Regulation of heme oxygenase activity in Cyanidium caldarium by light, glucose, and phycobilin precursors

Cyanobacteria, red algae, and cryptophytes contain phycobiliproteins which function as photosynthetic light-harvesting pigments. The chromophores of phycobiliproteins are phycobilins, open-chain tetrapyrroles that are synthesized from protoheme. The first step of phycobilin formation is the conversion of protoheme to biliverdin IX alpha in a reaction that is catalyzed by heme oxygenase. In the unicellular red alga, Cyanidium caldarium, light is required for the accumulation of phycobiliproteins. It has been reported previously that the synthesis of the apoprotein components of allophycocyanin and phycocyanin is induced by light in C. caldarium, that the phycobilin precursors, delta-aminolevulinic acid (ALA), protoporphyrin IX, and protoheme can substitute for light, and that the regulation is exerted at the level of mRNA synthesis. We have determined that a key enzyme of phycobilin formation is induced by light in C. caldarium. Extractable heme oxygenase activity is low in dark-grown cells, and it increases approximately 6-fold during the first 24 h after the cells are illuminated. After 24 h, the activity decreases to a level approximately equal to the initial activity. Heme oxygenase is induced in unilluminated cells by administration of ALA. D-Glucose, which is known to inhibit phycocyanin accumulation in C. caldarium, inhibits the induction of heme oxygenase by light or ALA. Induction of heme oxygenase by light or ALA is blocked by cycloheximide, an inhibitor of cytoplasmic protein synthesis, but not by chloramphenicol, an inhibitor of chloroplast protein synthesis. Rifampicin, an inhibitor of algal chloroplast RNA synthesis, and gabaculine, a competitive inhibitor of ALA biosynthesis, block the induction of heme oxygenase by light but not by ALA. These results indicate that heme oxygenase in C. caldarium is induced by phycobilin precursors. The induction by light and the repression of the induction by D-glucose are probably indirect effects mediated by the effects of light and D-glucose on phycobilin precursor formation. The results also indicate that heme oxygenase is encoded by a nuclear gene and is synthesized on cytoplasmic ribosomes.

Enhanced cellular oxidant stress by the interaction of advanced glycation end products with their receptors/binding proteins

Attack by reactive oxygen intermediates, common to many kinds of cell/tissue injury, has been implicated in the development of diabetic and other vascular diseases. Such oxygen-free radicals can be generated by advanced glycation end products (AGEs), which are nonenzymatically glycated and oxidized proteins. Since cellular interactions of AGEs are mediated by specific cellular binding proteins, receptor for AGE (RAGE) and the lactoferrin-like polypeptide (LF-L), we tested the hypothesis that AGE ligands tethered to the complex of RAGE and LF-L could induce oxidant stress. AGE albumin or AGEs immunoisolated from diabetic plasma resulted in induction of endothelial cell (EC) oxidant stress, including the generation of thiobarbituric acid reactive substances (TBARS) and resulted in the activation of NF-kappa B, each of which was blocked by antibodies to AGE receptor polypeptides and by antioxidants. Infusion of AGE albumin into normal animals led to the appearance of malondialdehyde determinants in the vessel wall and increased TBARS in the tissues, activation of NF-kappa B, and induction of heme oxygenase mRNA. AGE-induced oxidant stress was inhibited by pretreatment of animals with either antibodies to the AGE receptor/binding proteins or antioxidants. These data indicate that interaction of AGEs with cellular targets, such as ECs, leads to oxidant stress resulting in changes in gene expression and other cellular properties, potentially contributing to the development of vascular lesions. Further studies will be required to dissect whether oxidant stress occurs on the cell surface or at an intracellular locus.

Tissue lipid peroxidation may be triggered by increased formation of bilirubin in vivo

Following the administration of phenylhydrazine, cadmium chloride and ethanol to rats there was a marked increase in the concentration of liver lipid peroxides and a sharp decline in GSH levels. The oxidative stress generated by the action of these toxic compounds led to the induction of liver heme oxygenase, which exhibited a 3-fold increase in activity over the control value. Patients with various forms of liver disorders showed increased levels of plasma lipid peroxides as well as hyperbilirubinemia. In view of the known ability of bilirubin to cause lipid peroxidation in illuminated erythrocyte membranes, the results of the present paper suggest that in severely impaired liver, as in some liver diseases, lipid peroxides may be also produced by a mechanism involving heme oxygenase.

Heme oxygenase 1 mediates an adaptive response to oxidative stress in human skin fibroblasts

Oxidative stress of human skin fibroblasts by treatment with ultraviolet A (UVA) radiation has been shown to lead to an increase in levels of the heme catabolizing enzyme heme oxygenase 1 [heme, hydrogen-donor:oxygen oxidoreductase (alpha-methene-oxidizing, hydroxylating), EC 1.14.99.3] and the iron storage protein ferritin. Here we show that human skin fibroblasts, preirradiated with UVA, sustain less membrane damage during a subsequent exposure to UVA radiation than cells that had not been preirradiated. Pretreating cells with heme oxygenase 1 antisense oligonucleotide inhibited the irradiation-dependent induction of both the heme oxygenase I enzyme and ferritin and abolished the protective effect of preirradiation. Inhibition of the UVA preirradiation-dependent increase in ferritin, but not heme oxygenase, with desferrioxamine also abolished the protection. This identifies heme oxygenase 1 as a crucial enzymatic intermediate in an oxidant stress-inducible antioxidant defense mechanism, involving ferritin, in human skin fibroblasts.

Fasciola hepatica: liver microsomal membrane functions in host rat

Changes in different microsomal membrane functions were measured in the liver of rats 3, 6, or 9 weeks following an oral infection with 20 metacercariae of Fasciola hepatica. The parasitic pathology noted at autopsy was accompanied by increased levels in both plasma aspartate aminotransferase (EC 2.6.1.1) and microsomal gamma-glutamyltransferase (EC 2.3.2.2). Heme oxygenase activity of microsomes was significantly decreased by Weeks 3 and 6 postinfection and this decrease correlates with those of total microsomal cytochrome P450 and certain P450-dependent monooxygenase activities, namely, benzphetamine demethylation, ethoxycoumarin deethylation, and benzopyrene hydroxylation. Microsomal epoxide hydrolase (EC 3.3.2.3) was only altered 6 weeks after the infection. During the early stages of the parasitism, there were decreases in both microsomal calcium uptake and calcium ATPphosphohydrolase activity (EC 2.6.1.1), whereas membrane fluidity, estimated by the order parameter S, was lower in the infected rats than that in the controls. These alterations could be related to the already described increase in liver cytosolic calcium or lipid peroxidation which occurs in experimental fascioliasis.

