Structural studies on bovine spleen heme oxygenase. Immunological and structural diversity among mammalian heme oxygenase enzymes

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

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.

Selectivity of imidazole-dioxolane compounds for in vitro inhibition of microsomal haem oxygenase isoforms

Haem oxygenases (HO) are involved in the catalytic breakdown of haem to generate carbon monoxide (CO), iron and biliverdin. It is widely accepted that products of haem catabolism are involved in biological signaling in many physiological processes. Conclusions to most studies in this field have gained support from the judicious use of synthetic metalloporphyrins such as chromium mesoporphyrin (CrMP) to selectively inhibit HO. However, metalloporphyrins have also been found to inhibit other haem-dependent enzymes, such as nitric oxide synthase (NOS), cytochromes P-450 (CYPs) and soluble guanylyl cyclase (sGC), induce the expression of HO-1 or exhibit varied toxic effects. To obviate some of these problems, we have been examining non-porphyrin HO inhibitors and the present study describes imidazole-dioxolane compounds with high selectivity for inhibition of HO-1 (rat spleen microsomes) compared to HO-2 (rat brain microsomes) in vitro. (2R,4R)-2-[2-(4-chlorophenyl)ethyl]-2-[(1H-imidazol-1-yl)methyl]-4-methyl-1,3-dioxolane hydrochloride) was identified as the most selective inhibitor with a concentration of 0.6 microM inhibiting HO-1(inducible) by 50% compared with 394 microM for HO-2 (constitutive). These compounds were found to have no effects on the catalytic activities of rat brain NOS and lung sGC, but were potent inhibitors of microsomal CYP2E1 and CYP3A1/3A2 activities. In conclusion, we have identified imidazole-dioxolanes that are able to inhibit microsomal HO in vitro with high selectivity for HO-1 compared to HO-2, and little or no effect on the activities of neuronal NOS and sGC. These molecules could be used to facilitate studies on the elucidation of physiological roles of HO/CO in biological systems.

Heme oxygenase-2 is a hemoprotein and binds heme through heme regulatory motifs that are not involved in heme catalysis

The heme oxygenase (HO) system degrades heme to biliverdin and CO and releases chelated iron. In the primary sequence of the constitutive form, HO-2, there are three potential heme binding sites: two heme regulatory motifs (HRMs) with the absolutely conserved Cys-Pro pair, and a conserved 24-residue heme catalytic pocket with a histidine residue, His151 in rat HO-2. The visible and pyridine hemochromogen spectra suggest that the Escherichia coli expressed purified HO-2 is a hemoprotein. The absorption spectrum, heme fluorescence quenching, and heme titration analysis of the wild-type protein versus those of purified double cysteine mutant (Cys264/Cys281 --> Ala/Ala) suggest a role of the HRMs in heme binding. While the His151 --> Ala mutation inactivates HO-2, Cys264 --> Ala and Cys281 --> Ala mutations individually or together (HO-2 mut) do not decrease HO activity. Also, Pro265 --> Ala or Pro282 --> Ala mutation does not alter HO-2 activity. Northern blot analysis of ptk cells indicates that HO-2 mRNA is not regulated by heme. The findings, together with other salient features of HO-2 and the ability of heme-protein complexes to generate oxygen radicals, are consistent with HO-2, like five other HRM-containing proteins, having a regulatory function in the cell.

The heme oxygenase system: a regulator of second messenger gases

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.

