A two-molecule mechanism of haem degradation

Free heme toxicity and its detoxification systems in human

Severe hemolysis or myolysis occurring during pathological states, such as sickle cell disease, ischemia reperfusion, and malaria results in high levels of free heme, causing undesirable toxicity leading to organ, tissue, and cellular injury. Free heme catalyzes the oxidation, covalent cross-linking and aggregate formation of protein and its degradation to small peptides. It also catalyzes the formation of cytotoxic lipid peroxide via lipid peroxidation and damages DNA through oxidative stress. Heme being a lipophilic molecule intercalates in the membrane and impairs lipid bilayers and organelles, such as mitochondria and nuclei, and destabilizes the cytoskeleton. Heme is a potent hemolytic agent and alters the conformation of cytoskeletal protein in red cells. Free heme causes endothelial cell injury, leading to vascular inflammatory disorders and stimulates the expression of intracellular adhesion molecules. Heme acts as a pro-inflammatory molecule and heme-induced inflammation is involved in the pathology of diverse conditions; such as renal failure, arteriosclerosis, and complications after artificial blood transfusion, peritoneal endometriosis, and heart transplant failure. Heme offers severe toxic effects to kidney, liver, central nervous system and cardiac tissue. Although heme oxygenase is primarily responsible to detoxify free heme but other extra heme oxygenase systems also play a significant role to detoxify heme. A brief account of free heme toxicity and its detoxification systems along with mechanistic details are presented.

Separation and identification of the regioisomers of verdoheme by reversed-phase ion-pair high-performance liquid chromatography, and characterization of their complexes with heme oxygenase

We report an HPLC method for separating the four regioisomers of verdoheme formed in the coupled oxidation of hemin with oxygen and ascorbate in aqueous pyridine. The reversed-phase ion-pair system uses hexafluoroacetone and pyridine as ion-pair agents. The regiochemistry of the separated isomers was established both by HPLC of the corresponding biliverdin IX derivatives and by 1H NMR of each isomer. Optical spectra of the pyridine verdohemochrome isomers were similar to each other, but showed differences in the absorption maxima in the red region, which appear at 680, 663, 648 and 660 nm for the alpha, beta, gamma, and delta-isomers, respectively. Each of the four isomers was incorporated anaerobically into heme oxygenase-1, yielding the corresponding verdoheme-enzyme complex. The ferrous forms had absorption maxima at 690, 667, 655, and 663 nm, and their CO-bound forms had maxima at 638, 624, 616, and 626 nm for alpha, beta, gamma, and delta-isomer, respectively. Addition of ferricyanide to the alpha-verdoheme-heme oxygenase complex brought about a ferric low-spin heme-like signal, which is identical with the ferric alpha-verdoheme complexed with the heme oxygenase that was observed in the heme oxygenase reaction.

In Situ Monitoring of the Degradation of Iron Porphyrins by Dioxygen with Hydrazine as Sacrificial Reductant. Detection of Paramagnetic Intermediates in the Coupled Oxidation Process by (1)H NMR Spectroscopy

