The heme oxygenase (HO) system consists of two forms identified to date: the oxidative stress-inducible protein HO-1 (HSP32) and the constitutive isozyme HO-2. These proteins, which are different gene products, have little in common in primary structure, regulation, or tissue distribution. Both, however, catalyze oxidation of heme to biologically active molecules: iron, a gene regulator; biliverdin, an antioxidant; and carbon monoxide, a heme ligand. Finding the impressive heme-degrading activity of brain led to the suggestion that "HO in brain has functions aside from heme degradation" and to subsequent exploration of carbon monoxide as a promising and potentially significant messenger molecule. There is much parallelism between the biological actions and functions of the CO- and NO-generating systems; and their regulation is intimately linked. This review highlights the current information on molecular and biochemical properties of HO-1 and HO-2 and addresses the possible mechanisms for mutual regulatory interactions between the CO- and NO-generating systems.

THE HEME OXYGENASE SYSTEM:A Regulator of Second Messenger Gases

Heme-Containing Oxygenases

The heme oxygenases, which consist of constitutive and inducible isozymes (HO-1, HO-2), catalyze the rate-limiting step in the metabolic conversion of heme to the bile pigments (i.e., biliverdin and bilirubin) and thus constitute a major intracellular source of iron and carbon monoxide (CO). In recent years, endogenously produced CO has been shown to possess intriguing signaling properties affecting numerous critical cellular functions including but not limited to inflammation, cellular proliferation, and apoptotic cell death. The era of gaseous molecules in biomedical research and human diseases initiated with the discovery that the endothelial cell-derived relaxing factor was identical to the gaseous molecule nitric oxide (NO). The discovery that endogenously produced gaseous molecules such as NO and now CO can impart potent physiological and biological effector functions truly represented a paradigm shift and unraveled new avenues of intense investigations. This review covers the molecular and biochemical characterization of HOs, with a discussion on the mechanisms of signal transduction and gene regulation that mediate the induction of HO-1 by environmental stress. Furthermore, the current understanding of the functional significance of HO shall be discussed from the perspective of each of the metabolic by-products, with a special emphasis on CO. Finally, this presentation aspires to lay a foundation for potential future clinical applications of these systems.

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Heme oxygenase-1/carbon monoxide: from basic science to therapeutic applications.

The stress-inducible protein heme oxygenase-1 provides protection against oxidative stress. The anti-inflammatory properties of heme oxygenase-1 may serve as a basis for this cytoprotection. We demonstrate here that carbon monoxide, a by-product of heme catabolism by heme oxygenase, mediates potent anti-inflammatory effects. Both in vivo and in vitro, carbon monoxide at low concentrations differentially and selectively inhibited the expression of lipopolysaccharide-induced pro-inflammatory cytokines tumor necrosis factor-α, interleukin-1β, and macrophage inflammatory protein-1β and increased the lipopolysaccharide-induced expression of the anti-inflammatory cytokine interleukin-10. Carbon monoxide mediated these anti-inflammatory effects not through a guanylyl cyclase–cGMP or nitric oxide pathway, but instead through a pathway involving the mitogen-activated protein kinases. These data indicate the possibility that carbon monoxide may have an important protective function in inflammatory disease states and thus has potential therapeutic uses.

Carbon monoxide has anti-inflammatory effects involving the mitogen-activated protein kinase pathway

In biological systems oxidation of heme is carried out by two isozymes of the microsomal heme oxygenase, HO-1 and HO-2. HO-1 is the commonly known heme oxygenase, the activity of which can be induced by up to 100-fold in response to a wide variety of stimuli (metals, heme, hormones, etc.). HO-2 was only recently discovered, and the isozyme appears to be uninducible. The two forms are products of two different genes and differ in their tissue expression. The primary structure of HO-1 and an HO-2 fragment of 91 amino acid residues show only 58% homology, but share a region with 100% secondary structure homology. This region is believed to be the catalytic site. Most likely, HO-1 gene is regulated in the same manner as metallothione in the gene. HO-1 has a heat shock regulatory element, and possibly many promoter elements, which bind to respective inducers and cause transcription of the gene. In vivo induction of HO-1 activity in the liver is accompanied by decreases in the total P-450 levels and, in a reconstituted system, cytochrome P-450b heme can be quantitatively converted to biliverdin by HO-1 and HO-2. The enzyme activity is inhibited in vivo for extended periods subsequent to binding of Zn- and Sn- protoporphyrins. This property appears useful for the suppression of bilirubin production. The metalloporphyrins, however, are not innocuous and cause major disruptions in cellular metabolism. In this review recent findings on heme oxygenase are highlighted.

Heme oxygenase: function, multiplicity, regulatory mechanisms, and clinical applications.

The enzymatic conversion of heme to bilirubin by microsomal heme oxygenase

Seventy-one male Sprague-Dawley rats weighing from 250 to 400 g were maintained on Purina Laboratory Chow, except for a 24 hour period immediately preceding the experiment. During this period, they were given 36.5 g glucose per kg together with 7.5 g ethanol per kg or, in the experiment shown in Table III, 50 g per kg fructose, administered by means of 5 gastric tube feedings of equal volume. Approximately 3 hours after the last gastric feeding, the animals were decapitated. The liver was quickly removed, and slices were prepared in the cold with a Stadie-Riggs slicer (4). The C14-labeled 1 and unlabeled substrates were dissolved in 5 ml isotonic KrebsRinger bicarbonate buffer, pH 7.4 (6), and the final concentration of the substrates was expressed as millimoles per liter. An aliquot of each incubation mixture was added to a toluene solution containing 12 per cent methanol, 0.4 per cent diphenyloxazole (DPO) and 0.005 per

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The effect of ethanol on fatty acid metabolism; stimulation of hepatic fatty acid synthesis in vitro*

/pdf/chronic-progressive-myopathy-with-myoglobinuria-fk7mn1f1e2.pdf

Chronic progressive myopathy with myoglobinuria: demonstration of a glycogenolytic defect in the muscle*

Reduced nicotinamide adenine dinucleotide phosphate dependent biliverdin reductase. Partial purification and characterization

It has recently been reported that bilirubin is excreted in the bile as a water-soluble glucuronide (1-3). The conjugation of bilirubin is believed to occur mainly in the liver (4, 5). In vitro, formation of bilirubin glucuronide has been demonstrated with a system containing liver microsomes as the enzyme source and uridine diphosphate glucuronic acid as the glucuronic acid donor (5, 6). Since conjugation of bilirubin appears to be essential for its excretion in the bile, a defect in the glucuronide forming mechanism would result in impaired pigment excretion and, hence, in retention of free bilirubin in the blood. In preliminary communications (7, 8), such defects in glucuronide formation have been reported in congenital nonhemolytic nonobstructive jaundice in man (9) and in hereditary nonhemolytic jaundice in rats (10). The present report is concerned with the details of the murine studies, while the human syndrome will be considered in a separate communication.

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Rudi Schmid

Papers

The enzymatic conversion of heme to bilirubin by microsomal heme oxygenase

The effect of ethanol on fatty acid metabolism; stimulation of hepatic fatty acid synthesis in vitro*

Chronic progressive myopathy with myoglobinuria: demonstration of a glycogenolytic defect in the muscle*

Reduced nicotinamide adenine dinucleotide phosphate dependent biliverdin reductase. Partial purification and characterization

Congenital Jaundice in Rats, Due to a Defect in Glucuronide Formation