Mechanism of allylisopropylacetamide-induced increase of -aminolevulinate synthetase in liver mitochondria. V. Mechanism of regulation by hemin of the level of -aminolevulinate synthetase in rat liver mitochondria

Heme: a regulator of rat hepatic tryptophan 2,3-dioxygenase?

The hepatic cytosolic hemoprotein tryptophan 2,3-dioxygenase (TDO) is the rate-limiting enzyme in tryptophan catabolism and thus plays a key role in regulating the physiological flux of tryptophan into relevant metabolic pathways. The TDO protein is induced by corticosteroids such as dexamethasone (DEX) and is stabilized by its prosthetic heme. In rats, acute chemically induced hepatic heme depletion reduces the functional hepatic TDO levels to 25-30% of basal levels within 1 h, and this decrease persists beyond 28 h of heme depletion at which time only 25-30% of the protein is available for heme incorporation. Since this could stem from impaired de novo synthesis and/or instability of the newly synthesized apoTDO protein in the absence of heme, we examined the specific role of heme in these events in a previously validated rat model of acute hepatic heme depletion triggered by the P450 suicide substrate 3, 5-dicarbethoxy 2,6-dimethyl-4-ethyl-1,4-dihydropyridine. We now show that exogenous heme can reverse the functional impairment of the enzyme observed during hepatic heme depletion and fully restore the impaired DEX-mediated induction of the enzyme to normal. Furthermore, through Northern/slot blot analyses coupled with nuclear run-on studies, we now document that this heme regulation of TDO is exerted primarily at the transcriptional level. Immunoblotting analyses also reveal corresponding changes in the TDO protein, thereby establishing that heme is necessary for DEX-inducible TDO mRNA transcription and subsequent translation. Thus, the TDO gene may contain heme-regulatory elements in addition to the reported glucocorticoid-responsive elements. Together, these findings suggest that clinically, hepatic heme deficiency may enhance the tryptophan flux into synthetic (serotonergic) pathways, not only by depriving prosthetic heme for a functionally competent TDO hemoprotein, its primary catabolic enzyme, but also by impairing the de novo synthesis of this enzyme.

Role of interleukin-1beta in the modulations of cytochrome P450 and heme metabolism in rat liver

The effect of recombinant human interleukin-1beta (IL-1beta) on the modulation of hepatic cytochrome P450 (P450) was investigated by in vivo subcutaneous dosing studies in male Sprague-Dawley rats. To assess the effect of IL-1beta on heme metabolism, we determined the delta-aminolevulinic acid synthetase (delta-ALAS) and heme oxygenase activities in the liver. IL-1beta suppressed the microsomal total P450 and heme contents and delta-ALAS activity in the liver. In contrast, microsomal heme oxygenase activity was significantly increased by the IL-1beta treatments. Western blot analysis and marker enzyme activities for individual P450 isoforms demonstrated that IL-1beta suppressed CYP2C6, 2C13, 2E1, and 3A2, whereas CYP2A, 2B1/2, 2C11, and 4A1 were not influenced by the treatments. IL-1beta inhibited both allylisopropylamide- and phenobarbital-inducible delta-ALAS activities in the liver. These results indicate that IL-1beta has differential effects on the constitutive P450, and also on delta-ALAS and heme oxygenase activities in rat liver. Thus, the modulation of hepatic P450 by IL-1beta is complex, and IL-1beta may be involved in the regulation of both apoprotein synthesis for each P450 isoform and the heme pools in the liver.

Mechanism of the synergistic induction of CYP2H by isopentanol plus ethanol: comparison to glutethimide and relation to induction of 5-aminolevulinate synthase

We had previously found that combined treatment with isopentanol and ethanol synergistically induced CYP2H protein and activity in cultured chick nepatoytes. Here we investigated the mechanism of induction of CYP2H by the alcohols and whether they caused a coordinate induction of 5-aminolevulinate synthase (ALAS) mRNA. Treatment with isopentanol alone or in combination with ethanol resulted in coordinate increases in CYP2H1 and ALAS mRNAs. With isopentanol alone, the amounts of CYP2H1 and ALAS mRNAs at 4 to 6 h were similar to those observed after treatment with the alcohol combination, but declined by 11 h. Readdition of isopentanol at 11 h again increased the expression of both mRNAs, indicating that the decreases at 11 h were due to limiting amounts of inducer. Similar results were observed in cells exposed to low concentrations of glutethimide. In the combined alcohol treatment, increases in CYP2H1 and ALAS mRNAs were sustained from 4 h to 11 h after addition of the alcohols, but decreased to control levels by 24 h. Using pulse labeling to measure de novo synthesis of CYP2H1/2 protein, we found that the increases in CYP2H1/2 protein reflected the increases in CYP2H1 mRNA. The half-life of CYP2H1/2 protein, measured from pulse-chase experiments, was approximately twofold greater than the half-life of CYP2H1 mRNA. Our results indicate that the alcohols and glutethimide coordinately increase ALAS and CYP2H1 mRNA, and that increases in CYP2H1/2 protein arise from increases in its mRNA.

