Serial review: regulatory an cytoprotective aspects of lipid hydroperoxide metabolismRegulation of enzymatic lipid peroxidation: the interplay of peroxidizing and peroxide reducing enzymes1
Introduction
Lipid peroxidation is commonly regarded as a deleterious process [1], [2] leading to structural modification of complex lipid protein assemblies, such as biomembranes and lipoproteins, and is usually associated with cellular malfunction. During lipid peroxidation a polar oxygen moiety (hydroperoxy group) is introduced into the hydrophobic tails of unsaturated fatty acids This process is of dual consequence: (i) the presence of the hydroperoxy group disturbs hydrophobic lipid/lipid and lipid/protein interactions, which leads to structural alterations of biomembranes and lipoproteins; (ii) hydroperoxy lipids are sources for the formation of free radicals, which may induce secondary modification of other membrane and/or lipoprotein constituents. When the lipid bilayer of biomembranes is oxidized, it may lose its barrier function and thus put the integrity of subcellular organelles or of the entire cell in danger. However, if lipid peroxidation proceeds in a controlled manner and is restricted to certain cellular compartments, it may have beneficial effects for cells and the whole organism (Fig. 1). Local destructive processes are integral parts of normal cellular metabolism. Cell differentiation and/or maturation are characterized by a breakdown of subcellular organelles and remodeling of cellular and subcellular membranes [3]. Membrane remodeling is also involved in trafficking of intracellular vesicles, phagocytosis, degranulation, antigen presentation, receptor-mediated ligand uptake, and many other processes, and all of them require temporary and local destabilization of the membrane structures. Lipid peroxidation, which alters the noncovalent interactions within the membrane bilayer, may contribute to local membrane destabilization. An essential precondition for a potential beneficial effect of local lipid peroxidation appears to be the requirement for a tight regulation. There must be the possibility to turn on lipid peroxidation when it is needed and to switch it off when it starts to become harmful. Such a tight regulation may not be possible with nonenzymatic reactions. In our aerobic world, evolution has created a variety of enzymatic and nonenzymatic antioxidative defense systems protecting living organisms from the deleterious effects of oxidizing compounds [4], [5]. However, nonenzymatic systems appear to be unsuitable for the targeted formation of oxidizing species because they may not be versatile enough. In contrast, enzymatic lipid peroxidizing systems, which can be up- or downregulated on transcriptional, translational, and posttranslational levels, appear to be well adapted for the above described biological requirements.
Enzymatic oxidation of lipophilic cellular constituents is a universal principle in the metabolism of living organisms, and a variety of enzyme systems have been created for this purpose. Among these systems, lipoxygenases (LOXs) are somewhat unique because they catalyze the specific dioxygenation of polyenoic fatty acids using atmospheric dioxygen as second substrate. In contrast to lipid monooxygenases (e.g., cytochrome P-450), which mainly catalyze substrate hydroxylation, LOXs introduce molecular dioxygen, forming reactive fatty acid hydroperoxides. Alternatively, formation of lipid peroxides can also be induced by free radicals formed as reaction intermediates of other oxidative metabolic pathways. In these latter cases, lipid peroxidation may be considered as a minor side-reaction of an enzymatic process, and multiple protective systems (antioxidants) are available to counteract such “metabolic accidents.” In contrast, LOXs appear to be specifically designed for catalyzing lipid peroxidation and many naturally occurring antioxidative defense systems are ineffective in modulating the lipoxygenase reaction.
LOXs emerged early on in the development of living organisms. Since they have not been eliminated during evolution, there appears to be a high selection pressure on these proteins. In fact, the diversity of the LOX family has increased during evolution. Starting from a putative ancient precursor, a heterogeneous enzyme family has evolved and today LOXs are widely distributed in plants [6], [7] and animals [8], [9]. Among the different LOX isoforms only 12/15-LOXs are capable of oxidizing complex ester lipids even when they are bound to biomembranes or lipoproteins. Thus, this enzyme subfamily may be of particular importance for inducing structural alteration of complex lipid/protein assemblies. The phospholipid hydroperoxide glutathione peroxidase (phGPx) is the metabolic antipode of 12/15-LOXs. phGPx is unique among the different types of seleno glutathione peroxidases (GPxs) because of its capability of reducing membrane-bound ester lipid hydroperoxides. In this respect, phGPx constitutes the antioxidative counterplayer of the pro-oxidative 12/15-LOXs. Thus, the two enzymes constitute a pair of antagonizing proteins within the oxidative metabolism of complex ester lipids (Fig. 2).
This review is aimed at summarizing the recent trends in 12/15-LOX and phGPx research, with emphasis on cytokine-dependent regulation and the regulatory interaction of the two enzymes. Because of space limitations, other members of the LOX and GPx families are only briefly described. It should be stressed that this article is not intended to extend the controversial discussion on the biological importance of various LOX- and GPx-isoforms. The interested reader is referred to recent review articles on this topic [6], [7], [8], [9], [10], [11], [12], [13].
