Showing papers on "Heme oxygenase published in 1977"

Metals as regulators of heme metabolism

Abstract: Heme is essential for cell respiration, energy generation, and oxidative biotransformations. The latter function is exemplified by the oxidative metabolism of various endogenous and exogenous chemicals catalyzed by the heme protein cytochrome P-450. Recent studies have established that metal ions directly regulate cellular content of heme, and thus of heme proteins by controlling production of delta-aminolevulinate synthetase and heme oxygenase, the rate-limiting enzymes for heme synthesis and degradation, respectively. Metal ions also alter cellular content of glutathione. In excess amounts, metal ions greatly accelerate the turnover and degradation of heme and substantially impair the oxidative functions of cells--particularly those dependent on cytochrone P-450. As a result, the biological impact of chemicals which are detoxified or metabolically transformed by the P-450 system is greatly altered.

Enzymatic oxidation of cobalt protoporphyrin ix. observations on the mechanism of heme oxygenase action.

Abstract: Studies on the enzymatic mechanism of microsomal heme oxygenase were made utilizing various porphyrins and metalloporphyrins of different ring substituents and central metal ions. Co-heme (cobalt protoporphyrin IX) was shown to be a substrate for the enzyme and the product of its oxidative metabolism was identified as the natural bile pigment, biliverdin IXalpha isomer. Metalloporphyrins, which do not bind molecular oxygen (Ni, Mn, and Sn protoporphyrin IX), were not substrates for heme oxygenase, although they could competitively inhibit oxidation of reactive substrates for the enzyme. The presence of lipophilic substitutents on pyrrole rings I and II, as well as a central metal atom, were required for the heme oxidation reaction to occur. The oxidative cleavage of Co-heme displayed typical characteristics of an enzyme-mediated reaction, and the oxidation of this substrate, as well as that of Fe-heme (iron protoporphyrin IX), could be supported with either reduced nicotinamide adenine dinucleotide phosphate or reduced nicotinamide adenine dinucleotide. A hypothesis is proposed on the mode of action of heme oxygenase in which the enzyme and its substrate are considered to form a "transitory" hemoprotein which can activate molecular oxygen for cleavage of the heme tetrapyrrole ring. In this formulation, heme as substrate for heme oxygenase is synonymous with heme as prosthetic group for the enzyme.

Regulation of heme pathway enzymes and cellular glutathione content by metals that do not chelate with tetrapyrroles: blockade of metal effects by thiols.

Abstract: The trace metals nickel and platinum, which are not substrates for ferrochelatase and thus do not form heme in biological systems, were found to act similaryl to cobalt, and heme itself, in regulating heme metabolism in liver and kidney. These metals induced heme oxygenase activity in both organs with the peak of induced enzyme activity reached approximately 16 hr after single injections in rats. Both metals caused transient depression of cellular glutathione content followed by increases above normal after 12 hr in liver. Nickel and platinum were more potent inducers of heme oxygenase in kidney than in liver (10-13 times normal versus 5-6 times normal). At high concentrations, they inhibited heme oxygenase [heme, hydrogen-donor:oxygen oxidoreductase (alpha-methene-oxidizing, hydroxylating), EC 1.14.99.3] in vitro. Both were active in regulating heme metabolism only when administered in the ionic form. Complexing of the metals with sulfhydryl agents completely blocked their actions on heme metabolism. Administration of cysteine orally prior to or shortly after administration of the metals had a similar blocking effect. Nickel and platinum produced depression of delta-aminolevulinate synthase [succinyl-CoA:glycine c-succinyltransferase (decarboxylating), EC 2.3.1.37] activity in liver, but neigther inhibited this rate-limiting ennzyme for heme synthesis in vitro. Furthermore, despite the substantial decreases in cellular heme and hemoprotein contents mediated by the metal, production of delta-amimolevulinate synthase did not undergo the compensatory increase that would be expected if there were a direct reciprocal feedback relationship between cellular heme level and synthesis of this enzyme. These findings indicate that it is not necessary for metal ions to be chelated in the porphyrin ring in order to regulate the enzymes of heme synthesis and heme oxidation. Accordingly, it is suggested that the iron atom of heme is the proximately active regulator of delta-aminolevulinate synthase and heme oxygenase--actions generally ascribed to the iron-tetrapyrrole complex itself--and that the tetrapyrrole moiety of the complex functions primarily as a means of transport of the metal to regulatory sites in cells.

Effects of Cadmium on Microsomal Hemoproteins and Heme Oxygenase in Rat Liver

Abstract: Various parameters of drug metabolism were measured in rats following a single intraperitoneal injection of cadmium acetate dihydrate (2.0 mg/kg; 7.5 µmoles/kg). Three days after treatment with Cd2+, the hexobarbital-induced sleeping time was increased to 240% of control; the microsomal contents of cytochromes P-450 and b5 were decreased by 44% and 27%, respectively. Seven days after treatment, the contents of cytochromes P-450 and b5 had partially returned to normal but each was 15-20% below control levels. Aminopyrine demethylase activity in hepatic microsomes was decreased by 47% and 37% at 3 and 7 days, respectively, after treatment with Cd2+; aniline hydroxylase was decreased by 32% and 23% at 3 and 7 days, respectively. Cadmium, given 3 days prior to injection of [3H]δ-aminolevulinic acid, decreased the half-life of the heme in CO-binding particles (microsomes devoid of cytochrome b5) from 8 hr to less than 2.5 hr in the fast-phase component, and from 60 hr to 32 hr in the slow-phase component. The greatest increase in microsomal heme oxygenase (to 350% of control) occurred 36-48 hr after treatment with Cd2+, and the enzymatic activity returned essentially to normal at 7 days. The activity of biliverdin reductase of the cytosol was not altered 3 or 7 days after treatment with Cd2+.

