Diphosphatidylglycerol is required for optimal activity of beef heart cytochrome c oxidase

Novel Antineoplastic Inducers of Mitochondrial Apoptosis in Human Cancer Cells

I propose a new strategy to suppress human cancer completely with two entirely new drug compounds exploiting cancer's Warburg effect characterized by a defective mitochondrial aerobic respiration, substituted by cytosolic aerobic fermentation/glycolysis of D-(+)-glucose into L-(+)-lactic acid. The two essentially new drugs, compound 1 [P(op)T(est)162] and compound 3 (PT167), represent new highly symmetric, four-bladed propeller-shaped polyammonium cations. The in vitro antineoplastic highly efficacious drug compound 3 represents a covalent combination of compound 1 and compound 2 (PT166). The intermediate drug compound 2 is an entirely new colchic(in)oid derivative synthesized from colchicine. Compound 2's structure was determined using X-ray crystallography. Compound 1 and compound 3 were active in vitro versus 60 human cancer cell lines of the National Cancer Institute (NCI) Developmental Therapeutics Program (DTP) 60-cancer cell testing. Compound 1 and compound 3 not only stop the growth of cancer cells to ±0% (cancerostatic effect) but completely kill nearly all 60 cancer cells to a level of almost -100% (tumoricidal effect). Compound 1 and compound 3 induce mitochondrial apoptosis (under cytochrome c release) in all cancer cells tested by (re)activating (in most cancers impaired) p53 function, which results in a decrease in cancer's dysregulated cyclin D1 and an induction of the cell cycle-halting cyclin-dependent kinase inhibitor p21Waf1/p21Cip1.

Understanding differential aspects of microdiffusion (channeling) in the Coenzyme Q and Cytochrome c regions of the mitochondrial respiratory system

Over the past decades, models of the organization of mitochondrial respiratory system have been controversial. The goal of this perspective is to assess this "conflict of models" by focusing on specific kinetic evidence in the two distinct segments of Coenzyme Q- and Cytochrome c-mediated electron transfer. Respiratory supercomplexes provide kinetic advantage by allowing a restricted diffusion of Coenzyme Q and Cytochrome c, and short-range interaction with their partner enzymes. In particular, electron transfer from NADH is compartmentalized by channeling of Coenzyme Q within supercomplexes, whereas succinate oxidation proceeds separately using the free Coenzyme Q pool. Previous evidence favoring Coenzyme Q random diffusion in the NADH-dependent electron transfer is due to downstream flux interference and misinterpretation of results. Indeed, electron transfer by complexes III and IV via Cytochrome c is less strictly dependent on substrate channeling in mammalian mitochondria. We briefly describe these differences and their physiological implications.

Cardiolipin, and not monolysocardiolipin, preferentially binds to the interface of complexes III and IV

The mitochondrial electron transport chain comprises a series of protein complexes embedded in the inner mitochondrial membrane that generate a proton motive force via oxidative phosphorylation, ultimately generating ATP. These protein complexes can oligomerize to form larger structures called supercomplexes. Cardiolipin (CL), a conical lipid, unique within eukaryotes to the inner mitochondrial membrane, has proven essential in maintaining the stability and function of supercomplexes. Monolysocardiolipin (MLCL) is a CL variant that accumulates in people with Barth syndrome (BTHS). BTHS is caused by defects in CL biosynthesis and characterised by abnormal mitochondrial bioenergetics and destabilised supercomplexes. However, the mechanisms by which MLCL causes pathogenesis remain unclear. Here, multiscale molecular dynamics characterise the interactions of CL and MLCL with yeast and mammalian mitochondrial supercomplexes containing complex III (CIII) and complex IV (CIV). Coarse-grained simulations reveal that both CL and MLCL bind to sites at the interface between CIII and CIV of the supercomplex. Free energy perturbation calculations show that MLCL interaction is weaker than that of CL and suggest that interaction with CIV drives this difference. Atomistic contact analyses show that, although interaction with CIII is similar for CL and MLCL, CIV makes more contacts with CL than MLCL, demonstrating that CL is a more successful "glue" between the two complexes. Simulations of the human CIII₂CIV supercomplex show that this interface site is maintained between species. Our study suggests that MLCL accumulation in people with BTHS disrupts supercomplex stability by formation of relatively weak interactions at the interface lipid binding site.