Tin-mediated heme oxygenase gene activation and cytochrome P450 arachidonate hydroxylase inhibition in spontaneously hypertensive rats

The effect of SnCl2 on the transcription of the heme oxygenase gene in spontaneously hypertensive rats was examined using cDNA for the rat heme oxygenase (HO-1). An increase in renal HO-1 mRNA levels was observed in response to SnCl2 treatment. Quantitative evaluation by scanning densitometry demonstrated a maximal increase in HO-1 mRNA 24-fold over control at 8 hours after SnCl2 administration. Nuclear runoff assay using isolated renal nuclei from SnCl2-treated rats revealed an active HO-1 gene transcription. Transcription of HO-1 in rat kidney was greatly increased within 3 hours of administration of SnCl2, as evidenced by the level of [alpha 32P]UTP incorporation into nuclear RNA. As a consequence of activation of the HO-1 gene transcription, renal enzyme activity increased eightfold at 16 hours after SnCl2, and reached maximal activity of 16-fold over control at 32 hours after injection. No significant change in cytochrome P450 fatty acid omega-hydroxylase (P450 4A) mRNA was observed after SnCl2 administration. Cytochrome P450-arachidonic acid omega/omega-1 hydroxylase(s) activity (formation of 20- and 19-HETE) was significantly reduced 24 hours after SnCl2 administration and remained lower than the control level 48 and 72 hours after injection. In addition, blood pressure was reduced from 151 +/- 2.5 mm Hg to 133 +/- 2.3 mm Hg after 48 hours of SnCl2 treatment. The reduction in blood pressure preceded natriuresis. It is concluded that SnCl2 induces activation of the HO-1 gene, which is followed by elevation in enzyme activity and a decrease in cytochrome P450-arachidonic acid omega-hydroxylase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of aluminum chloride on some steps of heme biosynthesis in rats after oral exposure

Certain disturbances in heme biosynthesis induced by aluminum chloride were examined. The experiment was performed on female rats that received AlCl3 orally at the dose 100 mg Al/kg daily for 21 d. The effects of aluminum on the activity of delta-aminolevulinic acid synthetase (ALA-S), dehydratase (ALA-D), and heme oxygenase (O.H.) were observed on 3, 7, 14, and 21 d in liver and kidneys of rats. Also the activity of ALA-D in blood and the concentration of delta-aminolevulinic acid (ALA-U) in urine were observed. Orally administered aluminum caused increase in the activity of ALA-D in the liver and blood, and parallel decrease of ALA-U in urine (r = -0.85) of rats. Aluminum chloride also induced an increase of ALA-S and O.H. in the liver but not in the kidneys. The changes of the enzymes activity participating in heme biosynthesis after administration of aluminum may be correlated with anemia and iron metabolism in rats.

Sources of carbon monoxide (CO) in biological systems and applications of CO detection technologies

Carbon monoxide is produced from a variety of sources in biological systems. Heme oxygenase and heme oxygenase-like activity is the predominant source in mammals, and may be equally important in plants and lower animals. The enzyme appears to be ubiquitous, highly conserved throughout phylogeny, and tightly regulated during development. This and other evidence suggests that heme oxygenase has an important physiological role, of which CO production may be a part. Other minor sources of CO include the oxidation of organic molecules. This includes the following: (1) auto-oxidation of phenols, flavenoids, and halomethanes; (2) photo-oxidation of organic compounds; and (3) lipid peroxidation of membrane lipids. No longer thought of as a waste product only, recent studies suggest that in the central nervous system cellular CO production can influence cGMP levels through effects on soluble guanylyl cyclase activity. Cellular CO production may also be linked to cell-cell interactions, and may be important in the cell's response to environmental changes. Whether CO will have a place similar to nitric oxide in cellular metabolism is still unclear, but it is apparent that these metabolic relationships will become increasingly complex. Cellular heme oxygenase activity results in the equimolar production of CO and bilirubin for each molecule of heme degraded. The CO thus formed diffuses into the blood, is carried via hemoglobin, and is excreted in the lungs. Therefore, CO production can be assessed clinically by measuring the rate of total body CO excretion, blood COHb levels, and end-tidal CO concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Heme-heme oxygenase complex. Structure of the catalytic site and its implication for oxygen activation

Heme oxygenase, a central monooxygenase enzyme of the heme catabolism and the associated generation of carbon monoxide, forms a 1:1 stoichiometric complex with iron protoporphyrin IX, which is a prosthetic active center and at the same time the substrate of the enzyme. By using EPR, resonance Raman, and optical absorption spectroscopic techniques, we have determined the axial ligand coordination of the enzyme-heme complex. The ferric heme iron in the heme-enzyme complex at neutral pH is six-coordinate high spin, while at alkaline pH (pKa 7.6), the complex becomes low spin. Spectra of ferrous forms of the complex indicate that histidine serves as the iron proximal axial ligand and that the residue is in its neutral imidazole rather than its imidazolate protonation state. Thus, the active site of the heme-heme oxygenase complex has a myoglobin-like structure rather than an active site similar to the large cytochrome P-450 class of monooxygenases. As a consequence, the activated form of the heme-heme oxygenase complex, a peroxo intermediate, is different from that of the cytochrome P-450 monooxygenases, in which the activated form is an oxo intermediate. The overall catalytic mechanism is probably more closely related to that of other monooxygenases with myoglobin-like active sites, such as secondary amine monooxygenase.

Heme oxygenase expression in Swiss 3T3 cells following exposure to aqueous cigarette smoke fractions

A dose-dependent and transiently elevated expression of a cytoplasmic 32 kDa protein was observed in Swiss albino 3T3 fibroblasts exposed to mainstream cigarette smoke (CS) trapped in phosphate-buffered saline solutions (smoke-bubbled PBS). The protein was identified as heme oxygenase (HO) (heme, hydrogen donor:oxygen oxidoreductase, EC 1.14.99.3) by Western blotting using an anti-rodent HO-specific antibody. Kinetic investigations revealed that HO protein and its mRNA were detectable in smoke-bubbled PBS-treated cells between 1 and 24 h after exposure to 0.03 puffs (approximately 1 cm3) CS per ml medium. As a result of transcriptional activation, a nearly 50-fold increase in the amount of HO mRNA was determined after 8 h exposure compared to control levels. Since literature data indicate that there is a link between glutathione depletion and HO expression, the same was assumed for cells exposed to smoke-bubbled PBS, as a decrease of more than 60% in glutathione levels was observed after the exposure. This was further supported by the observation that no elevated amounts of HO mRNA appeared in smoke-bubbled PBS-treated cells when cysteine was exogenously added. However, although these effects may be attributable to the formation of hydroxyl radicals (which have been shown to induce HO and to deplete glutathione levels and which appear in aqueous smoke-containing solutions via the iron-catalysed Fenton reaction) neither catalase nor the iron cation chelating agent o-phenanthroline were able to suppress or even to reduce HO expression in smoke-bubbled PBS-treated cells. On the contrary, at comparable concentrations both compounds were found to be potent inhibitors of smoke-dependent DNA strand breaks. Hence, reactive species other than Fenton reaction-derived hydroxyl radicals are responsible for the effects observed in the present study.

Rat liver heme oxygenase. High level expression of a truncated soluble form and nature of the meso-hydroxylating species

A rat heme oxygenase (HO-1) gene without the sequence coding for the last 23 amino acids has been constructed and expressed behind the pho A promoter in Escherichia coli. The enzyme is expressed at high levels as a soluble catalytically active protein that causes the bacterial cells to accumulate biliverdin. The purified truncated heme-heme oxygenase complex is spectroscopically indistinguishable from the complex with the native enzyme and converts heme to biliverdin when reconstituted with rat liver cytochrome P450 reductase. Reaction of the recombinant heme-heme oxygenase complex with H2O2 produces a species with the spectroscopic properties of verdoheme. Unidentified products are obtained when this intermediate is directly extracted from the protein, but biliverdin is obtained if the verdoheme-protein complex is exposed to cytochrome P450 reductase and NADPH before the extraction step. In contrast, reaction of the heme-heme oxygenase complex with meta-chloroperbenzoic acid (mCPBA), tert-butylhydroperoxide, or cumene hydroperoxide yields a ferryl (FeIV = O) complex. Reaction of the heme-heme oxygenase complex with mCPBA also produces an EPR-detectable protein radical. In accord with formation of a ferryl intermediate, recombinant heme oxygenase catalyzes the mCPBA- and alkylhydroperoxide-dependent peroxidation of 2-methoxyphenol (guaiacol). Guaiacol oxidation is not observed during turnover of the enzyme by cytochrome P450 reductase/NADPH or H2O2. Conversely, biliverdin is not formed with tert-butylhydroperoxide or mCPBA. H2O2 thus supports the first step of the normal catalytic oxidation of heme by heme oxygenase, but alkyl and acyl hydroperoxides do not. These results suggest that the alpha-meso-hydroxylation required for biliverdin formation is mediated by the distal of the two oxygens in the iron-dioxygen intermediate (Fe-O-O) engendered by reaction with either cytochrome P450 reductase/NADPH or H2O2.