Hepatic porphyrias

The porphyrias are metabolic disorders characterized by abnormal heme biosynthesis with excessive accumulation and excretion of porphyrias or porphyrin precursors. Defects in the enzymes of the heme biosynthetic pathway result in porphyria. Several of the disorders have been classified as hepatic because the major site of the biochemical defect has been localized to the liver. This article describes the enzymes of the heme biosynthetic pathway, the clinical features of the hepatic porphyrias and management of the disorders.

Novel regulation of delta-aminolevulinate synthase in the rat harderian gland

The mode of expression of delta-aminolevulinate synthase (ALAS), as well as that of mRNAs for other heme pathway enzymes, was examined in the rat Harderian gland. Northern blot and in situ hybridization analyses demonstrated that the non-specific ALAS (ALAS-N) mRNA is highly expressed in this tissue, whereas the erythroid-specific ALAS (ALAS-E) mRNA is not. Immunoblot analysis of ALAS also confirmed this finding at the protein level. ALAS-N mRNA was maximally induced in the Harderian gland and was not increased further by treatment of animals with 2-allyl-2-isopropylacetamide (AIA). The levels of mRNAs for other heme pathway enzymes, i.e., delta-aminolevulinate dehydratase, porphobilinogen deaminase, uroporphyrinogen decarboxylase, and coproporphyrinogen oxidase, also were increased markedly in the Harderian gland and not influenced by AIA treatment. The level of ferrochelatase (FeC) mRNA in the gland was, however, lower than that in the liver. The gland contained an extremely high level of protoporphyrin, while heme was undetectable. Microsomal heme oxygenase-1 (HO-1) mRNA levels were significantly higher in the Harderian gland than in the liver. When isolated glands were incubated with hemin in vitro in organ cultures, the level of HO-1 mRNA was increased, whereas the ALAS-N mRNA level was not. These findings indicate that markedly elevated levels of protoporphyrin and extremely low levels of heme in the Harderian gland are the results of both decreased expression of FeC and markedly increased expression of ALAS-N and HO-1. The constitutive expression of the ALAS-N gene in the Harderian gland suggests a novel transcriptional control mechanism of this gene.

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.

Heme regulates hepatic 5-aminolevulinate synthase mRNA expression by decreasing mRNA half-life and not by altering its rate of transcription

Hepatic 5-aminolevulinate (ALA) synthase, the first and rate-limiting enzyme in the heme biosynthetic pathway, is known to be feedback repressed by the end product of the pathway, heme. We investigated whether heme regulates ALA synthase mRNA expression transcriptionally or post-transcriptionally in primary cultures of chick embryo hepatocytes. 2-Propyl-2-isopropylacetamide increased the rate of transcription of the ALA synthase gene, whereas heme or an inhibitor of heme biosynthesis, desferrioximine, had no effect on the drug-induced transcription rate. Heme decreased the half-life of ALA synthase mRNA from approximately 3.5 h to 1.2 as recently reported by Drew and Ades (1989, Biochem. Biophys. Res. Commun. 162, 102-107). We also found that the heme-mediated decrease in mRNA stability was prevented by cycloheximide treatment, suggesting that the heme effect was mediated by a labile protein. These results support a model for hepatic ALA synthase regulation in which inducing drugs directly stimulate ALA synthase gene transcription, whereas heme regulates ALA synthase expression post-transcriptionally by modulating mRNA stability as well as by blocking translocation of ALA synthase enzyme into the mitochondrion.

Erythroid 5-aminolaevulinate synthase activity during normal and iron deficient erythropoiesis

Reduced erythroblast 5-aminolaevulinate (ALA) synthase activity was observed during iron/haem deficient erythropoiesis. Enzyme activity was reduced approximately threefold to levels similar to those previously detected during sideroblastic erythropoiesis. This response would appear to be erythroblast specific as haem deficiency is known to stimulate hepatic ALA synthase activity. It is, however, unclear as to whether this reduced enzyme activity relates to iron deficiency or to the consequent haem deficiency.