Cardiolipin, conformation, and respiratory complex-dependent oligomerization of the major mitochondrial ADP/ATP carrier in yeast

The phospholipid cardiolipin has pleiotropic structural and functional roles that are collectively essential for mitochondrial biology. Yet, the molecular details of how this lipid supports the structure and function of proteins and protein complexes are poorly understood. To address this property of cardiolipin, we use the mitochondrial adenosine 5'-diphosphate/adenosine 5'-triphosphate carrier (Aac) as a model. Here, we have determined that cardiolipin is critical for both the tertiary and quaternary assembly of the major yeast Aac isoform Aac2 as well as its conformation. Notably, these cardiolipin-provided structural roles are separable. In addition, we show that multiple copies of Aac2 engage in shared complexes that are largely dependent on the presence of assembled respiratory complexes III and IV or respiratory supercomplexes. Intriguingly, the assembly state of Aac2 is sensitive to its transport-related conformation. Together, these results expand our understanding of the numerous structural roles provided by cardiolipin for mitochondrial membrane proteins.

Tetra-linoleoyl cardiolipin depletion plays a major role in the pathogenesis of sarcopenia

Sarcopenia, the progressive loss of muscle mass, strength, and physical performance that occurs during aging, is highly prevalent among the elderly. Sarcopenia increases the risk of falls, disability, and death. The biological basis for sarcopenia is not well understood. There are no specific preventive or therapeutic strategies for sarcopenia except exercise. The elucidation of biological pathways and identification of therapeutic targets for treating or preventing sarcopenia remain a high priority in aging research. Mitochondria play a critical role in skeletal muscle by providing energy in the form of ATP, regulation of signaling, calcium homeostasis, autophagy, and other functions. Cardiolipin, a unique dimeric phospholipid specific to mitochondria and an essential component of mitochondrial membranes, is involved in mitochondrial protein transport, maintaining structural organization of mitochondrial membranes, cellular signaling, regulating enzymes involved in β-oxidation of fatty acids, and facilitating normal electron transport chain (ETC) function and generation of ATP. The fatty acid species composition of cardiolipin is critical to mitochondrial bioenergetics, as cardiolipin affects membrane biophysical properties, binds and stabilizes ETC protein complexes, and shapes the curvature of the mitochondrial cristae. Tetra-linoleoyl cardiolipin (18:2)₄ comprises ∼80% of cardiolipin in mitochondria in normal human skeletal and cardiac muscle and is optimal for effective ETC function and ATP generation. Aging is associated with a decrease in cardiolipin content, decrease in tetra-linoleoyl cardiolipin (18:2)₄ and replacement of linoleic acid (18:2) with other fatty acids in cardiolipin composition, decline of ETC function, and increased generation of reactive oxygen species in muscle. Together, these findings from the literature prompt the hypothesis that depletion of the cardiolipin (18:2)₄ species may be at the root of mitochondrial dysfunction with aging, in turn leading to sarcopenia. Corroboration of the tetra-linoleoyl cardiolipin depletion hypothesis suggests new leads for the prevention and treatment of sarcopenia by enhancing the biosynthesis, accretion, and integrity of tetra-linoleoyl cardiolipin.