Endothelial-cell heme uptake from heme proteins: induction of sensitization and desensitization to oxidant damage

Iron-derived reactive oxygen species are implicated in the pathogenesis of various vascular disorders including atherosclerosis, vasculitis, and reperfusion injury. The present studies examine whether heme, when liganded to physiologically relevant proteins as in hemoglobin, can provide potentially damaging iron to intact endothelium. We demonstrate that reduced ferrohemoglobin, while relatively innocuous to cultured endothelial cells, when oxidized to ferrihemoglobin (methemoglobin), greatly amplifies oxidant (H2O2)-mediated endothelial-cell injury. Drawing upon our previous observation that free heme similarly primes endothelium for oxidant damage, we posited that methemoglobin, but not ferrohemoglobin, releases its hemes that can then be incorporated into endothelial cells. In support, cultured endothelial cells exposed to methemoglobin--in contrast to exposure to ferrohemoglobin, cytochrome c, or metmyoglobin--rapidly increased their heme oxygenase mRNA and enzyme activity, thereby supporting heme uptake; ferritin production was also markedly increased after such exposure, thus attesting to eventual incorporation of Fe. These cellular methemoglobin effects were inhibited by the heme-scavenging protein hemopexin and by haptoglobin or cyanide, agents that strengthen the liganding between heme and globin. If the endothelium is exposed to methemoglobin for a more prolonged period (16 hr), it accumulates large amounts of ferritin; concomitantly, and presumably associated with iron sequestration by this protein, the endothelium converts from hypersusceptible to hyperresistant to oxidative damage. We conclude that when oxidation of hemoglobin facilitates release of its heme groups, catalytically active iron is provided to neighboring tissue environments. The effect of this relinquished heme on the vasculature is determined both by extracellular factors--i.e., plasma proteins, such as haptoglobin and hemopexin--as well as intracellular factors, including heme oxygenase and ferritin. Acutely, if both extra- and intracellular defenses are overwhelmed, cellular toxicity arises; chronically, when ferritin is induced, resistance to oxidative injury may supervene.

Inhibitory effect of CO on internal anal sphincter: heme oxygenase inhibitor inhibits NANC relaxation

We examined the effect and role of CO in opossum internal anal sphincter (IAS) relaxation in response to nonadrenergic noncholinergic (NANC) nerve stimulation. Effects of NANC nerve stimulation on the IAS tension and second messengers (cAMP and cGMP) were examined before and after the selective heme oxygenase (HO) inhibitor zinc protoporphyrin IX (Zn PP-IX). The HO activity of the IAS smooth muscle was determined before and after NANC nerve stimulation. CO caused a concentration-dependent and tetrodotoxin-resistant fall in the resting tension of the IAS. The direct action of CO was confirmed by its relaxant action on the isolated smooth muscle cells. Furthermore, CO caused an increase in the tissue cGMP levels comparable to that observed with nerve stimulation. Zn PP-IX caused suppression of IAS relaxation caused by NANC nerve stimulation and vasoactive intestinal polypeptide (VIP) but not by peptide histidine-isoleucine and suppression of the increase in cGMP in response to NANC nerve stimulation. Zn PP-IX had no significant effect on the IAS responses to CO, nitric oxide (NO), and the beta-adrenoceptor agonist isoproterenol. The IAS responses to CO were not modified by the NO synthase inhibitor NG-nitro-L-arginine. Significant HO activity was detected in the IAS, which increased further in response to NANC nerve stimulation and VIP. The direct relaxant actions of CO and the suppression of NANC-mediated relaxation of the IAS by the HO inhibitor suggest the involvement of CO in the neurally mediated IAS relaxation.

Biliverdin reductase is heat resistant and coexpressed with constitutive and heat shock forms of heme oxygenase in brain

Two heme oxygenase (HO) isozymes--HO-1, which is a heat shock protein (HSP32), and HO-2--catalyze the isomer-specific production of biliverdin IX alpha and carbon monoxide. The latter has the potential of functioning as a neurotransmitter, whereas the reduced form of biliverdin, bilirubin, has potent antioxidant activity. Formation of bilirubin is catalyzed by biliverdin reductase (BVR). The reductase is a unique enzyme in being dual pyridine nucleotide and dual pH dependent. Here, we show that the reductase is resistant to thermal stress at both the protein and message level. We further demonstrate that the reductase is coexpressed in cells that display HO-1 and/or HO-2 under normal conditions, as well as in regions and cell types that have the potential to express heat shock-inducible HO-1 protein. Exposure of male rats to 42 degrees C for 20 min did not decrease brain BVR activity, but caused a slight increase in NADPH- and NADH-dependent activities at 1 and 6 h following hyperthermia. High levels of the approximately 1.5-kb BVR mRNA were detected in control brain; it too displayed thermal tolerance. Similarly, the pattern of multiplicity of net charge variants of the enzyme purified from brain of heat-shocked rats did not differ from the control pattern. Immunochemical localization of BVR protein in normal brain correlated well with the presence of HO-1 and/or HO-2 throughout the forebrain, diencephalon, cerebellum, and brainstem regions. There were select neuronal and nonneuronal cells in the substantia nigra and cerebellum that did express the reductase under normal conditions, wherein no HO isozymes could be detected.(ABSTRACT TRUNCATED AT 250 WORDS)

Heme oxygenase is not expressed as a stress protein after renal ischemia

Stressful stimuli such as heat, oxidative stress, heavy metals, and tissue trauma induce the expression of a family of proteins commonly referred to as stress proteins or heat shock proteins. The functions of these proteins are varied but include glycolysis, antioxidant defense, and several postulated "chaperone" functions involving the folding, unfolding, and translocation of other proteins. Heme oxygenase, the enzyme that catalyzes the degradation of heme to biliverdin, is also heat inducible and is, therefore, a heat shock protein. In the kidney, ischemia has been observed by several investigators to induce expression of the more commonly studied heat shock proteins HSP 70 and HSP 72. In addition, exposure of the kidney to myoglobin after glycerol injection induced heme oxygenase. The purpose of this study was to determine whether heme oxygenase is expressed as a stress protein after renal ischemia. Renal ischemia was induced in rats after right nephrectomy by clamping the renal artery for 40 minutes. Gene expression was evaluated after 60 minutes to 96 hours of postischemic reperfusion. There was essentially no expression of heme oxygenase at any of the time points evaluated. The absence of heme oxygenase expression was in striking contrast to the prompt and dramatic expression of HSP 70. This finding is consistent with the concept that all "stress proteins" are not equivalent and that, although there is considerable overlap between heat-sensitive gene promoters and oxidant stress-sensitive gene promoters, there is specificity for the type of stimulus that is able to activate any given stress protein gene.