The effects of using hydrazine rather than ascorbic acid on the coupled oxidation of (OEP)Fe(II)(py)(2) (OEP is the dianion of octaethylporphyrin, py is pyridine) have been investigated with the goal of directly detecting reactive intermediates during the process of heme degradation by dioxygen. The reaction products, [(OEOP)Fe(II)(py)(2)]Cl and (OEB)Fe(III)(py)(2) (OEOP is the monoanion of octaethyl-5-oxaporphyrin and OEB is the trianion of octaethylbilindione), and their yields are similar to those of the standard coupled oxidation process. The reaction has been monitored in situ in pyridine/dichloromethane mixtures by (1)H NMR spectroscopy. The recently isolated and crystallographically characterized complex, (OEPO)Fe(py)(2) (OEOP is the trianion of octaethyloxaphlorin), has been identified as a key intermediate. Addition of dioxygen to (OEP)Fe(II)(py)(2) in pyridine with hydrazine present also produces two new transient species: (OEPO)Fe(py)(N(2)D(4)) and (OEPO)Fe(N(2)D(4))(2). These complexes have also be produced independently by low-temperature titration of hydrazine into a solution of {(OEPO)Fe}(2). Thus, hydrazine acts as an axial ligand during the early stages of the coupled oxidation process. However, the two hydrazine-containing complexes eventually are converted into (OEPO)Fe(py)(2) before [(OEOP)Fe(II)(py)(2)]Cl and (OEB)Fe(III)(py)(2) are formed. The observations reported here suggest that the coupled oxidation process can be divided into two stages. The first stage involves the meso C-H bond and results in introduction of oxygen at that site with the formation of the three intermediates: (OEPO)Fe(N(2)H(4))(2), (OEPO)Fe(N(2)H(2))(py), and (OEPO)Fe(py)(2). The second stage of the coupled oxidation process involves C-C bond breaking and the conversion of the hydroxylated heme, (OEPO)Fe(py)(2), into the final products, [(OEOP)Fe(II)(py)(2)](+) and (OEB)Fe(III)(py)(2).

Carbon monoxide: a role in carotid body chemoreception

Carbon monoxide (CO), produced endogenously by heme oxygenase, has been implicated as a neuronal messenger. Carotid bodies are sensory organs that regulate ventilation by responding to alterations of blood oxygen, CO2, and pH. Changes in blood gases are sensed by glomus cells in the carotid body that synapse on afferent terminals of the carotid sinus nerve that projects to respiratory-related neurons in the brainstem. Using immunocytochemistry, we demonstrate that heme oxygenase 2 is localized to glomus cells in the cat and rat carotid bodies. Physiological studies show that zinc protoporphyrin IX, a potent heme oxygenase inhibitor, markedly increases carotid body sensory activity, while copper protoporphyrin IX, which does not inhibit the enzyme, is inactive. Exogenous CO reverses the stimulatory effects of zinc protoporphyrin IX. These results suggest that glomus cells are capable of synthesizing CO and endogenous CO appears to be a physiologic regulator of carotid body sensory activity.

Effect of cis-platinum on heme, drug, and steroid metabolism pathways: possible involvement in nephrotoxicity and infertility

cis-Platinum, a coordination complex of platinum, is highly effective against a number of human tumors, including steroid-dependent tumors such as testicular and prostatic cancers. It is generally assumed that DNA is the cellular target responsible for the antitumor activity of the drug. Much evidence, however, has been gathered in recent years suggesting that cis-platinum has major effects on the endocrine system, particularly the hypothalamic-pituitary-testis steroidogenesis axis, and severely disrupts the normal production of testosterone. In the axis, testis is the prime target, where the LH receptor-binding capacity of Leydig cells is decreased by nearly 80%. Within the testis, the mitochondrial cytochrome P-450scc and side-chain cleavage activity are markedly depressed and the microsomal 17 alpha-hydroxylase activity and cytochrome P-450 concentration are decreased; side-chain cleavage activity is rate-limiting in testosterone production. The effects are not limited to the testis cytochrome P-450, but extend to other organs including the liver and the kidney cytochromes. In the liver, a feminization of the cytochromes P-450 profile is produced, and hence the biotransformation of endogenous steroids as well as that of exogenous chemicals is affected. In the kidney, cis-platinum appears to be the most effective inducer of cytochrome P-450, whereby the biotransformation of the prostaglandins and fatty acids could be altered. The spectrum of the effects of cis-platinum on cytochrome P-450-dependent drug metabolism and steroid hydroxylation activity mimic those produced by hypophysectomy and are for the most part reversed by the anterior pituitary hormones. These findings suggest the possibility that general feminization of steroidogenesis caused by cis-platinum may significantly contribute to the activity of cis-platinum against hormone-dependent tumors.