Molecular and topological reorganizations in mitochondrial architecture interplay during Bax-mediated steps of apoptosis

During apoptosis, Bcl-2 proteins such as Bax and Bak mediate the release of pro-apoptotic proteins from the mitochondria by clustering on the outer mitochondrial membrane and thereby permeabilizing it. However, it remains unclear how outer membrane openings form. Here, we combined different correlative microscopy and electron cryo-tomography approaches to visualize the effects of Bax activity on mitochondria in human cells. Our data show that Bax clusters localize near outer membrane ruptures of highly variable size. Bax clusters contain structural elements suggesting a higher order organization of their components. Furthermore, unfolding of inner membrane cristae is coupled to changes in the supramolecular assembly of ATP synthases, particularly pronounced at membrane segments exposed to the cytosol by ruptures. Based on our results, we propose a comprehensive model in which molecular reorganizations of the inner membrane and sequestration of outer membrane components into Bax clusters interplay in the formation of outer membrane ruptures.

Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Role of cardiolipin in stability of integral membrane proteins

Cardiolipin (CL) is a unique phospholipid with a dimeric structure having four acyl chains and two phosphate groups found almost exclusively in certain membranes of bacteria and of mitochondria of eukaryotes. CL interacts with numerous proteins and has been implicated in function and stabilization of several integral membrane proteins (IMPs). While both functional and stabilization roles of CL in IMPs has been generally acknowledged, there are, in fact, only limited number of quantitative analysis that support this function of CL. This is likely caused by relatively complex determination of parameters characterizing stability of IMPs and particularly intricate assessment of role of specific phospholipids such as CL in IMPs stability. This review aims to summarize quantitative findings regarding stabilization role of CL in IMPs reported up to now.

Kinetic and Structural Characterization of the Effects of Membrane on the Complex of Cytochrome b 5 and Cytochrome c

Cytochrome b₅ (cytb₅) is a membrane protein vital for the regulation of cytochrome P450 (cytP450) metabolism and is capable of electron transfer to many redox partners. Here, using cyt c as a surrogate for cytP450, we report the effect of membrane on the interaction between full-length cytb₅ and cyt c for the first time. As shown through stopped-flow kinetic experiments, electron transfer capable cytb₅ - cyt c complexes were formed in the presence of bicelles and nanodiscs. Experimentally measured NMR parameters were used to map the cytb₅-cyt c binding interface. Our experimental results identify differences in the binding epitope of cytb₅ in the presence and absence of membrane. Notably, in the presence of membrane, cytb₅ only engaged cyt c at its lower and upper clefts while the membrane-free cytb₅ also uses a distal region. Using restraints generated from both cytb₅ and cyt c, a complex structure was generated and a potential electron transfer pathway was identified. These results demonstrate the importance of studying protein-protein complex formation in membrane mimetic systems. Our results also demonstrate the successful preparation of novel peptide-based lipid nanodiscs, which are detergent-free and possesses size flexibility, and their use for NMR structural studies of membrane proteins.

Croix CM, Watkins S, Shen Z, Li Y, Greenberg ML, Tokarska-Schlattner M, Boissan M, Lacombe ML, Epand RM, Chu CT, Mallampalli RK, Bayır H, Schlattner U. NDPK-D (NM23-H4)-mediated externalization of cardiolipin enables elimination of depolarized mitochondria by mitophagy

Mitophagy is critical for cell homeostasis. Externalization of the inner mitochondrial membrane phospholipid, cardiolipin (CL), to the surface of the outer mitochondrial membrane (OMM) was identified as a mitophageal signal recognized by the microtubule-associated protein 1 light chain 3. However, the CL-translocating machinery remains unknown. Here we demonstrate that a hexameric intermembrane space protein, NDPK-D (or NM23-H4), binds CL and facilitates its redistribution to the OMM. We found that mitophagy induced by a protonophoric uncoupler, carbonyl cyanide m-chlorophenylhydrazone (CCCP), caused externalization of CL to the surface of mitochondria in murine lung epithelial MLE-12 cells and human cervical adenocarcinoma HeLa cells. RNAi knockdown of endogenous NDPK-D decreased CCCP-induced CL externalization and mitochondrial degradation. A R90D NDPK-D mutant that does not bind CL was inactive in promoting mitophagy. Similarly, rotenone and 6-hydroxydopamine triggered mitophagy in SH-SY5Y cells was also suppressed by knocking down of NDPK-D. In situ proximity ligation assay (PLA) showed that mitophagy-inducing CL-transfer activity of NDPK-D is closely associated with the dynamin-like GTPase OPA1, implicating fission-fusion dynamics in mitophagy regulation.