Oxidative stress resulting from ultraviolet A irradiation of human skin fibroblasts leads to a heme oxygenase-dependent increase in ferritin

Heme oxygenase-1 mRNA levels increase following exposure of many mammalian cell lines to oxidative stress such as ultraviolet A (UVA) irradiation. Here we demonstrate a 4-fold increase in microsomal heme oxygenase activity and a 40% decrease in microsomal heme content 14 h after treatment of human skin fibroblasts (FEK4) with 250 kJ m-2 of UVA radiation. Paralleling this was a 2-fold increase in ferritin levels that was sustained for at least 46 h after UVA irradiation. Treatment of fibroblasts with the iron chelating agent desferrioxamine, after the UVA-dependent induction of heme oxygenase, prevented the increase in ferritin levels. Treatment of fibroblasts with Sn-protoporphyrin IX (an inhibitor of heme oxygenase) also prevented the effect of UVA radiation on ferritin levels. Thus we conclude that the effect of UVA radiation on ferritin levels is via the heme oxygenase-dependent release of iron from endogenous heme sources. We propose that the increase in ferritin that follows UVA irradiation would decrease intracellular free iron such that iron-catalyzed free radical reactions would be restricted during periods of subsequent oxidative stress.

Enhancement of heme oxygenase expression and activity in A431 squamous carcinoma multicellular tumor spheroids

We have investigated the effects of the growth of A431 human squamous carcinoma cells as three-dimensional aggregates (multicellular tumor spheroids) on the expression and enzyme activity of heme oxygenase (HO). We demonstrate that A431 squamous carcinoma cells grown as day 4 spheroids selectively increase the expression of heme oxygenase 1 (HO-1), caused, directly or indirectly, by three-dimensional cell-cell contact effects. Steady-state levels of both mRNA and protein are significantly enhanced in spheroids compared with day 4 monolayers (approximately 13-fold). Because of the similarity of apparent half-lives between monolayers (2.7 h) and spheroids (2.1 h), it appears that the increases are caused at least partly by altered transcriptional rates. Total HO enzyme activity, measured by carbon monoxide production, is also up-regulated (2.6-fold) in spheroids, compared to that in monolayers. This increase indicates that the up-regulation in HO-1 protein expression corresponds to an increase in functional enzyme levels. We propose that HO may play a more complex role in cellular metabolism than would be evident from studies using two-dimensional monolayer cultures.

Nature of heme metabolizing enzymes in a mutant rat with hyperbilirubinuria

Eisai hyperbilirubinuria rat (EHBR) is a mutant strain which is originated from Sprague Dawley rats. The activities of heme metabolizing enzymes in EHBR were compared with those of Sprague Dawley rats as the control. The activity of delta-aminolevulinic acid (delta-ALA) synthetase, the initial and rate-limiting enzyme in heme biosynthesis, was significantly higher in the liver of EHBR. However, no significant difference in the activity of heme oxygenase, the first and rate-limiting enzyme in heme degradation, was observed in EHBR as compared to control rats. DDC, a porphyrinogenic agent, caused marked and prolonged increase in the activity of delta-ALA synthetase in EHBR. However, the contents of microsomal total heme and cytochrome P-450 were significantly decreased in EHBR 24 hrs after DDC injection. The activity of heme oxygenase by CdCl2, a inducer of heme oxygenase, was considerably higher in EHBR than in control rats. These results indicate some specific hypersensitivity for heme metabolizing enzymes in EHBR.

Domains of rat heme oxygenase-2: the amino terminus and histidine 151 are required for heme oxidation

Cleavage of heme b (Fe-protoporphyrin IX) at the alpha-meso carbon bridge is catalyzed by heme oxygenase isozymes, HO-1 and HO-2, to form biliverdin IX alpha. Currently, we have examined the requirement for the amino terminus and the hydrophobic carboxy terminus of rat HO-2 expressed in Escherichia coli for heme degradation activity and have assessed the importance of His 151 for this activity. His 151 is in the longest span of amino acids (24 residues) which are present, with only a single conservative substitution, in seven cloned heme oxygenases including the apparent single isozyme in chicken. We show His 151 is essential for cleavage of heme, as substitution of alanine for this residue by site-directed mutagenesis resulted in expression of an inactive protein with immunoreactivity toward antibody to rat HO-2. A cDNA construct in which nucleotides encoding the 33 N-terminal amino acid residues were deleted, when expressed, produced a protein of predicted size and immunoreactivity with antibody to HO-2 but also devoid of heme degrading activity. The presence of additional residues at this terminus, for the most part, accounts for the larger size of HO-2 compared to HO-1. Conversely, the hydrophobic region at the carboxy terminus did not appear to be essential for heme degradation. A construct in which the sequence encoding the primarily hydrophobic amino acids of the carboxy terminus was replaced by a sequence encoding predominantly hydrophilic residues expressed a protein which retained full capability to convert heme to biliverdin. Further, the construct with a hydrophilic carboxy terminus was not appreciably associated with bacterial membranes, suggesting that the carboxy terminus in the wild-type protein serves as a membrane anchor for this enzyme.

Regulation of heme oxygenase and metallothionein gene expression by the heme analogs, cobalt-, and tin-protoporphyrin

Two heme analogs, cobalt- and tin-protoporphyrin (CoPP and SnPP, respectively) have been used to probe the heme-hemopexin interaction, hemopexin receptor binding, and the mechanism of regulation of heme oxygenase (HO) and metallothionein-1 (MT-1) gene expression by hemopexin. Both CoPP and SnPP are HO inhibitors and hemopexin binds SnPP (Morgan, W. T., Alam, J., Deaciuc, V., Muster, P., Tatum, F. M., and Smith, A. (1988) J. Biol. Chem. 263, 8226-8231) and CoPP. The association of CoPP with hemopexin produces characteristic changes in the absorbance spectrum of CoPP and quenches the intrinsic fluorescence of hemopexin. Binding of CoPP is tight (Kd ca. 3 x 10(-7) M) although of lower affinity than heme itself (Kd < pM); and CoPP binding, like heme, produces conformational changes in hemopexin shown by an increase in the molar ellipticity at 233 nm and affords protection from proteolysis of the hinge region between the two structural domains of hemopexin. The coordination of the central cobalt atom is predicted to be similar to that of heme and to involve His56 and His127 of rabbit hemopexin. Furthermore, CoPP-hemopexin, like SnPP-hemopexin, binds to the hemopexin receptor as shown by competitive inhibition studies with radioactive heme-hemopexin. The effect of free heme analogs and their hemopexin complexes on HO and MT gene regulation was investigated and compared with the extent of induction by heme and heme-hemopexin. Free CoPP is a more effective inducer of HO steady state mRNA levels than free heme and produces a 5-fold increase within 1 h compared to only a 2-fold increase with heme, but free SnPP (up to 10 microM) produces no detectable increase in HO mRNA. In contrast, by 3 h heme-hemopexin and SnPP-hemopexin increase HO mRNA levels 11- and 6-fold, respectively; but the CoPP-hemopexin complex causes no detectable change in HO mRNA levels. The complexes of hemopexin with heme or either of the two heme analogs are effective inducers of metallothionein (MT) mRNA. Induction of MT mRNA by heme-hemopexin is rapid, increasing 4-fold within 1 h and 14-fold by 3-4 h. Strikingly, an even more rapid and slightly more extensive induction of MT mRNA is seen in response to either CoPP- or SnPP-hemopexin complexes, with MT mRNA rising 8-fold within 1 h. In contrast, free heme and the free analogs are far less effective inducers, increasing MT and mRNA levels and in vitro transcription rates only 3-4-fold and declining after 2-3 h.(ABSTRACT TRUNCATED AT 400 WORDS)

Phagocytosis of Staphylococcus aureus induces a selective stress response in human monocytes-macrophages (M phi): modulation by M phi differentiation and by iron

Phagocytosis of microorganisms represents a stress not only for the phagocytosed agent but also for the host cell. We have investigated the stress response induced in human monocytes-macrophages (M phi) phagocytosing inactivated Staphylococcus aureus. Exposure of human M phi to S. aureus induced in these cells (i) a threefold increase in superoxide dismutase activity, (ii) a selective and differentiation-dependent induction of host heat shock protein synthesis (HSP70 but not HSP65), and (iii) de novo synthesis of heme oxygenase, but only when exogenous iron was added to the cultures. The coordinate upregulation of two scavenging enzymes and of HSP70 suggests that all three are part of cellular protective mechanisms against phagocytosis-related oxidative injury to host cells.

