Reaction of malonaldehyde with mitochondrial membranes

Formation of malondialdehyde adducts in livers of rats exposed to ethanol: role in ethanol-mediated inhibition of cytochrome c oxidase

BACKGROUND

Previous studies in our laboratory demonstrated that short-term ethanol consumption by maternal rats increased the hepatic levels of 4-hydroxynonenal (HNE) in both the adult and the fetus. Additionally, HNE inhibited cytochrome c oxidase (COX) by forming adducts with the enzyme subunits. The present study examined modification of COX by another major aldehydic lipid peroxidation product, malondialdehyde (MDA), and its role in COX inhibition by ethanol.

METHODS AND RESULTS

It is demonstrated in vitro that MDA inhibits the activity of purified COX while forming adducts with the enzyme. Compared with HNE, MDA is a more potent inhibitor of COX. Overnight incubation at room temperature caused an 80% decrease in COX activity by MDA versus a 67% decrease by HNE. MDA produced marked inhibition of COX activity at physiologically relevant concentrations, e.g., 43% inhibition at 10 microM. Although our previous studies documented that HNE formed adducts primarily with subunit IV of COX via histidine residues, the current report showed that MDA forms adducts with both subunit IV and subunit V via lysine residues. Furthermore, both aldehydes induce carbonyl formation in subunit IV. The in vivo role of MDA in the impairment of COX by ethanol is assessed in both adult and fetal liver after maternal ethanol consumption.

CONCLUSIONS

The results showed that: (1) there are significant increases in MDA levels in liver homogenate as well as mitochondria in both adult and fetal livers after ethanol exposure; (2) these MDA levels are in the nanomole/mg protein range, in contrast to picomole/mg protein range of HNE in identical setting; and (3) ethanol-induced production of MDA is accompanied by enhanced formation of MDA adducts with COX. These findings suggest that MDA may play at least as equally an important role as HNE in ethanol-induced inhibition of COX.

Surface oxidase and oxidative stress propagation in aging

This report summarizes new evidence for a plasma-membrane-associated hydroquinone oxidase designated as CNOX (constitutive plasma membrane NADH oxidase) that functions as a terminal oxidase for a plasma membrane oxidoreductase (PMOR) electron transport chain to link the accumulation of lesions in mitochondrial DNA to cell-surface accumulations of reactive oxygen species. Previous considerations of plasma membrane redox changes during aging have lacked evidence for a specific terminal oxidase to catalyze a flow of electrons from cytosolic NADH to molecular oxygen (or to protein disulfides). Cells with functionally deficient mitochondria become characterized by an anaerobic metabolism. As a result, NADH accumulates from the glycolytic production of ATP. Elevated PMOR activity has been shown to be necessary to maintain the NAD(+)/NADH homeostasis essential for survival. Our findings demonstrate that the hyperactivity of the PMOR system results in an NADH oxidase (NOX) activity capable of generating reactive oxygen species at the cell surface. This would serve to propagate the aging cascade both to adjacent cells and to circulating blood components. The generation of superoxide by NOX forms associated with aging is inhibited by coenzyme Q and provides a rational basis for the anti-aging activity of circulating coenzyme Q.

Detoxification of reactive aldehydes in mitochondria: effects of age and dietary restriction

Previously, we proposed that reactive aldehydic products generated from lipid peroxidation might be the deleterious cause of mitochondrial dysfunction during aging. Our present study focuses on the roles that aging and dietary restriction (DR) play in the elimination of 4-hydroxynonenal (HNE) in rat liver by exploring three enzymatic systems: aldehyde dehydrogenase (ALDH), glutathione S-transferase (GST), and alcohol dehydrogenase (ADH). Results show that the main pathways of HNE elimination in mitochondria are through ALDH-catalyzed oxidation, and the GST-catalyzed conjugation of HNE. Findings also show that age reduces both ALDH and GST activities; mitochondrial HNE oxidation by ALDH declines at 18 and 24 months of age, and the glutathione conjugation of HNE reduces at 24 months of age. However, these enzymatic processes were found to be well-preserved in DR animals throughout their life span, supporting the evidence of less HNE accumulation in the membranes of restricted rats. These findings are consistent with our earlier proposal that indicates an age-associated decrease in mitochondrial detoxification as a major underlying process for malondialdehyde and lipofuscin accumulation in older animals. They also indicate that the prevention of the age-associated decrease in aldehyde detoxification by DR may be an important mechanism underlying enhanced aldehyde elimination, thus minimizing the functional deterioration observed in mitochondria of old animals.