Influence of tissue, diet, and enzymatic remodeling on cardiolipin fatty acyl profile

Cardiolipin is a specialized phospholipid found primarily in the inner mitochondrial membrane. Because of its unique dimeric structure, cardiolipin plays an important role in mitochondrial function, stability, and membrane fluidity. As such, cardiolipin is subject to a high degree of remodeling by phospholipases, acyltransferases, and transacylases that create a fatty acyl profile that tends to be highly tissue-specific. Despite this overarching regulation, the molecular species of cardiolipin produced are also influenced by dietary lipid composition. A number of studies have characterized the tissue-specific profile of cardiolipin species and have investigated the specific nature of cardiolipin remodeling, including the role of both enzymes and diet. The aim of this review is to highlight tissue specific differences in cardiolipin composition and, collectively, the enzymatic and dietary factors that contribute to these differences. Consequences of aberrant cardiolipin fatty acyl remodeling are also discussed.

Cardiolipin linoleic acid content and mitochondrial cytochrome c oxidase activity are associated in rat skeletal muscle

Cardiolipin (CL) is an inner-mitochondrial membrane phospholipid that is important for optimal mitochondrial function. Specifically, CL and CL linoleic (18:2ω6) content are known to be positively associated with cytochrome c oxidase (COX) activity. However, this association has not been examined in skeletal muscle. In this study, rats were fed high-fat diets with a naturally occurring gradient in linoleic acid (coconut oil [CO], 5.8%; flaxseed oil [FO], 13.2%; safflower oil [SO], 75.1%) in an attempt to alter both mitochondrial CL fatty acyl composition and COX activity in rat mixed hind-limb muscle. In general, mitochondrial membrane lipid composition was fairly resistant to dietary treatments as only modest changes in fatty acyl composition were detected in CL and other major mitochondrial phospholipids such as phosphatidylcholine (PC) and phosphatidylethanolamine (PE). As a result of this resistance, CL 18:2ω6 content was not different between the dietary groups. Consistent with the lack of changes in CL 18:2ω6 content, mitochondrial COX activity was also not different between the dietary groups. However, correlational analysis using data obtained from rats across the dietary groups showed a significant relationship (p = 0.009, R(2) = 0.21). Specifically, our results suggest that CL 18:2ω6 content may positively influence mitochondrial COX activity thereby making this lipid molecule a potential factor related to mitochondrial health and function in skeletal muscle.

Role of phospholipids of subunit III in the regulation of structural rearrangements in cytochrome c oxidase of Rhodobacter sphaeroides

Subunit III of cytochrome c oxidase possesses structural domains that contain conserved phospholipid binding sites. Mutations within these domains induce a loss of phospholipid binding, coinciding with decreased electron transfer activity. Functional and structural roles for phospholipids in the enzyme from Rhodobacter sphaeroides have been investigated. Upon the removal of intrinsic lipids using phospholipase A2, electron transfer activity was decreased 30-50%. Moreover, the delipidated enzyme exhibited turnover-induced, suicide inactivation, which was reversed by the addition of exogenous lipids, most specifically by cardiolipin. Cardiolipin exhibited two sites of interaction with the delipidated enzyme, a high-affinity site (Km = 0.14 μM) and a low-affinity site (Km = 26 μM). Subunit I of the delipidated enzyme exhibited a faster digestion rate when it was treated with α-chymotrypsin compared to that of the wild-type enzyme, suggesting that lipid removal induces a conformational change to expose the digestion sites further. Upon reaction of subunit III of the enzyme with a fluorophore (AEDANS), fluorescence anisotropy showed an increased rotational rate of the fluorophore in the absence of lipids, indicating increased flexibility of subunit III within the enzyme's tertiary structure. Additionally, Förster resonance energy transfer between AEDANS and a fluorescently labeled cardiolipin revealed that cardiolipin binds in the v-shaped cleft of subunit III in the delipidated enzyme and that it moves closer to the active site in subunit I upon a change in the redox state of the enzyme. In conclusion, these results show that the phospholipids regulate events occurring during electron transfer activity by maintaining the structural integrity of the enzyme at the active site.