Elevation of delta-aminolevulinic acid synthase and cytochrome PB1 P450 messenger RNA levels by dihydropyridines, dihydroquinolines, sydnones, and N-ethylprotoporphyrin IX

A series of compounds that increase the activity of delta-aminolevulinic acid synthase (ALAS) in chick embryo hepatocyte cultures were studied for their effects on steady-state levels of mRNA for ALAS and phenobarbital-inducible cytochrome PB1 P450. N-Ethylprotoporphyrin IX (N-EtPP), which is believed to lower heme levels by inhibition of ferrochelatase (FC), had little effect on steady-state ALAS mRNA levels. 3,5-Diethoxycarbonyl-1,4-dihydro-2,6-dimethyl-4- isobutylpyridine (4-isobutyl DDC), which is believed to lower heme levels by repetitive destruction of the heme moiety of cytochrome P450, increased steady-state levels of ALAS mRNA levels approximately 2-fold. 3,5-Diethoxycarbonyl-1,4-dihydro-2,6-dimethyl-4-ethylpyridine (4-ethyl DCC) which inhibits FC activity and destroys the heme moiety of cytochrome P450, increased ALAS mRNA levels approximately 4-fold. A combination of N-EtPP and 4-isobutyl DDC produced a synergistic increase in ALAS mRNA levels to approximately 6-fold over control levels. The synergistic increase in ALAS activity observed previously with this combination can be explained, at least in part, by a synergistic increase in ALAS mRNA levels. Other porphyrinogenic agents, which function as mechanism-based inactivators of cytochrome P450 and elevate ALAS activity, were found to elevate ALAS mRNA. These compounds included 3-[2-(2,4,6-trimethylphenyl)thioethyl]-4-methylsydnone (TTMS), 2,4-diethyl-2-methyl-1,2-dihydroquinoline (DMDQ), and 2,2,4-trimethyl-1,2,dihydroquinoline (TMDQ). The elevation of ALAS mRNA by these porphyrinogenic agents is probably due to their lowering of cellular heme levels by a combination of ferrochelatase inhibition and repetitive destruction of the heme moiety of cytochrome P450. The lowering of heme levels should result in an enhancement of ALAS mRNA half-life as it has been demonstrated by others that heme shortens the half-life of ALAS mRNA. It was of interest that some of these drug treatments also caused an elevation in steady-state levels of cytochrome PB1 P450 mRNA; the exception was TTMS, which along with its analogue 3-(2-phenylethyl)-4-methylsydnone (PEMS), did not alter cytochrome PB1 P450 mRNA levels. Increases in steady-state levels of cytochrome PB1 P450 mRNA subsequent to increases in steady-state levels of ALAS mRNA were observed with 4-ethyl DDC, 4-isobutyl DDC, DMDQ, and TMDQ. The data obtained with N-EtPP and a combination of N-EtPP and 4-isobutyl DDC on cytochrome PB1 P450 mRNA levels do not support the contention that heme functions as a positive regulator of cytochrome P450 gene expression.

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.

Effects of 3-(2-phenylethyl)-4-methylsydnone and related sydnones on heme biosynthesis

3-[2-(2,4,6-Trimethylphenyl)thioethyl]-4-methylsydnone (TTMS) and 3-(2-phenylethyl)-4-methylsydnone (PEMS) cause mechanism-based inactivation of rat hepatic microsomal cytochrome P-450 and the formation of N-alkylprotoporphyrins in rat liver. In the present study, we have shown that both TTMS and PEMS cause mechanism-based inactivation of chick embryo hepatic microsomal cytochrome P-450. TTMS also caused the inhibition of ferrochelatase activity, the accumulation of protoporphyrin IX, and an increase in the activity of delta-aminolevulinic acid synthase in chick embryo liver cell culture. PEMS was devoid of effect on ferrochelatase activity, porphyrin accumulation, and delta-aminolevulinic acid synthase activity. There are two possible explanations for the lack of effect of PEMS on heme biosynthesis: (1) the ring-A- and/or ring-B-substituted regiosomers of the N-phenylethyl- and N-phenylethenylprotoporphyrins which are produced during the mechanism-based inactivation of cytochrome P-450 by PEMS are too bulky to fit into the active site of ferrochelatase to inhibit its activity, in contrast to the N-vinylprotoporphyrin formed from TTMS; and (2) the N-alkylprotoporphyrins produced consist of the ring-C- and/or ring-D-substituted regioisomers, which are not inhibitors of ferrochelatase activity.