Membrane topology of liver microsomal cytochrome P450 2B4 determined via monoclonal antibodies directed to the halt-transfer signal

The membrane topology of cytochrome P450 2B4 from the endoplasmic reticulum has been studied with highly-purified liver microsomes in a site-directed immunochemical approach. Microsomes were prepared from phenobarbital-induced rabbits, and the resulting microsomal fraction was washed 6 additional times with 0.1 M pyrophosphate buffer to effect removal of significant quantities of adventitiously-bound protein. Monoclonal antibodies were prepared against residues 18-29 of P450 2B4 (Leu18-Leu-Phe-Arg-Gly-His-Pro-Lys-Ala-His-Gly-Arg29), essentially corresponding to the halt-transfer signal. This region was chosen due to its mutually-exclusive location in the two alternative membrane topology models currently tenable [Black, S.D. (1992) FASEB J.6, 680-685]. Model "A" contains a single transmembrane anchor peptide with the amino terminus projecting into the lumen of the endoplasmic reticulum, while model "B" exhibits a hairpin loop of the first approximately 46 residues inserted into the membrane with the amino terminus located on the cytosolic side of the lipid bilayer; the halt-transfer signal peptide would be located at the cytosolic surface of the membrane in model "A" or as a loop on the lumenal side of the membrane in model "B". Nine antibodies, denoted as MmAbA, MmAbC, MmAbD, MmAbF, MmAbH, MmAbI, MmAbK, MmAbL, and MmAbP, were produced, and all were identified as IgM/kappa subtypes. Western blotting demonstrated that the antibodies could readily recognize P450 2B4 in microsomes. ELISA assays showed that all of the antibodies exhibited strong binding to intact microsomes.(ABSTRACT TRUNCATED AT 250 WORDS)

Mitochondrial biogenesis in striated muscle

Mitochondrial biogenesis (synthesis) has been observed to occur in skeletal muscle in response to chronic use. It also occurs in cardiac muscle during growth and hypertrophy, and it may be impaired during the aging process. This review summarizes the literature on the processes of mitochondrial biogenesis at the biochemical and molecular levels, with particular reference to striated muscles. Mitochondrial biogenesis involves the expression of nuclear and mitochondrial genes and the coordination of these two genomes, the synthesis of proteins and phospholipids and their import into the organelle, and the incorporation of these lipids and proteins into their appropriate locations within the matrix, inner or outer membranes. The emphasis is on the regulation of these events, with information derived in part from other cellular systems. Although descriptions of mitochondrial content changes in heart and skeletal muscle during altered physiological states are plentiful, much work is needed at the molecular level to investigate the regulatory processes involved. A knowledge of biochemical and molecular biology techniques is essential for continued progress in the field. This is a promising area, and potential new avenues for future research are suggested.

Characterization of a cDNA-encoding rabbit brain heme oxygenase-2 and identification of a conserved domain among mammalian heme oxygenase isozymes: possible heme-binding site?

A 1.3-kb rat testis cDNA clone for heme oxygenase-2 (HO-2) was used as a Northern blot hybridization probe, and a single homologous mRNA species, of approximately 1.3 kb in rabbit brain and testis was detected. This contrasted with the observation made with rat brain in which two HO-2 transcripts of approximately 1.3 and 1.9 kb were detected. Use of the same rat HO-2 probe to screen a rabbit brain cDNA library in lambda gt11 resulted in the recovery of a single 1.2-kb cDNA clone. This cDNA exhibits 84% overall nucleotide sequence homology with rat HO-2 and encodes a protein of 35,352 Da, displaying 88% amino acid sequence homology with rat testis HO-2. Furthermore, when expressed in Escherichia coli, the rabbit cDNA-encoded protein displays heme oxygenase activity and cross-reactivity with antibody to rat HO-2. Based on findings obtained through Western immunoblot analysis of partially purified HO-2 protein prepared from rabbit testis and brain, the 35- to 36-kDa molecular form appears to be the major HO-2 form detected in the brain, whereas a 42-kDa species is the predominant form observed in rabbit testis. Having deduced the amino acid sequence of rabbit brain HO-2, we provide a comparison of this sequence with those of rat, mouse, and human HO-1 and rat HO-2, and thereby identify a 24-amino-acid-long peptide region which, except for one residue, is identical in all five species of HO-1 and HO-2 compared (96% similarity), and exhibits 100% similarity in predicted secondary structure (for this region) in all five proteins. We propose that this peptide may be important to the heme binding and isomer-specific tetrapyrrole cleavage activities of the heme oxygenase isozymes.