABSTRACT TRUNCATED AT 400 WORDS)

Enzymatic defect in "X-linked" sideroblastic anemia: molecular evidence for erythroid delta-aminolevulinate synthase deficiency

Recently, the human gene encoding erythroid-specific delta-aminolevulinate synthase was localized to the chromosomal region Xp21-Xq21, identifying this gene as the logical candidate for the enzymatic defect causing "X-linked" sideroblastic anemia. To investigate this hypothesis, the 11 exonic coding regions of the delta-aminolevulinate synthase gene were amplified and sequenced from a 30-year-old Chinese male with a pyridoxine-responsive form of X-linked sideroblastic anemia. A single T----A transition was found in codon 471 in a highly conserved region of exon 9, resulting in an Ile----Asn substitution. This mutation interrupted contiguous hydrophobic residues and was predicted to transform a region of beta-sheet structure to a random-coil structure. Prokaryotic expression of the normal and mutant cDNAs revealed that the mutant construct expressed low levels of enzymatic activity that required higher concentrations of pyridoxal 5'-phosphate to achieve maximal activation than did the normal enzyme. The amino acid substitution occurred in the exon containing the putative pyridoxal 5'-phosphate binding site and may account for the reduced ability of the cofactor to catalyze the formation of delta-aminolevulinic acid.

Human delta-aminolevulinate synthase: assignment of the housekeeping gene to 3p21 and the erythroid-specific gene to the X chromosome

delta-Aminolevulinate synthase (ALAS) catalyzes the first committed step of heme biosynthesis. Previous studies suggested that there were erythroid and nonerythroid ALAS isozymes. We have isolated cDNAs encoding the ubiquitously expressed housekeeping ALAS isozyme and a related, but distinct, erythroid-specific isozyme. Using these different cDNAs, the human ALAS housekeeping gene (ALAS1) and the human erythroid-specific (ALAS2) gene have been localized to chromosomes 3p21 and X, respectively, by somatic cell hybrid and in situ hybridization techniques. The ALAS1 gene was concordant with chromosome 3 in all 26 human fibroblast/murine(RAG) somatic cell hybrid clones analyzed and was discordant with all other chromosomes in at least 6 of 26 clones. The regional localization of ALAS1 to 3p21 was accomplished by in situ hybridization using the 125I-labeled human ALAS1 cDNA. Of the 43 grains observed over chromosome 3, 63% were localized to the region 3p21. The gene encoding ALAS2 was assigned by examination of a DNA panel of 30 somatic cell hybrid lines hybridized with the ALAS2 cDNA. The ALAS2 gene segregated with the human X chromosome in all 30 hybrid cell lines analyzed and was discordant with all other chromosomes in at least 8 of the 30 hybrids. These results confirm the existence of two independent, but related, genes encoding human ALAS. Furthermore, the mapping of the ALAS2 gene to the X chromosome and the observed reduction in ALAS activity in X-linked sideroblastic anemia suggest that this disorder may be due to a mutation in the erythroid-specific gene.

Regulation of the genes for heme pathway enzymes in erythroid and in non-erythroid cells

There are eight enzymes in the heme biosynthetic pathway and three enzymes in the heme catabolic pathway. Enzymatic defects in heme biosynthesis lead to clinical conditions termed porphyrias. cDNAs for five of the eight enzymes in the heme biosynthetic pathway and two of the three enzymes in the heme catabolic pathway have been cloned and characterized in mammalian cells. At least two enzymes exist as isozymes between erythroid and non-erythroid tissues. One is delta-aminolevulinic acid synthase (ALAS), and the erythroid and hepatic isozymes are coded by two separate genes. The other is porphobilinogen deaminase (PBGD), and both the erythroid and the non-erythroid PBGD mRNA are transcribed from a single PBGD gene by alternate transcription and splicing. There is also a significant tissue-specific control of expression of the uroporphyrinogen decarboxylase gene which is expressed as a unique mRNA in all tissues.

delta-Aminolaevulinate synthase in human HepG2 hepatoma cells. Repression by haemin and induction by chemicals

delta-Aminolaevulinate (ALA) synthase, the rate-limiting enzyme in haem biosynthesis in the normal liver, was examined in human HepG2 hepatoma cells. Haemin, up to 100 microM, had no effect on ALA synthase activity in vitro; it did, however, exhibit a dose-dependent inhibitory action when added to cells growing in culture (half-maximal inhibition at 1 microM). The half-life of ALA synthase activity after haemin treatment was 2 h, which was similar to that found after treatment with cycloheximide. Cells treated with actinomycin D showed a longer half-life of the enzyme activity, i.e. 4 h, compared with haemin or cycloheximide treatment. Treatment of cells with succinylacetone markedly inhibited the activity of ALA dehydratase and 59Fe incorporation into haem, but in increased ALA synthase activity. Both the haemin-induced repression and the succinylacetone-mediated de-repression of ALA synthase activity were reversible within 4 h after replacing the medium with fresh medium without the chemical. In addition to succinylacetone, dimethyl sulphoxide and 3-methylcholanthrene induced the enzyme. Induction of ALA synthase by these chemicals was also suppressed by treatment of cells with haemin. These findings indicate that the level of ALA synthase in HepG2 cells is maintained by both synthesis and degradation of the enzyme, and that the synthesis of the enzyme is regulated by the concentration of regulatory free haem in the cell.