In vivo prevention and reversal by the antimitotic colchicine and other related compounds of some porphyrinogenic drug action on heme pathway

1. The effect of colchicine, vincristine and griseofulvin (GRIS) on the porphyrinogenic action of 2-allyl-2-isopropylacetamide (AIA) and veronal was studied in vivo and using the in vitro experimental model of tissue explant cultures. 2. Complete prevention by colchicine was found in liver and heart explant from animals treated with AIA and veronal. 3. Vincristine, GRIS and colchicine reversed AIA induction in liver explants, however reversal was partial or nil in skin and heart explants depending on the antimitotic and the tissue. 4. The usefulness of the combination of the in vivo experimental model and the in vitro explant tissue culture model, for this kind of studies is emphasized.

Synergistic induction of delta-aminolevulinic acid synthase activity by N-ethylprotoporphyrin IX and 3,5-diethoxycarbonyl-1,4-dihydro-2,6-dimethyl-4-isobutylpyridine

The levels of the cellular free heme pool in chick embryo hepatocyte culture were lowered using N-ethylprotoporphyrin IX (N-ethylPP) and analogues of 3,5-diethoxycarbonyl-1,4-dihydro-2,4,6-trimethylpyridine (DDC), and the effect on delta-aminolevulinic acid synthase (ALAS) was examined. N-EthylPP, which lowers cellular heme levels by inhibiting ferrochelatase activity, produced an induction of ALAS activity to 444% of control at 3 hr after its administration. 4-Ethyl DDC, which lowers heme levels by destroying the heme moiety of cytochrome P-450 and lowering ferrochelatase activity, caused an induction of ALAS to 565% of control at 12 hr after administration. 4-Isobutyl DDC, which lowers heme levels by destroying the heme moiety of cytochrome P-450, induced the activity of ALAS to 289% of control at 3 hr after administration. This indicates that ferrochelatase inhibition is a more important mechanism of heme lowering than alkylation of cytochrome P-450 heme when both heme-depleting mechanisms are acting in chick embryo liver cells. It was anticipated that administration of a combination of 4-isobutyl DDC plus N-ethylPP would mimic the effect of 4-ethyl DDC. However, this combination induced ALAS activity to levels that were much greater than those observed after 4-ethyl DDC (1257% of control at 12 hr). This synergistic induction may be attributable to lowering of free heme levels to the point where transcription, translation, and translocation of ALAS are all derepressed.

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.

Control of 5-aminolevulinate synthase in animals

The proposed mechanism by which hepatic ALV-synthase mitochondrial levels are regulated is outlined in Fig. 2. ALV-synthase catalyzes the first and rate-limiting step in the heme pathway and is normally present in low amounts. A cytosolic, regulatory free heme pool tightly controls the amount of ALV-synthase in two ways. In the primary mechanism of regulation, heme is proposed to inhibit the synthesis of ALV-synthase mRNA. Most likely this would be mediated through the action of specific heme-binding protein(s) which recognize regulatory control regions of the ALV-synthase gene. Gene activity therefore is significantly repressed most of the time. When there is an increased demand for heme by newly synthesized cellular hemoproteins, the free heme pool is reduced, leading to a derepression of ALV-synthase mRNA synthesis. Once the need for increased heme synthesis is satisfied, inhibitory heme levels build up again. When drugs such as phenobarbital are administered to animals, there is a rapid induction in the liver of both cytochrome P-450 and ALV-synthase. It is proposed that the heme pool governing ALV-synthase levels is lowered by the increased heme demand due to cytochrome P-450 apoprotein formation. The primary event in the drug induction of ALV-synthase is therefore the increased synthesis of cytochrome P-450 apoprotein. However, the mechanism by which this occurs is unknown, although drugs do increase the synthesis of mRNA for cytochrome P-450 (Fig. 2). (There is evidence that for the aromatic hydrocarbons a specific cytosolic receptor exists.) In the acute hepatic porphyria diseases, uncontrolled synthesis of hepatic ALV-synthase occurs. The various forms are characterized by reduced levels of one of the heme pathway enzymes other than ALV-synthase. Attacks of the disease are commonly precipitated by drugs which induce cytochrome P-450, and the uncontrolled accumulation of ALV-synthase which accompanies these attacks results from the combined action of the block in the heme pathway and the increased cytochrome P-450 levels. A major challenge which now exists is to understand at the molecular level how the genes for ALV-synthase and cytochrome P-450 are regulated in the liver and other tissues.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Inhibition of experimental porphyria with colchicine

