Chem. Res. Toxicol. 1993,6, 134-146
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Invited Review Sources and Role of Iron in Lipid Peroxidation Giorgio Minotti Institute of General Pathology, Catholic University School of Medicine, Largo F. Vito 1, 00168 Rome, Italy Received August 5, 1992
I. Introduction The formation of an intracellular pool of free iron can be viewed as a sort of double-edged sword. Some iron is necessary for cell functions, but excess iron triggers deleterious reactions such as the degradation of proteins and nucleic acids, or the peroxidative decomposition of polyunsaturated fatty acids (LH1.I In particular, lipid peroxidation has been repeatedly invoked as a possible mechanism of inflammation (I),postischemic reperfusion injury (2), atherosclerosis (3), ethanol toxicity (4), and cancer (5, 6). Moreover, there have been reports on a possible involvement of lipid peroxidation in the toxicities of drugs (7,8)and environmental pollutants (91, as well as in the acute and chronic consequences of traumas (10, 11). These represent only a few examples of the toxicities and pathologiesthat have been linked to lipid peroxidation. A complete list of lipid peroxidation-dependent diseases would be probably endless. Not surprisingly, some investigators have questioned that pathophysiological significance of lipid peroxidation and have suggested that it may represent a nonspecific consequence rather than a specific cause of diseases (12). The uncertainty as to the actual role of lipid peroxidation has been caused by the lack of appropriate ex vivo indices. Lipid peroxidation can be studied in vitro by monitoring the formation of conjugated dienes or thiobarbituric acidreactive substances (TBARS) from biological membranes or commerciallyavailable LH. Unfortunately, body fluids or tissue biopsies contain numerous substances which interfere with the spectrophotometric assays for conjugated dienes or TBARS and thus preclude adequate evaluation of lipid peroxidation which has occurred in vivo. These problems can be overcome by recent developments of more appropriate and sensitive techniques, e.g., capillary gas chromatography and mass spectrometry. These procedures have enabled Morrow et al. (13,14) to identify arachidonate metabolites that are similar to the prostaglandin Fzbut are formed in a cyclooxygenase-independent manner. The blood plasma and urine levels of these prostanoids increase significantly in animals exposed to oxidative stress (13),or in patients suffering from hepatorenal syndromes which are thought to be caused by lipid 'Abbreviations: LH, polyunsaturated fatty acids; TBARS, thiobarbituric acid reactive substances; LOOH, lipid hydroperoxides;L', lipid alkylradical;LOO-,lipid peroxyl radical;LO', lipid alkoxylradical;ADP, adenosine diphosphate;EDTA, ethylenediaminetetraacetic acid;DTP A, diethylenetetraaminepentaacetic acid; DXAS, dispersive X-rayabsorption spectroscopy;FMNH2,dihydroflavin mononucleotide; ADR=O, adriamycin; ADR-O.-, adriamycin semiquinone; P450, cytochrome P-450; t-BOOH, tert-butyl hydroperoxide;MIP, microsomal iron protein;ALA, hminolevulinic acid; SOD, superoxide dismutase.
peroxidation.2 Abrogation of the oxidative stress and amelioration of the related syndromes decrease the prostaglandin Fz-like compounds back to the normal levels.2 It seems that these prostanoids might become reliable markers to assess lipid peroxidation and ita pathological fluctuations in vivo. The precise mechanisms of lipid peroxidation are controversial. A direct reaction of molecular oxygen with LH is precluded by spin barriers and thermodynamic constraints (15). Hence, there is general agreement that lipid peroxidation must be preceded by modifications of the electronic structure of LH and/or oxygen. A role for transition metals such as iron or copper is also generally accepted, in view of the ease by which these metals can oscillate between reduced and oxidized states and perturb the redox balance of biological systems (15). Nevertheless, it has remained unclear whether redox-active transition metals may promote lipid peroxidation directly (Le., through the formation of metal-oxo intermediates), or by forming oxygen-centeredradicals (16). Moreover,possible sources of iron for lipid peroxidation have remained a matter of speculation (17,18). In this review I will briefly survey the role of iron in lipid peroxidation. Special emphasis will be given to the experimental systems which have used natural sources of iron such as ferritin, transferrin, or other newly identified iron proteins.