Expression of delta-aminolevulinate synthase in avian cells: separate genes encode erythroid-specific and nonspecific isozymes

A controversy has existed in the literature for the past several years regarding the number of vertebrate genes encoding the mitochondrial protein that initiates the first step in heme biosynthesis, delta-aminolevulinate synthase [ALAS; succinyl-CoA: glycine C-succinyltransferase (decarboxylating), EC 2.3.1.37]. By analysis of chicken ALAS cDNA clones isolated from both liver and erythroid cells, we show that at least two separate genes encode ALAS mRNAs. These experiments show that (i) two different genes encode the ALAS isozymes found in erythroid and in liver tissues, and (ii) while the product of the erythroid gene (ALASE) is expressed exclusively in erythroid cells, the hepatic form of the enzyme is expressed ubiquitously, suggesting that this is the nonspecific form (ALASN) found in all chicken tissues.

An immunochemical study of delta-aminolevulinate synthase and delta-aminolevulinate dehydratase in liver and erythroid cells of rat

The relationship between erythroid delta-aminolevulinate (ALA) synthase and hepatic ALA synthase in rat was analyzed immunochemically, using antibodies directed against rat liver ALA synthase and against chicken liver ALA synthase. Rat erythroid ALA synthase showed no cross-reactivity with anti-liver ALA synthase antibodies, but hepatic ALA synthases from rat, mouse, and chicken share substantial cross-reactivity with one another. These results clearly distinguish the isozyme relationship between erythroid ALA synthase and hepatic ALA synthase in rat and suggest that there may be at least two different ALA synthase genes in rat. ALA dehydratase in rat liver, on the other hand, could not be immunochemically distinguished from ALA dehydratase in rat erythroid cells when antibody against rat erythroid ALA dehydratase was used. The finding that erythroid ALA synthase is an entity different from hepatic ALA synthase may provide a clue to understanding the different features in hepatic and erythropoietic porphyrias.

Nucleotide sequence of mouse 5-aminolevulinic acid synthase cDNA and expression of its gene in hepatic and erythroid tissues

The cDNA coding for 5-aminolevulinic acid (ALA) synthase (EC 2.3.1.37) in both liver and anemic spleen of the mouse has been cloned. The liver clone was selected by complementation of an Escherichia coli hemA mutant. Erythroid clones were obtained by screening a cDNA library made from mouse anemic spleen RNA, using the liver cDNA as a probe. The sequences of the spleen-derived and liver-derived cDNAs are identical. The nucleotide sequence and predicted amino acid (aa) sequence of a 1.85-kb spleen-derived cDNA is presented. The mouse ALA synthase as sequence displays extensive homology to ALA synthase of chick embryonic liver. The ALA synthase mRNA, detected by Northern blot analysis, was the same size, approx. 2.3 kb, in mouse liver, anemic spleen, and mouse erythroleukemia cells. It is therefore unlikely that different isozymic forms of ALA synthase are present in mouse erythroid and hepatic tissue and this is not the basis for the different effects of heme and porphyrinogenic compounds on the expression of liver and erythroid ALA synthase.

Isolation of recombinant cDNAs encoding chicken erythroid delta-aminolevulinate synthase

We report the isolation of cDNA clones encoding delta-aminolevulinate synthase (ALA synthase; EC 2.3.1.37), the first enzyme in the heme biosynthetic pathway in animal cells. The gene was isolated from a chicken erythroid cDNA library prepared in the bacteriophage lambda fusion/expression vector gt11, using rabbit antibody raised against the relatively abundant chicken liver enzyme. The chicken liver and red cell ALA synthase isozymes share substantial crossreactivity to the antibody, thereby allowing isolation of the erythroid-specific gene by using the heterologous antibody in immune screening of the red cell cDNA library. Preliminary analysis documenting the tissue specificity of transcription indicates that the enzyme is encoded by a highly homologous set of messages, which appear to differ in size in various avian tissues. From analysis using strand-specific RNA probes, it appears that the different ALA synthase mRNAs detected may be transcribed from a family of genes that are closely related in nucleotide sequence and are each regulated in a developmentally specific manner.

delta-Aminolevulinate synthase isozymes in the liver and erythroid cells of chicken

Antibodies raised against the purified chicken liver delta-aminolevulinate synthase showed a partial cross-reactivity with the chicken erythroid delta-aminolevulinate synthase. delta-Aminolevulinate synthase synthesized in vitro using polysomes from erythroid cells showed a subunit molecular weight of 55,000, whereas the enzyme synthesized in vitro using liver polysomes had a subunit molecular weight of 73,000. delta-Aminolevulinate synthase isolated from mitochondria of erythroid cells showed a molecular weight of 53,000, while the enzyme in liver mitochondria had a value of 65,000. These observations imply that the erythroid delta-aminolevulinate synthase differs from the hepatic enzyme.

Induction of delta-aminolaevulinate synthetase under environmental-stress conditions

1. Exposure of rats to environmental-stress conditions of hypobaria, hypoxia and cold did not alter the activity of hepatic delta-aminolaevulinate synthetase. 2. Induction of the enzyme by diethoxycarbonyldihydrocollidine was inhibited when the rats were exposed to hypobaria before or during the treatment with the drug but not after the initial phase when the process of induction was initiated. Neither increased concentration of the drug nor the time of induction had any effect on the inhibition under hypobaria. 3. A period of 12-24h of pre-exposure to hypobaria gave the maximum inhibition, and on longer exposure the inhibitory effect was decreased. 4. The inhibition was not a permanent effect and could be substantially reversed in 12h of withdrawal to ambient pressure. 5. Inhibition of induction was found under hypobaria and hypoxia, but not on exposure to cold. This suggests a specific effect of lack of O(2) rather than a general effect of stress. 6. It appears possible that alteration of concentration of endogenous adenine nucleotides may control the process of diethoxycarbonyldihydrocollidine-mediated induction of delta-aminolaevulinate synthetase, since treatment with ATP, cyclic AMP or theophylline produced inhibition similar to that under hypobaria and hypoxia.