Section snippets
Classification of lipoxygenases
LOXs constitute a heterogeneous family of lipid peroxidizing enzymes that catalyze the oxygenation of polyenoic fatty acids. They are widely distributed in plants [6], [7] and in the animal kingdom [8], [9]. They can also be found in lower marine organisms, such as algae, sea urchin, star fish, surf clam, and corals [14], [15], [16], [17], as well as in fungi [18], [19]. Most recently [20], LOX sequences have also been detected in bacteria (P. aeruginosa, accession No. AE004547; S. cellulosum;
Classification of glutathione peroxidases
Glutathione peroxidases (GPxs) constitute a family of enzymes, which are capable of reducing a variety of organic and inorganic hydroperoxides to the corresponding hydroxy compounds, utilizing glutathione, and/or other reducing equivalents [10], [11], [12], [13]. There are selenium-containing and nonselenium glutathione peroxidases, but this review is focused on the selenoproteins only. Selenoperoxidases contain selenocysteine at their active site and this amino acid is involved in the
Regulation of cellular lipoxygenase activity
The intracellular 15-LOX activity is strongly regulated, and elements of transcriptional, translational, and posttranslational control have been reported. In humans and rabbits, the reticulocyte-type 15-LOX is not expressed in most cells under resting conditions. However, during experimental anemia the expression of the 15-LOX gene is strongly upregulated in a variety of cells and tissues, with the exception of brain and skeletal muscle [99]. A second element of transcriptional regulation is
Regulation of cellular phGPx activity
As indicated above, phGPx is highly ranked in the hierarchy of selenoproteins, since expression of the enzyme is kept constant even under the conditions of severe selenium deficiency. Moreover, there is a remarkable tissue ranking of this enzyme, with brain and testis being most resistant towards selenium deficiency [12]. Selenium depletion of H4-hepatoma cells strongly reduced the cGPx protein and its mRNA abundance. In contrast, the steady state concentration of phGPx mRNA was hardly affected
Regulatory interplay and cell physiological consequences
Although phGPx constitutes the counterplayer of 12/15-LOX in the peroxide metabolism of complex ester lipids (Fig. 2), it is important to note that the enzyme does not reverse the chemical alterations in membrane lipids induced by LOXs. Rather, the reactive hydroperoxy lipids are simply reduced to the corresponding hydroxy derivatives and this reduction removes a membrane-bound source for the formation of free radicals. Hydroxy lipids still carry a hydrophilic OH-group, which disturbs the
Perspectives
The regulatory interplay of 12/15-LOX and phGPx is of physiological relevance, since the intracellular peroxide tone appears to be an important regulator of the expression of redox-sensitive genes, such as adhesion molecules and heat shock proteins. An imbalance in the steady state of these metabolic counterparts leads to up- or downregulation of target genes and thus, may alter the functional status of the cells. In vitro experiments have already indicated that 12/15-LOX and phGPx interact
Acknowledgements
Financial aid of Deutsche Forschungsgemeinschaft (Ku 961/6-1 and Ku 961/7-1) and of the European Community (BMH4-CT98-3191) is acknowledged. The authors are indebted to Dr. T. Schewe (Düsseldorf) for critical reading of the manuscript and valuable discussion.
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- 1
This article is part of a series of reviews on “Regulatory and Cytoprotective Aspects of Lipid Hydroperoxide Metabolism.” The full list of papers may be found on the homepage of the journal.
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Hartmut Kühn was trained in Medicine at Charité (Medical faculty of Humboldt University) from 1974–79. After 1 year of clinical internship, he completed his doctoral thesis on the quasi-lipoxygenase activity of hemoglobin in 1981 and moved as a postdoctoral fellow to the Institute of Biochemistry (Charité) to study with Dr. T. Schewe the enzymology and physiology of the rabbit reticulocyte 15-lipoxygenase. In 1986 Dr. Kühn started a year of postdoctoral study at Vanderbilt University (Nashville, TN, USA) and after returning to Charité, he became a senior scientist and research group leader. In the early 1990s he spent 1 year sabbaticals in the United States (UCSF, Baylor) and Japan (Tokushima). Dr. Kühn is now a professor at the Charité and investigates the enzymology and molecular biology of lipoxygenases and their metabolic counterparts, the glutathione peroxidases.
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Astrid Borchert has been a graduate student in Dr. Kühn’s lab. She was trained in food chemistry at the Humboldt University and completed her thesis for diploma in 1992. In 1995 she moved to the lab of Dr. Kühn and has been studying the regulation of expression of phospholipid hydroperoxide glutathione peroxidase isoforms in mammals and the interaction of these enzymes with various lipoxygenase isoforms.