Transcriptional control of rat heme oxygenase by heat shock

A heat shock element is located in the 5'-flanking region of the rat heme oxygenase gene (HO gene). The incubation of rat glioma cells at 42 degrees C or with hemin at 37 degrees C increased the levels of heme oxygenase mRNA within 1 h and produced a maximum at 3 h (at least a 20-fold increase). In both treatments, the heme oxygenase activity started to increase after a lag period of about 1 h and reached a maximum value at 5 h. There was an apparent additive effect of both treatments on the heme oxygenase induction. Studies with actinomycin D and cycloheximide suggested that both heat shock and hemin acted at the transcriptional level to induce heme oxygenase. Therefore, we analyzed the transient expression of chimeric fusion genes harboring the promoter of the rat HO gene ligated to the Escherichia coli gene gpt in rat glioma cells and in K1735 mouse amelanotic melanoma cells. The 5'-flanking region of the rat HO gene bearing the heat shock element conferred the heat inducibility of gpt RNA production in both cell lines; however, hemin treatment did not induce gpt RNA. These results indicate that rat heme oxygenase is a heat shock protein and that hemin induces heme oxygenase through a different mechanism from heat shock.

Coal fly ash induces hepatic and pulmonary cytochrome P-450 and sigma-aminolevulinic acid synthetase in rats

The effect of intratracheal administration of coal fly ash, its benzene-soluble and benzene-insoluble fractions has been studied on the levels of hepatic and pulmonary cytochrome P-450, cytochrome b5, and the activities of sigma-aminolevulinic acid synthetase and heme oxygenase. Fly ash and both its fractions significantly increased the levels of hepatic and pulmonary cytochrome P-450. Benzene-soluble and benzene-insoluble fractions of coal fly ash significantly increased the levels of cytochrome b5 also in both lung and liver. Fly ash and both its fractions increased the activity of sigma-aminolevulinic acid synthetase and reduced the activity of heme oxygenase in lung and liver. Glass bead particles of similar size did not show any effect on hepatic and pulmonary cytochrome P-450 and cytochrome b5.

Relationship of suppression of the androgenic axis by cobalt-protoporphyrin to its effects on weight loss and hepatic heme oxygenase induction

Cobalt-protoporphyrin administration to adult male rats results in an intense induction of hepatic heme oxygenase, a pronounced decline of cytochrome P-450 content in liver and associated metabolic abnormalities, including a dose-dependent decrease in weight gain and a marked decline in serum concentrations of testosterone without a compensatory increase in serum luteinizing hormone levels. These abnormalities persist for at least 5-6 weeks after a single subcutaneous dose of the metalloporphyrin (25 mumol/kg b.w.). Experiments with pair-fed control and metalloporphyrin-treated rats indicated that the androgenic dysfunction produced by cobalt-protoporphyrin is not causally related to the associated weight loss produced by the compound. Hepatic heme oxygenase activity was markedly induced by cobalt-protoporphyrin as expected; the enzyme activity was not altered in hypothalami of treated rats but was elevated (approximately 5-fold) in pooled pituitaries. However, despite the expected decrease in hepatic cytochrome P-450 content, no changes were noted in cytochrome P-450 content of hypothalami or pituitaries. In experiments in which the enhanced heme oxygenase activity produced in liver by cobalt-protoporphyrin was completely antagonized by tin-protoporphyrin, a competitive inhibitor of the enzyme, neither the endocrine suppression nor the weight loss produced by cobalt-protoporphyrin was altered. These phenomena were thus clearly dissociated from the effects of cobalt-protoporphyrin on heme oxygenase. Whether or not cobalt-protoporphyrin acts centrally to impair both appetite and endocrine control mechanisms could not be determined in these experiments, but remains a possible explanation of the novel actions of this synthetic heme analogue.