Colchicine at the concentrations of 5 X 10(-7) - 5 X 10(-6) M decreased significantly both delta-aminolevulinic acid synthase activity and accumulation of porphyrins in monolayers of chick embryo liver cells induced by allyl-isopropylacetamide, by 3,5-diethoxycarbonyl-1,4-dihydrocollidine or by phenobarbitone. No effect was noted in non-induced cells. In rats, colchicine 0.3 mg/kg, reduced significantly the allyl-isopropylacetamide induced increase in the activity of delta-aminolevulinic acid synthase in the liver and the concentration of urinary porphyrins while it did not affect these parameters in non-induced rats.

Relation of the extra-sequence of the precursor form of chicken liver delta-aminolevulinate synthase to its quaternary structure and catalytic properties

Precursor and mature forms of delta-aminolevulinate (ALA) synthase were purified to near homogeneity from chicken liver mitochondria and cytosol, respectively, and their properties were compared. The enzyme purified from mitochondria had apparently the same subunit molecular weight (65,000) as that of the native mitochondrial enzyme. The enzyme purified from the cytosol fraction, however, showed a subunit molecular weight of about 71,000 which was somewhat smaller than that estimated for the native cytosolic enzyme (73,000). The enzyme purified from liver cytosol seems to have been partially degraded by some endogenous protease during the purification, but may have the major part of the signal sequence. On sucrose density gradient centrifugation, the purified mitochondrial and cytosolic ALA synthases showed an apparent molecular weight of about 140,000, indicating that both enzymes exist in a dimeric form. The ALA synthase synthesized in vitro was also shown to exist as a dimer. Apparently the extra-sequence does not interfere with the formation of dimeric form of the enzyme. The purified cytosolic ALA synthase had a specific activity comparable to that of the purified mitochondrial enzyme. Kinetic properties of the two enzymes, such as the pH optimum and the apparent Km values for glycine and succinyl-CoA, were quite similar. The extra-sequence does not appear to affect the catalytic properties of ALA synthase. The isoelectric point of the cytosolic ALA synthase was 7.5, whereas that of the mitochondrial enzyme was 7.1. This suggests that the extra-sequence in the cytosolic enzyme may be relatively rich in basic amino acids.

On the role of catalase in the oxidation of tissue fatty acids

The role of catalase in lipid metabolism has been studied by means of a comparison of the turnover characteristics of the major lipid classes in the normal mouse with those of animals in which the catalase activity had been inhibited and blocked by aminotriazole and allylisopropylacetamide. Double isotope ratios were determined in the lipid fractions of several tissues following the injection of labeled glycerol, and a number of significant differences were identified between these treatments. Since catalase is recognized as an integral component of the peroxisomal pathway of fatty acid oxidation, these results may be taken as indicating that interruption of the process of peroxisomal beta-oxidation in this manner cause extensive perturbations of lipid metabolism in the living animal, and these perturbations extend well beyond those tissues where the predominant localization of these organelles occurs. The concept which derives from these data--that of a significant regulatory role of peroxisomes in relation to the overall balance of lipid metabolism in the animal body--is described and discussed.

Hemin inhibits transfer of pre-delta-aminolevulinate synthase into chick embryo liver mitochondria

Pulse labelling studies in chick embryo livers show that hemin prevents the transfer of drug induced pre-delta-aminolevulinate synthase from the cytosol into the mitochondria, leading to an accumulation of precursor in the cytosol. No effect of hemin was observed on the transfer of pre-pyruvate carboxylase into mitochondria. These results eliminated a general toxic effect of hemin on mitochondrial import of proteins and are consistent with the view that hemin specifically inhibits the transfer of ALA synthase.

Inhibition by hemin of in vitro translocation of chicken liver delta-aminolevulinate synthase into mitochondria

The precursor form of chicken liver delta-aminolevulinate synthase was synthesized in a reticulocyte lysate cell-free translation system and then incubated with the homologous liver mitochondria. The precursor enzyme was incorporated into the mitochondria with an attendant processing to the mature enzyme. In this in vitro experimental system, both the transport and processing of the enzyme were significantly inhibited by the addition of hemin as low as about 3 microM. This provides further support to the view, which had been derived from the studies in vivo, that inhibition by hemin of the translocation of delta-aminolevulinate synthase into mitochondria could be one of the regulatory mechanisms for heme biosynthesis in the liver cells.