Ultraviolet A (320-380 nm) radiation causes an alteration in the binding of a specific protein/protein complex to a short region of the promoter of the human heme oxygenase 1 gene

Ultraviolet A (320-380 nm) radiation strongly stimulates expression of the human heme oxygenase 1 gene as a consequence of an enhancement in transcription rate (1). We have used a 147 bp fragment of the promoter of this gene as a probe for DNA binding activity in nuclear extracts prepared from untreated and UVA treated populations of cultured human skin fibroblasts. Analysis using gel electrophoresis mobility shift assays clearly demonstrates the appearance of a strong binding activity unique to UVA-treated extracts that is formed in the absence of de novo protein synthesis. Footprint analysis defines a binding region from -41 to -50 bp that partially overlaps with a region known to constitutively bind upstream stimulatory factor (USF). Further analysis using synthetic oligonucleotides and gel retardation has confirmed that the crucial sequence for binding the protein present in both control and UVA-treated extracts lies within a 26 bp sequence that includes the core USF binding site. UVA radiation appears to lead to a modification of the USF complex (or closely related proteins(s)) to give a structurally modified protein/protein complex which protects only the upstream half of a region originally defined by foot-printing of the promoter with USF.

Nucleotide sequence of cDNA for porcine heme oxygenase and its expression in Escherichia coli

The nucleotide sequence of a cDNA for porcine heme oxygenase was determined. The open reading frame encoded a polypeptide of 288 amino acid residues with a molecular mass of 33,074 Da. A prokaryotic expression plasmid carrying porcine heme oxygenase cDNA was constructed and transfected into Escherichia coli cells. The full-length heme oxygenase expressed was localized in the bacterial membranes. Two small-sized heme oxygenases with no membrane-bound properties were also detected, suggesting that in E. coli cells a considerable amount of the enzyme expressed was degraded.

Expression of haptoglobin receptors in human hepatoma cells

The uptake of radio-labeled hemoglobin-haptoglobin complex (Hb-Hp) by human hepatoma PLC/PRF/5 and HepG2 cells was investigated in an attempt to characterize the uptake process and intracellular transport. Human hepatoma cells took up Hb-Hp in a receptor-mediated manner. Scatchard analysis of binding revealed that PLC/PRF/5 and HepG2 cells exhibited about 21,000 and 63,000 haptoglobin receptors/cell, with a dissociation constant (Kd) of 8.0 and 17 nM, respectively. Human hepatocytes in primary culture also expressed about 84,000 receptors/cells, with a Kd of 7.4 nM. The hemoglobin-haptoglobin complex was internalized and subsequently the internalized Hb-Hp was slowly degraded in the cells. Preincubation of the cells with Hb-Hp resulted in a decrease in binding of the radioactive Hb-Hp to the cell surface, and was accompanied with an accumulation of intracellular receptors. The uptake of Hb-Hp by the cells was not inhibited by 100 microM chloroquine or by 10 mM methylamine, but was inhibited by 50 microM monodansylcadaverine. Hemoglobin-heme taken up by the cells induced microsomal heme oxygenase. Thus, human hepatoma PLC/PRF/5 and HepG2 cells can take up Hb-Hp by haptoglobin receptor-mediated endocytosis and Hb-Hp probably causes translocation of the haptoglobin receptors from the cell surface to the cell interior where they can be degraded. The internalized heme-moiety of hemoglobin can regulate the expression of heme oxygenase.

Ferritin: a cytoprotective antioxidant strategem of endothelium

Phagocyte-mediated oxidant damage to vascular endothelium is likely involved in various vasculopathies including atherosclerosis and pulmonary leak syndromes such as adult respiratory distress syndrome. We have shown that heme, a hydrophobic iron chelate, is rapidly incorporated into endothelial cells where, after as little as 1 h, it markedly aggravates cytotoxicity engendered by polymorphonuclear leukocyte oxidants or hydrogen peroxide (H2O2). In contrast, however, if cultured endothelial cells are briefly pulsed with heme and then allowed to incubate for a prolonged period (16 h), the cells become highly resistant to oxidant-mediated injury and to the accumulation of endothelial lipid peroxidation products. This protection is associated with the induction within 4 h of mRNAs for both heme oxygenase and ferritin. After 16 h heme oxygenase and ferritin have increased approximately 50-fold and 10-fold, respectively. Differential induction of these proteins determined that ferritin is probably the ultimate cytoprotectant. Ferritin inhibits oxidant-mediated cytolysis in direct relation to its intracellular concentration. Apoferritin, when added to cultured endothelial cells, is taken up in a dose-responsive manner and appears as cytoplasmic granules by immunofluorescence; in a similar dose-responsive manner, added apoferritin protects endothelial cells from oxidant-mediated cytolysis. Conversely, a site-directed mutant of ferritin (heavy chain Glu62----Lys; His65----Gly) which lacks ferroxidase activity and is deficient in iron sequestering capacity, is completely ineffectual as a cytoprotectant. We conclude that endothelium and perhaps other cell types may be protected from oxidant damage through the iron sequestrant, ferritin.

Tin-protoporphyrin-mediated disruption in vivo of heme oxygenase-2 protein integrity and activity in rat brain

The ability of synthetic metalloporphyrins to suppress heme oxygenase activity and bilirubin formation has recently become of considerable clinical and experimental interest for suppression of jaundice in humans, including neonatal hyperbilirubinemia. The present investigation compares the biochemical effects of Sn- and Zn-protoporphyrins on the predominant heme oxygenase isozyme present in the brain (HO-2) at activity, protein, and transcript levels and describes the ability of Sn-protoporphyrin to adversely affect this isozyme. Specifically, 6 h after a modest dose (50 mumol/kg, i.v.) of Sn-protoporphyrin, heme oxygenase activity in rat brain was nearly undetectable. In addition, as revealed by Western blot analysis, HO-2 protein level was decreased by 20% and the electrophoretic behavior of the protein in the microsomal membranes was altered. Moreover, the activity of NADPH-cytochrome P-450 reductase, which is required for the oxidation of heme molecule, was markedly decreased (60% of control). Western immunoblot analysis revealed also a pronounced decrease in the reductase protein level. The inducible form of heme oxygenase, HO-1, was not detectable by immunoblotting in brain microsomes of either control or Sn-protoporphyrin-treated animals. Northern blot analyses did not reveal decreases in the levels of the single HO-1 mRNA (1.8 kb) or the two HO-2 transcripts (1.3 and 1.9 kb), suggesting that Sn-protoporphyrin mediates its effects on heme oxygenase isozymes at the protein level. Zn-protoporphyrin, on the other hand, had no deleterious effect on brain parameters presently investigated.(ABSTRACT TRUNCATED AT 250 WORDS)

Heme biosynthesis pathway regulation in a model of hepatocarcinogenesis pre-initiation

1. Heme regulation before the appearance of hyperplastic nodules was investigated in mice models of hepatocarcinogenesis. 2. With this aim 5-aminolaevulinate synthetase (ALA-S), microsomal heme-oxygenase (MHO), mitochondrial and cytoplasmic rhodanese activities were examined throughout a period of 35 days in animals exposed to dietary p-dimethylaminoazobenzene (DAB). 3. ALA-S activity was significantly diminished (50%) on day 14, then showing a sharply rising profile from day 28 onwards, and reaching 350% on day 35. 4. A similar profile was observed for mitochondrial rhodanese activity. 5. Changes in MHO and cytoplasmic rhodanese activities were almost the opposite to those observed for ALA-S. 6. The distinctive alteration in mitochondrial and cytoplasmic rhodanese would suggest that it plays a subtle role in ALA-S regulation during carcinogenesis initiation through a mechanism that appears to involve subcellular localization controls perhaps by means of the breakage of cystine trisulphide postulated to act as an ALA-S activator. 7. Taking into account the present results, we suggest a probable mechanism for the onset of hepatocarcinogenesis that includes a primary activating liver status, provoking biochemical aberration leading to the stage of initiation of hepatocarcinogenesis involving the whole organ.