Induction of xanthine oxidase and heme oxygenase and depression of liver drug metabolism by interferon: a study with different recombinant interferons

Induction of xanthine oxidase in mouse liver by interferon (IFN) was studied with three different recombinant human leukocyte IFN molecules: IFLrA, IFLrD and the hybrid IFLrA/D(Bgl II). The ability of different IFN species to induce xanthine oxidase correlated with their ability to depress liver cytochrome P-450-dependent drug metabolism, supporting the hypothesis that reactive oxygen metabolites generated by xanthine oxidase might be responsible for this impairment of liver function by IFN. The antioxidant N-acetylcysteine protected in vivo against the depression of liver drug metabolism by IFLrA/D. IFLrA/D was also found to induce liver microsomal heme oxygenase, an effect that was probably secondary to the observed depression of cytochrome P-450.

Control of heme metabolism with synthetic metalloporphyrins

Studies with synthetic metal-porphyrin complexes in which the central iron atom of heme is replaced by other elements indicate that those heme analogues that cannot be enzymatically degraded to bile pigments possess novel biological properties that may have considerable clinical as well as experimental value. Such studies have revealed the important role that the central metal atom plays in determining the physiological and pharmacological properties of metal-porphyrin complexes; and they have demonstrated that the form in which animals and humans are exposed to trace metals, i.e., inorganic, organified, porphyrin-chelated, etc., can be of great importance in determining the biological responses that such elements elicit, especially with respect to actions on heme synthesis and degradation and cytochrome P-450 formation and function. Study of the biological properties of synthetic metalloporphyrins represents a potentially fruitful area of research and the results may have significant value for basic as well as clinical disciplines.

Heme regulation of HeLa cell transferrin receptor number

The number of diferic transferrin receptors on HeLa cells decreases when cells are grown in iron-supplemented media. The experiments reported here suggest that heme is the iron-containing compound which serves as the signal for receptor number regulation. When HeLa cells were grown in the presence of hemin, transferrin receptor number decreased to a greater degree than when cells were grown in equivalent amounts of iron supplied as ferric ammonium citrate. Incubation of cells in conditions which increased cellular heme content resulted in a decrease in cellular transferrin receptors. Incubating cells with 5-aminolevulinic acid (thus bypassing the rate-limiting step in heme biosynthesis, 5-aminolevulinic acid synthase) led to a decrease in transferrin receptor number. Incubation of cells with an inhibitor of heme oxygenase, Sn-protoporphyrin IX, also led to a decrease in transferrin receptor number. When cellular heme content was decreased by inhibiting heme synthesis with succinylacetone (an inhibitor of 5-aminolevulinic acid dehydratase), or by depriving cells of iron with deferoxamine, an increase in HeLa cell transferrin receptor number was seen. When HeLa cells were incubated with inducers of heme oxygenase (CoCl2, SnCl2, Co-protoporphyrin IX), transferrin receptor number also increased. The effects of all compounds which alter transferrin receptor number were dependent on the concentration of the supplement, as well as the duration of the supplementation. These experiments suggest that intracellular heme content may be an important signal controlling transferrin receptor number.

Effects of 1,2-dibromo-3-chloropropane on hepatic heme synthesis

Previous studies showed that 1,2-dibromo-3-chloropropane (DBCP) caused a decrease in hepatic microsomal cytochrome P-450 [D.E. Moody, B. Head, and E.A. Smuckler (1979) J. Environ. Pathol. Toxicol. 3, 177-190; D.E. Moody, G.A. Clawson, C.H. Woo, and E.A. Smuckler (1982) Toxicol. Appl. Pharmacol. 66, 278-279], suggesting that hepatic heme metabolism may be affected by DBCP treatment. This study tested this hypothesis. Various parameters of hepatic heme synthesis were measured at intervals ranging from 0 to 72 hr in male Sprague-Dawley rats given a single oral dose (200 mg/kg) of DBCP. Incorporation of the radiolabeled heme precursor [delta-14C]aminolevulinic acid (14C-ALA) into liver, protein, extracted heme, and subcellular fractions of liver homogenates was significantly decreased to 75, 58, and 81% of controls, respectively, at 24 hr. At 48 and 72 hr after DBCP treatment, the accumulation of 14C-ALA label after 4 hr in liver homogenates and subcellular fractions was significantly increased in comparison to controls. These changes in 14C-ALA uptake were accompanied by decreases in total liver and microsomal heme, but not mitochondrial heme. Decreases were found in the spectral content of two heme proteins, cytochromes P-450 and b5, and the activity of another heme protein, catalase. Heme oxygenase activity increased to 130, 151, 209, and 186% of control values at 12, 24, 48, and 72 hr after DBCP, respectively. A slight, but significant, increase in ALA-synthetase to 112% of controls occurred at 24 hr, and slight, but significant, decreases in ALA-dehydratase to 90 and 80% of control occurred at 12 and 24 hr, respectively. No significant changes in uroporphyrinogen-1-synthetase or ferrochelatase at the time points tested was noted. The porphyrin content of liver was increased to 130% of control, while the serum and urine porphyrin levels were decreased to 30% of the control values at 24 hr. Liver ALA content was not significantly altered through the time period studied, but serum and urine levels were increased at 24 hr to 176 and 130% of the control values, respectively. In conclusion, the decreases in liver heme proteins following a single oral dose of DBCP are accompanied by alterations in heme turnover, particularly a prolonged increase in heme oxygenase activity.