Enzymes of heme metabolism is the kidney. Regulation by trace metals which do not form heme complexes

Abstract: The in vivo regulation by metal ions of the enzymes of heme metabolism in kidney-particularly of ALAS, the rate-limiting enzyme in heine formation- was investigated. Ni(2+) and Pt(4+), metals which do not enzymatically form metalloporphyrins, were found to regulate ALAS in kidney as they do in liver. The pattern of this regulation was generally similar to that observed with heme and metal ions in liver, i.e., a late increase in enzyme activity after an early period in which ALAS activity was unaltered or inhibited. The metals did not interact with the enzyme in vitro to alter its activity. In this study no direct reciprocal relationship between ALAS activity and total cellular heine content was demonstrated. The metal ions, particularly Pt(4+), also altered the activity of other enzymes of heme biosynthesis in kidney. Pt(4+) severely inhibited the activity of ALAD and UROS. Ni(2+) and Pt(4+) were potent inducers of heme oxygenase, the initial and rate-limiting enzyme in heine degradation. It is proposed that the physiological regulation of ALAS is mediated through the action of metal ions, rather than by the cellular content of heine, and that the regulation of ALAS by heine reflects the action of the central metal ion of heme rather than that of the entire metalloporphyrin complex. In this proposed mechanism for metal ion regulation of ALAS, the tetrapyrrole moiety of heine is considered to function principally as an efficient carrier of metal to the regulatory site for ALAS production, inasmuch as the tetrapyrrole ring itself has been shown in earlier studies not to have any effect on ALAS activity. The production of heine oxygenase is believed to be similarly regulated.

Bile pigment formation by skin heme oxygenase: studies on the response of the enzyme to heme compounds and tissue injury.

Abstract: Skin heme oxygenase is locally elevated by stimuli such as tissue injury and injections of whole blood, myoglobin, and hematin. The enzyme activity is also increased at the proximity of the injection site of chemicals such as cobalt and cobalt-protoporphyrin-IX (cobalt-heme). Protoporphyrin-IX, the tetrapyrrole nucleus of type-b heme compounds, was ineffective in altering the enzyme activity in vivo. The developmental pattern of heme oxygenase in skin was compared to that of the enzyme in liver. The enzyme activity in both organs was greatest during the 1st postpartum wk and declined to adult levels after 2 wk. The physiological implications of the increased activity of skin heme oxygenase are discussed, and it is concluded that the activity of the hepatic heme oxygenase system and that of the skin are regulated by the same mechanism.

Heme Oxygenase Purified to Apparent Homogeneity from Pig Spleen Microsomes

Abstract: Heme oxygenase was purified to apparent homogeneity from pig spleen microsomes. The purified heme oxygenase showed an apparent molecular weight of 157,000 +/- 7,000 daltons when estimated by gel filtration. On SDS-polyacrylamide gel electrophoresis, the heme oxygenase preparation gave a single protein band showing a minimum molecular weight of about 26,000 daltons. Heme oxygenase could readily bind with heme and the resulting heme complex gave an absorption maximum at 406 nm. The heme bound to the enzyme protein was found to be a good substrate for the heme oxygenase reaction.

The mechanism of degradation of endogenous heme and cytochrome p-450 by heme oxygenase

Abstract: 1 Endogenous heme is metabolized both in vivo and in vitro by the microsomal heme oxygenase (MHO) system Endogenous heme includes both the heme already incorporated into microsomal cytochromes as well as the “free heme” ie, the heme not committed as a prosthetic group of cytochromes or enzymes 2 The conversion of cytochrome P450 to P420 is a pre-requisite for the metabolism of its heme prosthetic group by the MHO 3 It is suggested that metals may increase the rate of microsomal P450 degradation not only by inducing MHO but also by increasing the rate of conversion of cyt P450 to P420 The latter is accomplished by a) increasing the cellular content of a native “denaturant” of cyt P450; b) modifying the affinity of apo-P450 for heme; c) metal direct denaturing of the cytochrome

Heme Metabolism: Factors Affecting the in Vivo Oxidation of Heme

Abstract: The mechanism underlying the conversion of heme compounds to bile pigments has been extensively explored since the mid-1940s. Although the relationship between the formation of bilirubin and the catabolism of heme (hemoglobin) were noted as early as 1847 (Virchow), later studies by investigators such as Mann and co-workers (1926), conclusively showing that bilirubin is derived from hemoglobin, as well as later work by Fischer and Hess (1931) which identified the correct structure of bilirubin, led to a new era of investigation on heme metabolism. This chapter presents a summary of some major features of the older literature on bile pigment formation and compares and analyzes more recent developments in the field of heme oxidation and the factors influencing this process.