Biosynthesis of phycobilins. Ferredoxin-supported nadph-independent heme oxygenase and phycobilin-forming activities from Cyanidium caldarium

The unicellular red alga, Cyanidium caldarium, synthesizes phycocyanobilin from protoheme via biliverdin IX alpha. In vitro transformation of protoheme to biliverdin IX alpha and biliverdin IX alpha to phycobilins were previously shown to require NADPH, ferredoxin, and ferredoxin-NADP+ reductase, as well as specific heme oxygenase and phycobilin formation enzymes. The role of NADPH in these reactions was investigated in this study. The C. caldarium enzymatic activities that catalyze biliverdin IX alpha formation from protoheme, and phycobilin formation from biliverdin IX alpha, were partially purified by differential (NH4)2SO4 precipitation. The enzyme fractions, when supplemented with a light-driven ferredoxin-reducing photosystem I fraction derived from spinach leaves, catalyzed light-dependent transformation of protoheme to biliverdin IX alpha and biliverdin IX alpha to phycobilins, with or without the addition of NADPH and ferredoxin-NADP+ reductase. In the dark, neither reaction occurred unless NADPH and ferredoxin-NADP+ reductase were supplied. These results indicate that the only role of NADPH in both reactions of phycobilin biosynthesis, in vitro, is to reduce ferredoxin via ferredoxin-NADP+ reductase and that reduced ferredoxin can directly supply the electrons needed to drive both steps in the transformation of protoheme to phycocyanobilin.

Differential regulation of heme oxygenase isozymes by Sn- and Zn-protoporphyrins: possible relevance to suppression of hyperbilirubinemia

Synthetic metalloporphyrins decrease heme oxygenase (HO)-dependent bilirubin formation. Presently, the effects in vivo and in vitro of Sn- and Zn-protoporphyrins on HO-1 (HSP-32) and HO-2 at the protein and transcript levels were examined. Western blot analysis of HO-2 in testes microsomes of Sn-protoporphyrin-treated rats revealed a dramatic disruption of the integrity of the HO-2 protein. Similar observations were made with the liver and adrenal HO-2 and the NADPH-cytochrome P-450 reductase of treated rats. Northern blot analysis, however, suggested unaltered tissue levels of HO-2 transcripts (approximately 1.9 and approximately 1.3 kb). The HO-1 protein integrity in organs of treated rats was less dramatically affected by the metalloporphyrin and an increase in its 1.8 kb mRNA level in the testes was detected. Zn-protoporphyrin also increased HO-1 mRNA level in the testes, but did not affect HO-2 protein integrity. In in vitro studies with purified HO-1 and HO-2, both Sn- and Zn-protoporphyrins were equally inhibitory to HO-1 activity; Sn-protoporphyrin, however, was by far more inhibitory to HO-2-dependent activity than to that of HO-1. Together, these findings and the fact that HO-2 under normal conditions is the predominant form of the enzyme in most organs suggest that loss of HO-2 protein integrity may to a significant degree account for suppression of bilirubin formation by Sn-protoporphyrin. These in turn may reflect differences between HO-1 and HO-2, both at the transcriptional level with HO-2 being noninducible, and in structure/composition of the isozymes, with HO-2 being more labile.

Normal and heat-induced patterns of expression of heme oxygenase-1 (HSP32) in rat brain: hyperthermia causes rapid induction of mRNA and protein

Most cells possess a variety of mechanisms, such as high levels of glutathione, that guard against cytotoxic free radicals, which are suspected in the etiology of various neurological deficits. Neurons, however, are deficient in this antioxidant source. The list of other potent antioxidants includes the bile pigments biliverdin and bilirubin. Heme oxygenase (HO) isozymes, HO-1 (HSP32) and HO-2, catalyze the rate-limiting step in the only biological pathway by which bile pigments are produced. In this study, heat shock is identified as the only stimulus reported to date that can alter expression in brain HO-1 of protein and mRNA in vivo. Using a HO-1 cDNA probe, we examined the level of HO-1 mRNA in normal rat brain and in brain 1 and 6 h following heat shock. Exposure of male rats to 42 degrees C for 20 min caused a 20-fold increase in brain HO-1 1.8-kb mRNA within 1 h after treatment. Quantification of brain HO-1 protein by HO-1 radioimmunoassay revealed a fourfold increase at 6 h posttreatment. In normal brain, HO-1 protein was sparsely expressed in few select neuronal and nonneuronal cell populations in forebrain, diencephalon, cerebellum, and brainstem regions. Six hours following heat shock, an intense increase in HO-1 protein in glia throughout the brain, ependyma lining the ventricles of the brain, paraventricular nucleus, Purkinje cell layer of the cerebellum, and cochlear nucleus of brainstem was observed. We suggest that increases in HO-1 transcript and protein reflect a means to elevate levels of antioxidants in cells with compromised defense mechanisms caused by stress.

Evidence for pteridine regulation of lead-mediated inhibition of uroporphyrinogen and heme formation in rat bone marrow

Uroporphyrin I (URO I) accumulation has been reported in the bone marrow of rats exposed to lead, suggesting a sensitivity of uroporphyrinogen III cosynthase (COSYN) to this heavy metal. Furthermore, it has been reported that a polyglutamated folate derivative may serve as a coenzyme for the catalytic action of hepatic uroporphyrinogen III cosynthase. These findings raised the question of whether depletion of polyglutamated folate could enhance the susceptibility of bone marrow COSYN to lead and potentially interfere with the formation of heme. Nitrous oxide, an anesthetic agent capable of causing bone marrow tetrahydrofolate deficiency, depressed total bone marrow polyglutamated folate content by 42% with significant reductions in all three chain lengths (5-7) identified in the bone marrow during an exposure period of 7 days at 4 hr/day. Lead acetate (15 mg/kg) administered by ip injection at Days 0 and 2 during a 7-day exposure to nitrous oxide resulted in an 84% increase of bone marrow URO I content, which was markedly higher than the increases of 22 and 38% seen with sole administration of lead or nitrous oxide, respectively. The combination of agents also produced a 48% rise in COPRO I, a 39 and 43% decrease in COPRO III and protoporphyrin, respectively, and a 42% decline in the activity of microsomal 7-ethoxycoumarin O-deethylase, which is hemoprotein, cytochrome P-450 mediated. Heme oxygenase activity was not altered by nitrous oxide, lead, or their combination. These results suggest that bone marrow folate deficiency may render COSYN more sensitive to lead as characterized by increased uroporphyrin I and coproporphyrin I isomer content, decreased coproporphyrin III and protoporphyrin content, and depressed microsomal hemoprotein, cytochrome P-450-mediated drug-metabolizing capability.