Translational inhibition by heme of the synthesis of hepatic delta-aminolevulinate synthase in a cell-free system

Synthesis of delta-aminolevulinate synthase in a rabbit reticulocyte lysate system directed by total polysomes from the liver of allylisopropylacetamide-treated rats was studied with the combined use of [3H]leucine and a specific rabbit antibody. The protein synthesis observed in the cell-free system employed represented mainly the peptide chain elongation and its termination rather than the net synthesis involving initiation. Synthesis of delta-aminolevulinate synthase in this cell-free system was inhibited progressively with the increased addition of hemin; the synthesis was reduced to about 40% by about 30 microM hemin. Synthesis of total protein, however, was not significantly affected by the addition of hemin. The data obtained suggest that heme inhibits a peptide chain elongation step in the synthesis of delta-aminolevulinate synthase.

Evidence relating the inhibitory effect of cobalt on the activity of delta-aminolevulinate synthase to the intracellular concentration of porphyrins

This study shows that the inhibition of ALAS activity caused by cobalt is directly correlated with the intracellular porphyrin concentration, thus indicating that cobalt exerts its inhibitory effect on ALAS activity as a result of the formation of cobalt-protoporphyrin.

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.

Regulation of rat hepatic delta-aminolevulinic acid synthetase and heme oxygenase activities: evidence for control by heme and against mediation by prosthetic iron

1. The effect of in vivo administration of 6 compounds on the activity of delta-aminolevulinic acid (ALA) synthetase and heme oxygenase were determined. 2. The order of decreasing potency in reducing ALA synthetase activity was heme, bilirubin, protoporphyrin IX, bilirubin dimethyl ester, CoCl2 and FeCl3. 3. The chelating agents EDTA and deferoxamine did not prevent heme's repression of ALA synthetase or induction of heme oxygenase activity. 4. The dose response, time course, enzyme subcellular distribution and chelation antagonism studies all suggest that heme itself, and not iron, regulates the rate limiting enzymatic steps of rat hepatic heme synthesis and degradation.

Transport of newly synthesized proteins into mitochondria - a review

This review examines the mechanism of translocation of cytoplasmically synthesized proteins into mitochondria. Approximately 10% of the mitochondrial proteins are synthesized within the organelles while most mitochondrial proteins are coded for by nuclear genes and synthesized on cytoplasmic ribosomes. Those mitochondrial proteins synthesized on cytoplasmic ribosomes have to be transferred at some point into one of the mitochondrial compartments, a process which would require their insertion through one or both mitochondrial membranes. Data accumulated during the past five years indicate that the cytoplasmically synthesized mitochondrial proteins are synthesized on free polysomes then released into the cytoplasm. Most of the proteins examined so far are synthesized in the cytoplasm as larger precursors whose conformations may differ from the conformations of their respective mature forms. These precursor proteins become translocated into mitochondrial post-translationally and processed to their mature forms either during or immediately following translocation into the organelles. The translocation step appears to require mitochondrial ATP. Some processing activities have been localized in the matrix fractions of mitochondria from liver and yeast and they appear to be associated with soluble endopeptidases which act selectively on precursors of mitochondrial proteins. Although it is not clear how the precursor proteins interact with or recognize mitochondrial membranes, studies in yeast indicate that the interactions occur at specific regions on the other mitochondrial membranes.

Liver damage in rats by allylisopropylacetamide (AIA), an inducer of delta-aminolevulinic acid synthetase (ALAS)

Twentyfour hours after the intraperitoneal injection of a single dose of allylisopropylacetamide (AIA) in amounts of 100 mg/kg or more to 30-day old male Wistar rats, the livers of most of the animals showed an irregular yellow coloration and a fragile consistency. No macroscopic changes were detected following AIA doses of 25 or 50 mg/kg. Bromsulphthalein retention was not significantly increased after the administration of 25 mg/kg AIA, but distinctly enhanced after 400 mg/kg. Succinate-dehydrogenase-activity in liver homogenate was not altered after 25 mg/kg, but significantly decreased after 400 mg/kg. Ethylmorphine-N-demethylation activity was enhanced after small doses without increase of cytochrome P-450 (cyt. P-450) concentration and decreased after higher doses, accompanied by a remarkable cyt. P-450 loss. GPT activity in serum and liver was not altered after both 25 and 400 mg/kg AIA. GOT activity was slightly but significantly enhanced both in serum and liver after the high dose of 400 mg/kg. Thus in addition to the well-known cyt. P-450 destruction other signs of hepatotoxicity could be demonstrated.