The Kinetics of Enzyme Mixtures

Even purified enzyme preparations are often heterogeneous. For example, preparations of aspartate aminotransferase or cytochrome oxidase can consist of several different forms of the enzyme. For this reason we consider how different the kinetics of the reactions catalysed by a mixture of forms of an enzyme must be to provide some indication of the characteristics of the species present. Based on the standard Michaelis-Menten model, we show that if the Michaelis constants (K_m) of two isoforms differ by a factor of at least 20 the steady-state kinetics can be used to characterise the mixture. However, even if heterogeneity is reflected in the kinetic data, the proportions of the different forms of the enzyme cannot be estimated from the kinetic data alone. Consequently, the heterogeneity of enzyme preparations is rarely reflected in measurements of their steady-state kinetics unless the species present have significantly different kinetic properties. This has two implications: (1) it is difficult, but not impossible, to detect molecular heterogeneity using kinetic data and (2) even when it is possible, a considerable quantity of high quality data is required.

The power of yeast to model diseases of the powerhouse of the cell

Mitochondria participate in a variety of cellular functions. As such, mitochondrial diseases exhibit numerous clinical phenotypes. Because mitochondrial functions are highly conserved between humans and Saccharomyces cerevisiae, yeast are an excellent model to study mitochondrial disease, providing insight into both physiological and pathophysiological processes.

Implication of liver cardiolipins in mitochondrial energy metabolism disorder in cancer cachexia

Mitochondrial membranes are essential for the good functioning of the organelle. For instance, the inner mitochondrial membrane contains the oxidative phosphorylation system that permits ATP synthesis. Phospholipids environment and especially cardiolipin are crucial for the mitochondrial energy metabolism. Indeed, cardiolipin is known to provide essential structural and functional support to several proteins involved in oxidative phosphorylation. Alterations in cardiolipin structure, content and fatty acids composition have been associated with mitochondrial dysfunction in several physiopathological conditions and diseases. Cancer cachexia is a complex and dynamic process characterized by a negative energy balance induced by anorexia and hypermetabolism which leads to a drastic loss in body weight that aggravate prognosis of cancer patients. The underlying mechanisms of hypermetabolism are not fully understood. Whether the mitochondrial energy metabolism is altered during this disease and may participate to hypermetabolism is not clear. This mini-review focuses on cardiolipin especially its biosynthesis and remodeling pathways, its relation with mitochondrial energy metabolism and its possible implication in the cancer cachexia syndrome.

Conformational properties of cardiolipin-bound cytochrome c

Interactions of cytochrome c (cyt c) with cardiolipin (CL) are important for both electron transfer and apoptotic functions of this protein. A sluggish peroxidase in its native state, when bound to CL, cyt c catalyzes CL peroxidation, which contributes to the protein apoptotic release. The heterogeneous CL-bound cyt c ensemble is difficult to characterize with traditional structural methods and ensemble-averaged probes. We have employed time-resolved FRET measurements to evaluate structural properties of the CL-bound protein in four dansyl (Dns)-labeled variants of horse heart cyt c. The Dns decay curves and extracted Dns-to-heme distance distributions P(r) reveal a conformational diversity of the CL-bound cyt c ensemble with distinct populations of the polypeptide structures that vary in their degree of protein unfolding. A fraction of the ensemble is substantially unfolded, with Dns-to-heme distances resembling those in the guanidine hydrochloride-denatured state. These largely open cyt c structures likely dominate the peroxidase activity of the CL-bound cyt c ensemble. Site variations in P(r) distributions uncover structural features of the CL-bound cyt c, rationalize previous findings, and implicate the prime role of electrostatic interactions, particularly with the protein C terminus, in the CL-induced unfolding.