Heme oxygenase is a positive acute-phase reactant in human Hep3B hepatoma cells

The effects of human interleukin-6 (hIL-6), the major acute-phase inducer, on the level of the transcript of microsomal heme oxygenase (HO) were examined in a human hepatoma cell line, Hep3B. Messenger RNAs (mRNAs) encoding HO and haptoglobin (Hpt) increased after hIL-6 treatment in a time- and dose-dependent manner. hIL-6 had no effect on the induction of heat-shock protein 70 (hsp70) mRNA, suggesting that the induction of HO by hIL-6 is regulated by a different mechanism from that which mediates the heat-shock induction of this enzyme. The hIL-6-mediated induction of HO mRNA was completely abrogated by simultaneous treatment of cells with actinomycin D, but not with cycloheximide, suggesting that the induction occurs at the level of transcription. A nuclear factor was shown both in untreated, and in the hIL-6-treated Hep3B cells that binds specifically to the IL-6-responsive element (IL6-RE) of the human HO gene. These findings suggest that HO is a positive acute-phase reactant in this human liver-derived cell line, and that the nuclear factor specific to the IL6-RE may be involved in the activation of the HO gene after hIL-6 treatment.

Importance of histidine residue 25 of rat heme oxygenase for its catalytic activity

A truncated, soluble, and enzymatically active rat heme oxygenase lacking its membrane-associative, C-terminal segment was expressed in E. coli strain JM109. The roles of its four histidine residues were examined by determining the enzymatic activities of mutant enzymes in which each of these residues in turn was replaced by alanine. Mutation of histidine residue 25 to alanine resulted in marked decrease in activity for heme breakdown, indicating that this histidine residue has an important role in the heme oxygenase reaction.

Tin-protoporphyrin: a potent inhibitor of hemoprotein-dependent steroidogenesis in rat adrenals and testes

The present investigation provides evidence of the ability of Sn-protoporphyrin to cause striking alterations in adrenal and testicular cytochrome P-450-dependent steroidogenesis and defines the potential of this metalloporphyrin to serve as a cellular toxin. Sn-protoporphyrin is currently used on an experimental basis for treatment of hyperbilirubinemias in humans, including newborn infants. Specifically, in the adrenals of rats treated with a moderate regimen of Sn-protoporphyrin (two doses of 50 mumol/kg, s.c.), marked decreases of 60 to 70% in the microsomal 21 alpha-hydroxylase and the mitochondrial 11 beta-hydroxylase activities were observed after 7 days. Concomitant with these decreases was a significant depression in the adrenal mitochondrial cytochrome P-450 content and a notable reduction (approximately 30%) in serum corticosterone levels. Similarly, in the testes, significant decreases in the microsomal and mitochondrial cytochrome P-450 contents and the microsomal 17 alpha-hydroxylase activity were observed. Serum testosterone level, however, was not decreased, reflecting an absence of decrease in side chain cleavage activity. Metalloporphyrin caused a striking decrease of 65 to 80% in the microsomal heme oxygenase activity in the testes and the adrenals, as well as significant reductions in NADPH-cytochrome P-450 reductase activity of the organs. The decrease in heme oxygenase activity, however, as suggested by Western immunoblotting, apparently resulted, to a large extent, from the loss of enzyme protein integrity rather than a competitive inhibition of activity. At the transcript level, Northern blot analysis using a full length rat testis cDNA probe for heme oxygenase-2 mRNA indicated that Sn-protoporphyrin treatment did not decrease the amount of message for either of the heme oxygenase-2 transcripts (1.3 and 1.9 Kb).(ABSTRACT TRUNCATED AT 250 WORDS)

Heme oxygenase: expression in human retina and modulation by stress agents in a human retinoblastoma cell model system

PCR and Southern blot analyses demonstrate that mRNA for heme oxygenase (HO), a well known "stress protein" in a number of tissues, is present in human retina. Western and northern blots show that the protein and mRNA are also expressed in human Y-79 retinoblastoma cells in culture and that the HO enzyme is rapidly induced by its substrate, heme. Moreover, HO is also induced by two chemicals, sodium arsenite and menadione, that act as agents of oxidative stress. HO is the regulatory enzyme in the heme degradative pathway and an increase in its activity could lead to the accumulation of bilirubin, an antioxidant, in the cell at the expense of heme, a prooxidant. The HO pathway may thus be of importance in protecting the retina against oxidative stress in vivo. Moreover, the Y-79 culture system should provide an excellent model for use in examining stress mechanisms in retinal cells at a molecular level.

A microassay for heme oxygenase activity using thin-layer chromatography

A sensitive and facile assay for heme oxygenase (HO) has been developed. The basis of the assay is the detection of [14C]bilirubin formation in a coupled enzyme assay involving HO and biliverdin reductase actions, respectively. Separation of substrate from product is accomplished by thin-layer chromatography with subsequent quantitation by liquid scintillation counting of radioactive material present on chromatograms. As little as 20 micrograms of total cellular protein derived from cells growing in a standard 25-cm2 culture flask is sufficient for detection of HO enzyme activity using this assay. The reaction is inhibited by tin-protoporphyrin (10 microM final concentration), a specific inhibitor of HO. The linearity of the enzyme reaction with respect to incubation time and amount of protein used was established. Comparison of the new HO assay with a spectrophotometric assay was made, and good agreement of the results from both methods was found. The assay described here should facilitate measurements of this important heme-degrading enzyme in tissue culture studies and cases where limited amounts of material are available.

Effect of acute and chronic treatment of tin on blood pressure in spontaneously hypertensive rats

Cytochrome P450-dependent metabolites of arachidonic acid (AA) are increased in the kidneys of spontaneously hypertensive rats (SHR) as compared to control rats (WKY) in the period of rapid elevation of blood pressure (BP) from 5 to 13 weeks. We treated rats with stannous chloride (SnCl2) (10 mg/100 g body weight/day for 4 days) to decrease selectively renal cytochrome P450 content through increasing renal heme oxygenase activity. A decrease in renal cytochrome P450-dependent AA metabolites was associated with decreased BP and increased urinary Na+ excretion in 7- but not in 20-week-old SHR rats. Chronic treatment with SnCl2 (10 mg/100 g body weight twice a week) from 5 to 20 weeks prevented the elevation of BP in SHR rats. Further, the antihypertensive effects of tin persisted for 7 weeks beyond its discontinuation. BP in WKY rats was unaffected by tin. Both the acute and chronic treatment with tin are the first studies to demonstrate amelioration of hypertension in SHR by an intervention which is targeted at a single enzyme system.

Regulation of heme pathway in regenerating mouse liver

1. delta-Aminolevulinic acid synthetase (ALA-S), rhodanese and microsomal heme oxygenase (MHO), were quantitated in Cl4C induced regenerating mouse liver. 2. Maximal hepatomegalia was observed at 48 hr after i.p. injection of a single dose of the toxin. 3. ALA-S activity decreased on day 2, and then significantly increased (50%) between days 3 and 7, returning afterwards to control values. 4. Cytoplasmic rhodanese, as well as MHO activities, exhibited a clear correlation as compared with the ALA-S activity profile. 5. Porphyrin biosynthesis from precursor delta-aminolevulinic acid (ALA) was significantly increased even after 15 days of intoxication. 6. Present results would indicate that Cl4C is acting in a dual fashion.

Renal heme metabolism in hereditary tyrosinemia: use of succinylacetone in rat renal tubules

Succinylacetone (SA), a metabolic end-product found in urine from individuals with hereditary tyrosinemia and associated renal Fanconi syndrome and a known inhibitor of hepatic 5-aminolevulinic acid dehydratase (ALAD), has been used to study heme metabolism in isolated rat renal tubules. Heme biosynthetic porphyrin precursors are increased selectively in the presence of 4 mmol/1 SA. Total porphyrin content of the tubules are increased approximately 2-fold, while both ferrochelatase and heme oxygenase activities remain unaffected by SA. Nonetheless, total heme content is reduced, as was incorporation of radioactive label from amino[14C]levulinic acid. Cytochrome P-450 content remained unaffected. Impairment of iron uptake and/or transport within the cell or enhancement of heme catabolism via a non-heme oxygenase-dependent pathway could explain the observations.