Iron and the liver. Acute and long-term effects of iron-loading on hepatic haem metabolism

We have determined the dose-response curves (100-900 mg of Fe/kg body wt.) and the time course over 84 days for the effects of a single injection of iron-dextran on rat hepatic 5-aminolaevulinate synthetase, cytochrome P-450, iron content, and GSH (reduced glutathione). Porphyrins in liver and urine have also been measured. (1) At 2 days after treatment, a dose of 500 mg of Fe/kg produced a 20-fold increase in iron concentration, which was maintained for 14 days. Total hepatic iron remained constant over 63 days, falling slightly by 84 days. (2) The activity of 5-aminolaevulinate synthetase was maximally increased (6-fold) 12-24 h after iron treatment. By 48 h the activity fell to less than twice the control value and thereafter remained slightly above the control value (1.1-1.5-fold) until 84 days after iron treatment. Liver GSH concentrations were unaffected by iron. Porphyrins in liver and urine were either unchanged or decreased. (3) Hepatic cytochrome P-450 decreased after iron treatment to a minimum (63% of control) at 48 h after iron administration and gradually returned to the control value by 28 days. (4) Iron-dextran potentiated 2 allyl-2-isopropyl-acetamide-induced synthesis of hepatic 5-aminolaevulinate. Potentiation occurred if the drug was given at the same time or 36 h after iron administration, but did not occur if the drug was given 14 or 64 days after iron administration. (5) The results are discussed in relation to proposed mechanisms for the effects of iron on hepatic haem metabolism.

Influence of puromycin and chloramphenicol on aminoketone synthesis ( delta-aminolevulinic acid synthesis) induction by allylisopropylacetamide in rat liver

In vitro induction of aminoketone synthesis (AKS) by allylisopropylacetamide (AIA) in liver slices from 30 day-old male rats was reversibly inhibited by puromycin (50 micrograms/ml incubation mixture). Normal AKS was initially stimulated and thereafter blocked. Chloramphenicol (350 micrograms/ml) inhibited AIA mediated induction of AKS only partially and only when added 1 h before AIA to the incubation mixture. In vivo puromycin (3 i.p. injections of 15 mg/kg in 1 h intervals) did not influence AKS or AIA mediated AKS induction in newborn rats, whereas chloramphenicol (500 mg/kg s.c. 30 min prior to AIA) inhibited AIA mediated AKS induction in newborn rats. In 40 day-old rats pretreatment with chloramphenicol completely inhibited AKS induction by 200 mg/kg AIA.

Tryptophan pyrrolase in haem regulation. Experiments with administered haematin and the relationship between the haem saturation of tryptophan pyrrolase and the activity of 5-aminolaevulinate synthase in rat liver

1. Administration of haematin to rats decreases 5-aminolaevulinate synthase activity in whole liver homogenates. 2. An inverse relationship between this decrease and the increase in saturation of apo-(tryptophan pyrrolase) with haem is observed during the initial phase of treatment with haematin. 3. Significant changes in both functions are caused by a 1 mg/kg dose of haematin, whereas the maximum effects are achieved by the 5 mg/kg dose. 4. Prevention by allopurinol of the conjugation of exogenously administered haematin with apo-(tryptophan pyrrolase) renders this haem available for further repression of 5 aminolaevulinate synthase. 5. The various aspects of the relationship between synthase activity and the haem saturation of tryptophan pyrrolase are discussed.

Haem control in experimental porphyria. The effect of haemin on the induction of delta-aminolaevulinate synthase in isolated chick-embryo liver cells