11. LOOH-Dependent and -Independent Lipid Peroxidation: Requirement for Fe(I1) Lipid peroxidation is the process by which LH are converted to lipid hydroperoxides (LOOH). However, commercial sources of LH are heavily contaminated by LOOH, which can also contaminate biological membranes as a consequence of oxidations occurring during tissue manipulation and cell disruption. The preexisting LOOH will strongly influence the mechanism(@ by which new LOOH can be formed in vitro. When the concentration of the preexisting LOOH is low, the formation of new LOOH proceeds predominantly via reactive species that overcome the dissociation energy of an allylic C-H bond of LH, involving hydrogen abstraction and formation of lipid alkyl radicals (LO). In contrast to LH, the corresponding L' radicals can couple with molecular oxygen, forming lipid peroxyl radicals (LOO.) which eventually liberate LOOH via a hydrogen abstraction from a neighboring allylic or bis-allylic bond. The formation of a first generation of LOOH is, therefore, paralleled by the formation of a second generation of L' radicals, which 2J.D. Morrow, personal communication.
Q893-228~/93/ 2706-Q134$04.QQ/Q 0 1993 American Chemical Society
Chem. Res. Toxicol., Vol. 6, No. 2, 1993 135
Invited Reviews INITIATION
J
\
LOOH-INDEPENDENT
1
LOOE-DEPENDENT
i
F e (11) + 0 2 / H 2 0 2
L?OH
R'
Fe (111)
J
\
111. Possible Sources of Fe(I1) for Lipid Peroxidation
L'
/ k 02.s/
/
LH
\ b "[Fe
C)
an alkoxylradical that readily undergoesB scission,forming a carbonyl and a carbon-centered radical. The latter may enter various reactions, including oxygen addition and formation of a peroxyl radical, or hydrogen abstraction from LH (20). In addition, Dix and Marnett (21) have shown that hematin (ferric protoporphyrin IX) catalyzes a one-electron reduction of linoleic acid hydroperoxide to an alkoxyl radical that cyclizes to an adjacent double bond to form an epoxy allylic radical. Subsequent oxygen addition to the epoxy allylic radical yields a peroxylradical which may have a major role in LOOH-dependent reactions (21). The effects of hematin on linoleicacid hydroperoxide may explain the contribution of ubiquitous heme complexes to peroxidative processes in vivo (21).
L'
r
+
(11)
F e (111)
LO'
c L'
PROPAGATION
Figure 1. Iron-dependent initiation a n d propagation of lipid peroxidation.
reinitiate the peroxidative process. This is the so-called "LOOH-independent initiation" (19). When the preexisting LOOH are abundant, the formation of new LOOH will proceed predominantly via an iron-catalyzed decomposition to highly reactive lipid alkoxyl radicals (LO'), which can form L' radicals by hydrogen abstraction from LH. This is the so-called "LOOH-dependent initiation" of lipid peroxidation (19). The reaction of non-heme Fe(I1) with molecular oxygen and/or HzOz is currently envisioned as the most likely source of the reactive species (R')which initiates the LOOH-independent peroxidation (Figure 1, panel A). Likewise,the decomposition of preformed LOOH is much faster with Fe(I1) than with Fe(II1) (Figure 1,panel B). Thus, the LOOH-dependent or -independent reactions share a requirement for Fe(I1). The latter can also contribute to the so-called "propagation" of lipid peroxidation, which consists of a decomposition of the newly formed LOOH to LO' radicals, similar to the LOOHdependent initiation (Figure 1,panel C). This simplified picture of lipid peroxidation highlights the importance of identifying possible sources of Fe(I1). The bimolecular reaction of LO' with LH may not be the only mechanism by which LOOH contributes to lipid peroxidation. Other mechanisms have been described, involving both heme and non-heme iron species. For example, Labeque and Marnett (20)have shown that the reaction of 10-hydroperoxy-8,12-octodecadienoic acid with either Fe(I1) or Fe(II1)plus a reductant like cysteine yields
Attempts to identify intracellular low molecular weight iron complexes have produced inconclusive or conflicting evidence, with the proposed iron ligands varying from adenosine diphosphate (ADP) to citrate (18). Similarly, the concentration of free iron in extracellular fluids has been estimated as low as lo-'* [for Fe(I1)I or