Expression of rat heme oxygenase in Escherichia coli as a catalytically active, full-length form that binds to bacterial membranes

A plasmid, pKK-RHO, was constructed by incorporating the coding sequence of a cDNA for rat heme oxygenase into the expression vector pKK233-2. Escherichia coli strain XL1-blue transformed with pKK-RHO produced a catalytically active, full-length heme oxygenase. The 32-kDa native enzyme expressed, was localized in the bacterial membranes, possibly due to the spontaneous membrane-binding properties of a hydrophobic segment in its C-terminal region. During cultivation, a few degraded forms of heme oxygenase that had lost their membrane-associative properties appeared. Probably, some bacterial proteases cut the native heme oxygenase at sites near its C-terminus and so release hydrophilic peptides of heme oxygenase from the membranes. A 30-kDa polypeptide, one of the degraded forms of heme oxygenase, retained ability to accept electrons from NADPH--cytochrome P450 reductase and also activity for catalyzing breakdown of heme to biliverdin. The cultured cells were pale green. From them we extracted green pigment(s), of which the absorption spectrum closely resembled that of biliverdin, suggesting that a large amount of the endogenous heme of E. coli was actually degraded to biliverdin by the expressed heme oxygenase.

Alterations in hepatic delta-aminolevulinic acid synthetase and heme oxygenase activities after chronic ethanol consumption in rats

In the present study, the effect of chronic ethanol consumption in rats on the hepatic heme metabolism was investigated. Male Wistar rats were fed a nutritionally adequate liquid diet containing ethanol as 36% of the total calories for 5 weeks. After an overnight fast, the livers were excised and centrifuged to obtain mitochondrial and microsomal fractions. Chronic ethanol feeding of rats resulted in about 19% hepatomegaly as represented by the increased liver/body weight ratio. There was no difference in the mitochondrial protein content between the ethanol-treated and control rats, but the microsomal protein content was significantly increased in the ethanol-treated rats. Hepatic microsomal content of cytochrome P-450 (P-450) was markedly enhanced by chronic ethanol ingestion. Microsomal contents of cytochrome b5 (b5) and total heme were also increased to a lesser extent. After chronic ethanol abuse, the hepatic activity of delta-aminolevulinic acid (ALA) synthetase, which is a rate-limiting enzyme for heme production, was significantly increased and that of the heme oxygenase was slightly increased. These data indicate that ALA synthetase activity is induced by the negative feedback mechanism in order to compensate the depletion of heme caused by the utilization of heme for P-450. It is also speculated that, in response to excessive production of heme as described above, heme oxygenase activity is secondarily induced to regulate the amount of heme.

Control of cytochromes P450 expression in Gunn rat liver: implication of the intracellular heme pool

The absence of changes in the overall hepatic cytochrome P450 content after administration of 3-methylcholanthrene (MC) to congenitally jaundiced Gunn rats is believed to be related to a limited heme availability in this strain of rat. The amount of available heme, estimated by tryptophan pyrrolase activity, shows a substantial decrease in control Gunn versus control Wistar rats. This reduction is moderately enhanced by MC treatment in Gunn rats but is abolished after phenobarbital administration. Heme oxygenase activity is diminished in Gunn rats and consequently is not responsible for the decrease in the hepatic heme availability. These data point out that the depletion of the intracellular heme can lead to a limitation in the synthesis of cytochrome P450 isoenzymes in the MC-induced Gunn rat.

Retinoic acid can enhance the stimulation by thyroid hormone of heme oxygenase activity in the liver of thyroidectomized rats

The effects of retinoic acid (RA) (50 micrograms/100 g body wt. per day) on hepatic heme oxygenase activity, delta-aminolevulinate synthase (ALAS) activity and on cytochrome P-450 content were determined in thyroidectomized rats treated with T3 (10 micrograms/100 g body wt. per day) or diluent. RA, when administered for 3 days, failed to influence significantly the activity of either heme oxygenase or ALAS, however, the retinoid depleted hepatic cytochrome P-450 content by 17% (P less than 0.01) and microsomal heme content by 47% (P less than 0.001). T3 administration enhanced heme oxygenase activity by 72% (P less than 0.001) and ALAS activity by 251% (P less than 0.001) above levels in diluent treated controls and depleted cytochrome P-450 levels by 55% (P less than 0.001) and heme levels by 75% (P less than 0.001). When RA and T3 were administered together, the retinoid markedly enhanced the T3 stimulation of heme oxygenase activity; 173% above controls (P less than 0.001), and 61% above T3 alone (P less than 0.001). However, RA failed to influence the effect of T3 on ALAS activity or cytochrome P-450 depletion. The results indicate that RA can influence the levels of hepatic cytochrome P-450 and can modulate the stimulation of heme oxygenase activity by thyroid hormone in vivo.

Co-regulation of heme oxygenase and erythropoietin genes

The mechanism responsible for the accumulation of heme oxygenase and erythropoietin (epo) transcripts due to cobalt chloride (CoCl2) administration was investigated in rat kidney using a rat heme oxygenase and mouse epo probes. We found an increase of heme oxygenase transcripts in kidney in response to CoCl2. Quantitative evaluation of the heme oxygenase mRNA changes, by scanning densitometry, indicated that the levels of mRNA encoding heme oxygenase were increased by about fiftyfold in rat kidney after administration of CoCl2. That the increase in heme oxygenase mRNA levels resulted from enhanced transcription of the heme oxygenase gene was confirmed by nuclear runoff using isolated rat kidney nuclei after CoCl2 administration. Transcription of the heme oxygenase gene is greatly increased in rat kidney within 1 hr of administration of CoCl2 as evidenced from the levels of 32P-UTP incorporation into the specific transcript. Time course studies showed that stimulation of transcription was increased about fortyfold 3 hr after CoCl2 administration. This stimulation is the most rapid transcriptional response to heavy metals yet described. In addition, Northern blot analysis demonstrated that epo mRNA was first detected 4 hr following CoCl2 administration and reached a maximum at 5 hr. On the other hand, PCR analysis indicated that epo mRNA was increased as early as 1 hr following CoCl2 administration. The fact that CoCl2 caused increased transcription of both the epo and heme oxygenase genes suggests that a common mechanism may be involved in the regulation of these two genes by the heavy metal ion.

Degradation of heme by a soluble peptide of heme oxygenase obtained from rat liver microsomes by mild trypsinization

A tryptic peptide of heme oxygenase obtained after solubilization of rat liver microsomes by mild trypsin treatment was purified. The purified peptide gave only a single protein band with a molecular mass of 28 kDa on SDS/PAGE. The tryptic peptide, like the native heme oxygenase, readily bound with substrate heme forming a hemeprotein transiently. The absorption spectra of the ferric, ferrous, ferrous-CO and ferrous-O2 forms of the resulting complex resembled those of the corresponding forms of the complex of heme and the native enzyme. Ferric heme bound to the tryptic peptide was quantitatively decomposed to biliverdin on incubation with a mixture of ascorbic acid and desferrioxamine, indicating that the tryptic peptide still retained catalytic activity. These observations suggest that heme oxygenase has two domains, a hydrophilic and a hydrophobic domain, and that the two domains are folded almost independently of each other. An NADPH-cytochrome-P-450 reductase system composed of NADPH and detergent-solubilized NADPH-cytochrome-P-450 reductase readily reduced the ferric heme bound to the tryptic peptide, but failed to transfer the second electron required for rapid heme degradation, suggesting that the hydrophobic domain of heme oxygenase is important for receiving the second electron from the reductase.