The potent induction of intestinal heme oxygenase by the organotin compound, bis(tri-n-butyltin)oxide

Oral administration of bis(tri-n-butyltin)oxide, an important organotin biocidal agent, produces a substantial elevation in heme oxygenase activity when measured at 16 hours in rat small intestine. An apparent Km for hemin of 100 microM is the same in both control and the organotin-induced 9,000 X g supernatant preparations. Concomitant with elevated heme oxygenase activity there occurs a substantial reduction in benzo(a)pyrene hydroxylase activity (approximately 20% of controls) and cytochrome P-450 concentration (approximately 60% of controls). These perturbations of heme metabolism in intestinal epithelium of the rat define an important new toxicological effect of organotins and raise the possibility that concurrent oral ingestion of environmental pollutants can directly affect the cytochrome P-450-dependent metabolism of other chemicals in the intestine.

Phenylhydrazine-mediated induction of haem oxygenase activity in rat liver and kidney and development of hyperbilirubinaemia. Inhibition by zinc-protoporphyrin

Phenylhydrazine was found to be a potent inducer of microsomal haem oxygenase activity in rat liver and kidney, but not in spleen. The phenylhydrazine-mediated increase in haem oxygenase activity was time-dependent. Maximum activity was attained 12h after treatment in the liver, and 24h after treatment in the kidney. The increases in the activity of haem oxygenase in the liver and the kidney could be inhibited by cycloheximide. Furthermore, the increases could not be elicited by the treatment of microsomal preparations in vitro with phenylhydrazine. In consonance with the increased haem oxygenase activity, a marked increase (16-fold) was observed in the serum total bilirubin concentration in phenylhydrazine-treated rats. The mechanism of haem degradation promoted by phenylhydrazine in vivo appears to differ from that in vitro; only in the former case is bilirubin formed as the end-product of haem degradation. When rats were given zinc-protoporphyrin (40 mumol/kg) 12h before and after phenylhydrazine treatment, the phenylhydrazine-mediated increases in haem oxygenase activity in the liver and the kidney were effectively blocked. Treatment of rats in vivo with the metalloporphyrin also inhibited the activity of splenic haem oxygenase, and promoted a major decrease in the serum bilirubin levels. In phenylhydrazine-treated animals, the microsomal content of cytochrome P-450 was significantly decreased in the absence of a decrease in the microsomal haem concentration. The decrease in cytochrome P-450 content was accompanied by an increased absorption in the 420nm region of the reduced CO-difference spectrum, suggesting the conversion of the cytochrome to an inactive form. The marked depletion of cellular glutathione levels suggests that this conversion may be related to the action of active intermediates and free radicals formed in the course of the interaction of phenylhydrazine with the haem moiety of cytochrome P-450.