Inhibition of adenine nucleotide translocator by lipid peroxidation products

Our previous data showed that aldehydic lipid peroxidation products, interacting with mitochondrial membrane lipids, could alter the physicochemical status of the membrane. This study was initiated to examine the interaction of these aldehydes with a major mitochondrial protein, the adenine nucleotide translocator (ANT). Our findings showed that the transporting activity of ANT in intact mitochondria was inhibited by two unsaturated aldehydes, 4-hydroxynonenal (HNE) and 4-hydroxyhexenal (HHE). To probe further into the underlying mechanism of this inhibition, a reconstituted ANT model was developed by incorporating isolated ANT into liposomes. Pretreatment of ANT with HNE prior to reconstitution resulted in decreased activity in the reconstituted ANT. Further investigation revealed that this decreased activity was probably due to loss of sulfhydryl groups, which are essential for ANT activity. Interestingly, pretreatment of the liposomes with HNE also caused a decrease in the reconstituted ANT activity by indirectly altering the physiochemical status of the lipid environment in which ANT was embedded. These results demonstrate that the reactive aldehydes derived from mitochondrial lipid peroxidation can impair the membrane function by interacting with both the protein and the lipid moieties in the membrane. Thus, the varied damaging effects associated with lipid peroxidation may be mediated by their secondary aldehydic byproducts.

Alterations in mitochondrial membrane fluidity by lipid peroxidation products

Age-related damage to the mitochondrial membrane, including decreased membrane fluidity, has been attributed to free radical reactions. Our previous studies point to lipid peroxidation as a primary cause in age-related changes in membrane fluidity. This report offers new evidence that lipid peroxidation-modulated decreases in membrane fluidity are mediated through two aldehydic lipid peroxidation products, 4-hydroxynonenal (HNE) and malondialdehyde (MDA). Hepatic mitochondria were isolated from both ad libitum fed (AL) and dietary restricted (DR) rats of different ages. Introduction of the aldehydes was found to decrease mitochondrial membrane fluidity, although the fluidity decrease induced by HNE was more pronounced than that induced by MDA. It seems likely that HNE modifies membrane fluidity by direct interaction with membrane phospholipids, as shown by the generation of a fluorescent complex between HNE and membrane phospholipids. Finally, HNE and MDA were isolated and quantitated in mitochondria. Their levels clearly differentiated between animals of different age and dietary groups. These data indicate that the reactive products of lipid peroxidation, especially HNE, may play an important role in mediating the decreased mitochondrial membrane fluidity observed in aging animals.

Mitochondrial production of pro-oxidants and cellular senescence

Mitochondria are the major intracellular producers of O2- and H2O2. The level of oxidative stress in cells, as indicated by the in vivo exhalation of alkanes and the concentration of molecular products of oxy-radical reactions, increases during aging in mammals as well as insects. In this paper, we discuss the relationship between mitochondrial generation of O2- and H2O2, and the aging process. The rate of mitochondrial O2- and H2O2 generation increases with age in houseflies and the brain, heart and liver of rat. This rate has been found to correspond to the life expectancy of flies and to the maximum life span potential (MLSP) of six different mammalian species, namely, mouse, rat, guinea pig, rabbit, pig and cow. In contrast, the level of antioxidant defenses provided by activities of superoxide dismutase, catalase, glutathione peroxidase and glutathione concentration neither uniformly declines with age nor corresponds to variations in MLSP of different mammalian species. It is argued that the rate of mitochondrial O2- and H2O2 generation rather than the antioxidant level may act as a longevity determinant.

Free radical formation in the oxidation of malondialdehyde and acetylacetone by peroxidase enzymes

Malondialdehyde, a product of lipid peroxidation, and acetylacetone undergo one-electron oxidation by peroxidase enzymes to form free radical metabolites, which were detected with ESR using the spin-trapping technique. The structures of the radical adducts were assigned using isotope substitution. With both malondialdehyde and acetylacetone and the enzymes myeloperoxidase and chloroperoxidase, carbon-centered radicals were detected. With horseradish peroxidase, a carbon-centered radical was initially trapped and then disappeared with the concomitant appearance of an iminoxyl radical.

Characterization of age-related malondialdehyde oxidation: the effect of modulation by food restriction

Age-related in vitro malondialdehyde (MDA) oxidation by liver was investigated. It was established that mitochondria are the major site of oxidation, and that the oxidation process was catalyzed by aldehyde dehydrogenase as indicated by inhibitory action of disulfiram. The reaction of MDA oxidation requires Mg2+ and NAD as cofactors. The capacity for MDA oxidation was progressively lost with increasing age and the extent of the loss was modulated by food restriction.

Modification of human serum low density lipoprotein by oxidation--characterization and pathophysiological implications

Plasma low density lipoprotein (LDL) can undergo free radical oxidation either catalyzed by divalent cations, such as Cu2+ or Fe2+ or promoted by incubation with cultured cells such as endothelial cells, smooth muscle cells and monocytes. The content of vitamin E, beta-carotene and unsaturated fatty acids is decreased in oxidized LDL. A breakdown of apolipoprotein-B (apoB), hydrolysis of the phospholipids, an increase of thiobarbituric acid reactive substances and the generation of aldehydes also occur. Changes in the ratio of lipid to protein, the electrophoretic mobility and the fluorescent properties have also been reported to accompany oxidation of this lipoprotein. The functional changes of oxidized LDL include its recognition by the scavenger receptor on macrophages, its cytotoxicity especially to proliferating cells, its chemotactic properties with respect to monocyte-macrophages and its regulation of platelet-derived growth factor-like protein (PDGFc) production by endothelial cells. In this article we summarize some of the contributions to this topic and present speculations relating oxidized LDL to pathological conditions such as atherosclerosis.