2-Allyl-2-isopropylacetamide-mediated induction of hepatic porphyria was studied in isolated chick-embryo liver cells. Increased delta-aminolaevulinate synthase activity occurred within 1h of induction and continued to increase for 8h. Protoporphyrins synthesized during this time accumulated to a concentration 10-fold greater than that in the control. Removal of 2-allyl-2-isopropylacetamide from the cells by washing at 3h immediately inhibited further increases in delta-aminolaevulinate synthase synthesis. However substitution of 2-allyl-2-isopropylacetamide at 3h by deferoxamine methane-sulphonate, an inhibitor of haem synthesis, allowed continued delta-aminolaevulinate synthase induction at an unaltered rate, even though this agent did not, by itself, induce enzyme synthesis. Exogenously added haemin was shown completely to inhibit 2-allyl-2-isopropylacetamide-mediated delta-aminolaevulinate synthase induction at concentrations as low as 20nm, a value that is less than the reported physiological one. The duration of inhibition was dependent on the concentration of added haemin and was followed by a period of delta-aminolaevulinate synthase synthesis at a rate similar to that of the control. These data are consistent with the hypothesis that delta-aminolaevulinate synthase synthesis is regulated by the concentration of intracellular haem and that induction is initiated by 2-allyl-2-isopropylacetamide-mediated destruction of haem. Induction of delta-aminolaevulinate synthase was shown to be dependent on both RNA and protein synthesis, and a study of the comparative effects of cordycepin, cycloheximide and haem has shown that, at haemin concentrations up to 50nm, the inhibition of delta-aminolaevulinate synthase synthesis followed kinetics similar to the effect of cordycepin, with no synergism between cordycepin and 50nm-haemin. However, at a haemin concentration of 2mum, the inhibition of delta-aminolaevulinate synthase synthesis followed similar kinetics to the effect of cycloheximide. These data demonstrate the control of delta-aminolaevulinate synthase synthesis by low concentrations of haemin and suggests that the primary effect of haemin is at the level of transcription.

Influence of cycloheximide on normal and induced delta-aminolevulinic acid synthetase (aminoketone synthesis) of rat liver in vivo and in vitro

2 and 5 mg/kg cycloheximide given i.p. 30 minutes prior to the inducer allylisopropylacetamide (AIA) diminished or prevented the enhancement of delta-aminolevulinic acid synthetase (ALAS) activity in newborn rats. In adult rats the small AIA effect 24 hours after pretreatment was enhanced by 2 mg/kg cycloheximide i.p., when the inhibitor was injected 2 hours prior to the inducer. The basic aminoketone synthesis was inhibited by cycloheximide alone. 150-600 microgram/ml cycloheximide had no effect on the aminoketone synthesis of rat liver slices in vitro during 4 hours of incubation. The in vitro induction by 0.6 mg/ml AIA was enhanced by cycloheximide, if the inducer was added 1 hour after preincubation with cycloheximide. It is supposed, that the cytoplasmically localized precursor-ALAS is activated at the beginning of the induction process prior to the initiation of de novo enzyme synthesis, at least in vitro.

Tryptophan pyrrolase, the regulatory free haem and hepatic porphyrias. Early depletion of haem by clinical and experimental exacerbators of porphyria

1. The importance of the early depletion of liver haem in the production of porphyria is discussed and further supporting evidence is presented from experiments with tryptophan pyrrolase, under conditions of exacerbation of experimental porphyria by therapeutic and other agents. 2. In addition to the early depletion of pyrrolase haem by porphyrogens, a further depletion is produced when rats are given a porphyrogen plus an analogue or one of 19 drugs known to exacerbate the human disease. 3. Non-exacerbators of human porphyrias do not cause a further early depletion of pyrrolase haem and it is suggested that this system may be used as a screening test for possible exacerbation of the disease by new and existing drugs. 4. A similar further early depletion of haem is produced by combined administration of lead acetate plus phenobarbitone, thus suggesting that the depletion is a more general phenomenon in experimental porphyria. 5. The relationship between tryptophan pyrrolase and the regulatory free haem is discussed. It is suggested that pyrrolase may play an important role in the regulation of haem biosynthesis.

Hepatic drug metabolism and haem biosynthesis in lead-poisoned rats

1 Pretreatment of rats with intraperitoneal injections of lead was shown to result in a depression of the microsomal mixed function oxidase system, as assessed by a decrease in hepatic microsomal P-450 and b5 content and by a decrease in the activity of the enzymes aniline hydroxylase and aminopyrine demethylase. Lead had a more marked effect on cytochrome P-450 than b5. 2 The activity of the rate-limiting enzyme of haem biosynthesis, delta-aminolaevulinic acid synthase, was inversely correlated with the microsomal cytochrome P-450 content. 3 The activity of the haem biosynthetic enzymes delta-aminolaevulinic acid dehydratase, coproporphyrinogen oxidase and ferrochelatase were decreased by increasing lead pretreatment. 4 The activity of the haem catabolic enzyme, haem oxygenase, was increased by lead pretreatment.