An induction of heme oxygenase and its possible relation to the decrease of cytochrome P-450 content during liver regeneration

The alterations of various enzymes responsible for drug metabolism and heme metabolism were examined in regenerating livers of male rats. Microsomal cytochrome P-450 content and aminopyrine demethylase activity were significantly decreased during liver regeneration. In contrast, microsomal heme oxygenase activity was markedly increased under the identical conditions. The increased heme oxygenase activity which appeared within 4 h and reached maximum at 1 d after partial hepatectomy was sustained for 5 d. In sham-operated rats, the changed patterns of these parameters were similar, but to a lesser extent as compared to partially hepatectomized rats. The increase of heme oxygenase activity following partial hepatectomy was blocked by the administration of cycloheximide or actinomycin D. Other enzymes involved in heme synthesis did not change appreciably during liver regeneration. The inverse relationship between the decrease of cytochrome P-450 content and the increase of heme oxygenase activity was also observed in female rats and male mice. These findings suggest that the increase of heme oxygenase activity in regenerating rodent liver would be correlated to the decrease of cytochrome P-450 content. Adrenalectomy enhanced the increase of heme oxygenase activity following partial hepatectomy, though the decrease of cytochrome P-450 content was less extensive under the experimental conditions. The results also suggest that the inverse relationship between the increase of heme oxygenase activity and the decrease of cytochrome P-450 content would be a biochemical phenomenon seen in regenerating liver and that the phenomenon would not be simply due to the surgical stress to the animals.

Synergistic induction of microsomal heme oxygenase activity in rat liver and kidney by diethyldithiocarbamate and nickel chloride

Microsomal heme oxygenase activity was measured in liver and kidney of rats killed after administration of sodium diethyldithiocarbamate (DDC) and nickel chloride (NiCl2), singly and in combinations (DDC dosages: 0.33 to 1.33 mmol/kg, im, 17 hr before death; NiCl2 dosages: 0.125 and 0.25 mmol/kg, sc, 17 hr before death). Synergistic induction was observed at all dosage combinations. At the highest dosages of DDC and NiCl2, the dual treatments induced heme oxygenase activity 11-fold in liver and 16-fold in kidney; at the same dosages given individually, DDC induction of heme oxygenase activity was 3-fold in liver and 2-fold in kidney, and NiCl2-induction was 1.3-fold in liver and 6-fold in kidney. Synergistic induction of heme oxygenase activity in liver occurred when DDC was injected 6 hr before to 6 hr after NiCl2; synergistic induction in kidney occurred when DDC was injected 6 hr before to 3 hr after NiCl2. Actinomycin D prevented the induction of heme oxygenase activity by DDC or NiCl2, given individually; the effect of actinomycin D on synergistic induction could not be measured, since the rats all died following treatment with DDC, NiCl2, and actinomycin D. Administration of cysteine to rats, po, 18 hr before death, partially suppressed the induction of hepatic heme oxygenase activity by DDC, singly or in combination with NiCl2. Synergistic induction of hepatic heme oxygenase activity also occurred in rats that received dual injections of DDC (1.33 mmol/kg, im) and hemoglobin (0.3 g/kg, iv); the synergism of DDC and hemoglobin, although statistically significant, was small in comparison to the striking synergistic effect of DDC and NiCl2.

Investigation of adrenergic and prostaglandin influences in the endotoxin alteration of hepatic heme oxygenase, microsomal mixed-function oxidase, and glucocorticoid-induced tryptophan oxygenase activities

The possible role for adrenergic influences or prostaglandins in the effects of endotoxin to inhibit the glucocorticoid induction of hepatic tryptophan oxygenase (TO) activity, to decrease the hepatic microsomal cytochrome P--450-dependent drug-metabolizing activity, and to induce heme oxygenase activity was examined. Administration of the alpha-adrenergic locking agents phenoxybenzamine or phentolamine attenuated the inhibitory effect of the bacterial lipopolysaccharide on the induction of TO activity by dexamethasone. Injection of a beta-adrenergic blocker, propranolol, or of indomethacin, an inhibitor of prostaglandin biosynthesis, accentuated the effect of endotoxin to inhibit TO induction. Neither phenoxybenzamine, propranolol, nor indomethacin altered the effect of endotoxin to decrease aniline hydroxylase activity, ethylmorphine N-demethylase activity, or the levels of cytochrome P--450. Also, dexamethasone administration did not significantly protect against the effects of endotoxin on the hepatic microsomal drug metabolizing enzyme system, and none of the pharmacological agents diminished the effects of endotoxin to induce hepatic heme oxygenase activity. Endotoxin administration was also shown to diminish, but not prevent, the induction of cytochrome P--450 and ethylmorphine N-demethylase activity produced by phenobarbital. The results indicate that alpha-adrenergic mechanisms are involved in the endotoxic inhibition of the glucocorticoid induction of TO activity and suggest that neither adrenergic influences nor prostaglandins play a significant role in the effect of endotoxin to decrease hepatic mixed-function oxidase activity.

Induction of heme oxygenase and inhibition of delta-aminolevulinic acid synthetase of rat liver by thioacetamide and thioacetamide-S-oxide

Thioacetamide and one of its metabolite, thioacetamide-S-oxide, were shown to increase heme oxygenase and to inhibit delta-aminolevulinic acid (ALA) synthetase when administered in vivo to male rats. Concomitant with the increase of heme oxygenase and the decrease of ALA synthetase, concentration of cytochrome P-450 and drug metabolizing enzyme activities were decreased by in vivo administration of thioacetamide and thioacetamide-S-oxide to rats. The results of these studies indicate that thioacetamide and thioacetamide-S-oxide are not only inhibitor of ALA synthetase, but also inducer of heme oxygenase in rats. Further, thioacetamide-S-oxide is generally more effective than thioacetamide with respect to the effects on cytochrome P-450, ALA synthetase and heme oxygenase.

Nickel induction of microsomal heme oxygenase activity in rodents

Heme oxygenase activity was measured in tissues of rats killed after administration of NiCl2 or Ni3S2. Induction of renal heme oxygenase activity occurred 6 hr after NiCl2 injection (0.25 mmol/kg, sc), reached a maximum of five to six times the baseline activity at 17 hr, and remained significantly increased at 72 hr. Heme oxygenase activities were also increased in liver, lung, and brain at 17 hr after the NiCl2 injection; heme oxygenase activities in spleen and intestinal mucosa were unchanged. The effects of NiCl2 on heme oxygenase activities in kidney and liver were dose-related from 0.06 to 0.75 mmol/kg, sc. Three Ni chelators were administered (1 mmol/kg, im) prior to injection of NiCl2 (0.25 mmol/kg, sc); d-penicillamine partially prevented Ni induction of renal heme oxygenase activity; triethylenetetramine had no effect; sodium diethyldithiocarbamate enhanced the Ni induction of renal heme oxygenase activity (three times greater than NiCl2 alone). Intrarenal injection of Ni3S2 (10 mg/rat) caused induction of renal heme oxygenase activity at 1 week but not at 2, 3, or 4 weeks; no correlation was observed between induction of renal heme oxygenase activity and erythropoietin-mediated erythrocytosis. Hypoxia (10% O2, 12 hr/day, 7 days) did not affect renal heme oxygenase activity. Induction of renal heme oxygenase activity was observed in mice, hamsters, and guinea pigs killed 17 hr after injection of NiCl2 (0.25 mmol/kg, sc). These studies established (a) the time course, dose-effect, organ selectivity, and species susceptibility relationships for Ni induction of microsomal heme oxygenase activity, (b) the effects of Ni chelators, and (c) the lack of relationship between induction of renal heme oxygenase activity and the erythrocytosis that develops after intrarenal injection of Ni3S2.

Metal ion-mediated regulation of heme oxygenase induction in cultured avian liver cells

The induction of heme oxygenase (EC 1.14.99.3) in response to various metal treatments was investigated in monolayer cultures of chick embryo liver cells maintained in a chemically defined serum-free medium. The most potent heme oxygenase-inducing action was exhibited by CO2+, Cd2+, Sb3+, As3+, and Au1+ followed by lesser induction observed with Cu2+, Fe2+, and Fe3+. Mn2+, Ni2+, Se4+, Sn2+, and Zn2+ were without effect. In contrast to the marked inducing effect of Co2+ on heme oxygenase, Co-protoporphyrin IX decreased the enzyme activity in a dose-dependent manner. Addition of Zn2+ (20 microM) to Co2+-treated liver cell cultures revealed a striking ability of Zn2+ to block completely Co2+-induced heme oxygenase. Simultaneous addition of Mn2+ (50 microM) to Co2+-treated cells also blocked Co2+-induced heme oxygenase (approximately 50%). These findings in tissue culture confirm those made earlier in whole animals (Drummond, G. S., and Kappas, A. (1979) Proc. Natl. Acad. Sci. U. S. A. 76, 5331-5335) and indicate that these effects of Zn2+ and Mn2+ are exerted directly in liver cells. Addition of cysteine (400 microM) to the cultures also inhibited heme oxygenase induction by Co2+ substantially. Cycloheximide and actinomycin D blocked the induction of heme oxygenase, indicating that increased heme oxygenase activity by metal treatment is dependent on fresh RNA and protein synthesis. The half-life of the enzyme was calculated to be approximately 15 h after treatment with cycloheximide. These findings provide further evidence that metal ions can regulate heme oxygenase synthesis directly in isolated liver cells and that the metal-metal interactions which lead to blockade of the enzyme induction do not involve extrahepatic tissues.

Differential responses to inducers of delta-aminolaevulinate synthase and haem oxygenase during pregnancy

The responses of hepatic delta-aminolaevulinate synthase and microsomal haem oxygenase to inducers were examined in pregnant rats. 2-Allyl-2-isopropylacetamide-mediated induction of delta-aminolaevulinate synthase was greatly decreased during pregnancy and in the early post-partum period. Administration of allylisopropylacetamide to pseudopregnant rats induced delta-aminolaevulinate synthase normally. Treatment of pregnant rats with cortisol failed to restore the drug-mediated induction of delta-aminolaevulinate synthase. Microsomal cytochrome P-450 content and the activities of drug-metabolizing enzymes such as aniline hydroxylase and ethylmorphine. N-demethylase were significantly lowered during pregnancy. In contrast with the greatly impaired induction of delta-aminolaevulinate synthase, the induction of haem oxygenase in response to CoCl2 remained unaltered in pregnant rats. The normal perturbations of delta-aminolaevulinate synthase, consisting of an initial inhibition followed by a rebound increase in the enzyme activity associated with CoCL2 treatment, were observed during pregnancy. These findings indicate that hormones and metabolic factors associated with gestation exert significant but differential controls on the induction patterns of delta-aminolaevulinate synthase and haem oxygenase.

The potent heme oxygenase inducing action of arsenic and parasiticidal arsenicals

The administration of trivalent arsenic, either as sodium arsenite or as the trypanocidal drug melarsoprol, to rats produced a profound induction of microsomal heme oxygenase (EC 1.14.99.3) in both liver and kidney and a concomitant decrease in cytochrome P-450 content. In addition, perturbations of delta-aminolevulinate synthase were observed which showed an initial decline followed by a rebound increase in the activity of this enzyme with arsenical treatment. Pentavalent arsenic did not induce hepatic heme oxygenase but did induce the enzyme in kidney, although to a lesser extent (50%) than trivalent arsenic. Treatment of isolated chick embryo liver cells in vitro with sodium arsenite or the parasiticidal drug melarsoprol also showed a potent induction of heme oxygenase. These findings describe a new and potent ability of arsenic and parasiticidal arsenicals to induce heme oxygenase resulting in enhanced degradation of cellular heme.

Adrenalectomy enhances the induction of heme oxygenase and the degradation of cytochrome P-450 in liver

The induction of hepatic heme oxygenase in response to cobaltous chloride (CoCl2) administration was examined in normal, sham-operated, and adrenalectomized rats. The basal level of heme oxygenase was elevated about 2-fold in adrenalectomized rats as compared to normal controls or sham-operated animals. The extent of heme oxygenase induction by CoCl2 was also increased about 2-fold above normal in adrenalectomized animals and was accompanied by an enhanced breakdown of cytochrome P-450. The initial decline (approximately 2 h) and the late rebound increase (approximately 16 h) of delta-aminolevulinate synthase activity caused by the metal administration were, however, similar for all three groups of animals. Hydrocortisone is known to restore the impaired inducibility of delta-aminolevulinate synthase by allylisopropylacetamide in adrenalectomized rats. In this study, treatment with hydrocortisone prevented the exaggerated metal induction of heme oxygenase but did not affect the associated initial decline or the late rebound of delta-aminolevulinate synthase. These data indicate that hydrocortisone and adrenalectomy can significantly influence the extent of the induction of heme oxygenase produced by CoCl2, but that both the initial decline and the rebound induction of delta-aminolevulinate synthase associated with this metal treatment are apparently independent of these endocrine controls.

Metal ion interactions in the control of haem oxygenase induction in liver and kidney

Mn2+ and Zn2+ exhibit a striking ability to block the induction by Sn2+ and Ni2+ of haem oxygenase (EC 1.14.99.3) in kidney. The blocking effects of Mn2+ and Zn2+ were found to be greatest on simultaneous administration, time-dependent when administered up to 8 h before the inducing metal ions, and ineffective when administered as little as 10 min after the inducing metal ions. The decreases in cytochrome P-450 and haem contents and the sequential changes in delta-aminolaevulinate synthase (EC 2.3.1.37) activity that occur concomitant with haem oxygenase induction were largely eliminated with simultaneous or prior treatment with Mn2+ or Zn2+, but not when Mn2+ or Zn2+ was administered after Sn2+ or Ni2+. Mn2+ and Zn2+ did not increase the catabolism of the enzyme in vivo. Zn2+ on simultaneous administration was also able substantially to block the induction of haem oxygenase by Co2+, Cd2+ and Ni2+ in liver. The Zn2+ blockade of Cd2+ induction was examined in detail, and prior or simultaneous administration of Zn2+ was found to be effective in blocking the induction of haem oxygenase and the concomitant decreases in cytochrome P-450 and haem contents, ethylmorphine demethylase activity and the sequential changes in delta-aminolaevulinate synthase activity. Zn2+ administration 10 min or more after Cd2+ was ineffective in preventing the occurrence of these perturbations in haem metabolism. These findings describe a new and striking biological property of Mn2+ and Zn2+, and indicate the existence of significant metal ion interactions in the control of haem metabolism.

Prolonged induction of hepatic haem oxygenase and decreases in cytochrome P-450 content by organotin compounds

The administration of organotin compounds to rats in single doses causes a significant and prolonged induction of haem oxygenase and a sustained decrease in haemoprotein content in the liver. The extent of induction of hepatic haem oxygenase varied between 3 and 5-fold at 72h after a single injection of water-insoluble organotins of differing structure. The alterations in haem metabolism produced by tricyclohexyltin hydroxide were studied in detail. The effects were dose-dependent, with doses as low as 3.75 mg/kg body wt. resulting in significant induction of haem oxygenase and a decrease in cytochrome P-450 and cytochrome b5 contents at 72h in the liver. The effects with time of a single dose of tricyclohexyltin on various parameters of liver haem metabolism were also examined. The organotin produced a substantial and very prolonged induction of haem oxygenase accompanied by a steady decline in cytochrome P-450 content for periods up to 8 days. The long duration of action of these organotins with respect to induction of haem oxygenase and depletion of cellular haemoprotein content provides a highly sensitive metabolic system with which to define further the toxic potential of organometals as well as to study the adaptive responses in liver to long-term perturbations of haem metabolism by foreign chemicals.

Intracellular site of synthesis of microsomal heme oxygenase in pig spleen

In the pig spleen the specific activity of heme oxygenase was two to three times higher in smooth microsomes than in rough microsomes, whereas the total heme oxygenase activities recovered in the two microsomal fractions were similar. Free and bound polysomes were isolated from pig spleen and nascent peptides on these polysomes were analyzed by employing [3H]puromycin and a heme oxygenase-specific rabbit antibody (IgG). It was shown that free polysomes are the major site of heme oxygenase synthesis. In addition, cell-free synthesis of heme oxygenase was performed in a reticulocyte lysate system with free and bound polysomes isolated from pig spleen, and the results obtained again indicated that heme oxygenase is synthesized predominantly on free polysomes. The heme oxygenase newly synthesized on free polysomes may be incorporated first into the rough portion of endoplasmic reticulum either before or after its release from polysomes, although the specific activity of this enzyme at the steady state is considerably higher in the smooth region.