Ribose 5-phosphate glycation reduces cytochrome c respiratory activity and membrane affinity

Spontaneous glycation of bovine heart cytochrome c (cyt c) by the sugar ribose 5-phosphate (R5P) weakens the ability of the heme protein to transfer electrons in the respiratory pathway and to bind to membranes. Trypsin fragmentation studies suggest the preferential sites of glycation include Lys72 and Lys87/88 of a cationic patch involved in the association of the protein with its respiratory chain partners and with cardiolipin-containing membranes. Reaction of bovine cyt c with R5P (50 mM) for 8 h modified the protein in a manner that weakened its ability to transfer electrons to cytochrome oxidase by 60%. An 18 h treatment with R5P decreased bovine cyt c's binding affinity with cardiolipin-containing liposomes by an estimated 8-fold. A similar weaker binding of glycated cyt c was observed with mitoplasts. The reversal of the effects of R5P on membrane binding by ATP further supports an A-site modification. A significant decrease in the rate of spin state change for ferro-cyt c, thought to be due to cardiolipin insertion disrupting the coordination of Met to heme, was found for the R5P-treated cyt c. This change occurred to a greater extent than what can be explained by the permanent attachment of the protein to the liposome. Turbidity changes resulting from the multilamellar liposome fusion that is readily promoted by cyt c binding were not seen for the R5P-glycated cyt c samples. Collectively, these results demonstrate the negative impact that R5P glycation can have on critical electron transfer and membrane association functions of cyt c.

The cellular and molecular origin of reactive oxygen species generation during myocardial ischemia and reperfusion

Myocardial ischemia-reperfusion injury is an important cause of impaired heart function in the early postoperative period subsequent to cardiac surgery. Reactive oxygen species (ROS) generation increases during both ischemia and reperfusion and it plays a central role in the pathophysiology of intraoperative myocardial injury. Unfortunately, the cellular source of these ROS during ischemia and reperfusion is often poorly defined. Similarly, individual ROS members tend to be grouped together as free radicals with a uniform reactivity towards biomolecules and with deleterious effects collectively ascribed under the vague umbrella of oxidative stress. This review aims to clarify the identity, origin, and progression of ROS during myocardial ischemia and reperfusion. Additionally, this review aims to describe the biochemical reactions and cellular processes that are initiated by specific ROS that work in concert to ultimately yield the clinical manifestations of myocardial ischemia-reperfusion. Lastly, this review provides an overview of several key cardioprotective strategies that target myocardial ischemia-reperfusion injury from the perspective of ROS generation. This overview is illustrated with example clinical studies that have attempted to translate these strategies to reduce the severity of ischemia-reperfusion injury during coronary artery bypass grafting surgery.

Permeabilization of the mitochondrial outer membrane by Bax/truncated Bid (tBid) proteins as sensitized by cardiolipin hydroperoxide translocation: mechanistic implications for the intrinsic pathway of oxidative apoptosis

Cytochrome c (cyt c) release upon oxidation of cardiolipin (CL) in the mitochondrial inner membrane (IM) under oxidative stress occurs early in the intrinsic apoptotic pathway. We postulated that CL oxidation mobilizes not only cyt c but also CL itself in the form of hydroperoxide (CLOOH) species. Relatively hydrophilic CLOOHs could assist in apoptotic signaling by translocating to the outer membrane (OM), thus promoting recruitment of the pro-apoptotic proteins truncated Bid (tBid) and Bax for generation of cyt c-traversable pores. Initial testing of these possibilities showed that CLOOH-containing liposomes were permeabilized more readily by tBid plus Ca(2+) than CL-containing counterparts. Moreover, CLOOH translocated more rapidly from IM-mimetic to OM-mimetic liposomes than CL and permitted more extensive OM permeabilization. We found that tBid bound more avidly to CLOOH-containing membranes than to CL counterparts, and binding increased with increasing CLOOH content. Permeabilization of CLOOH-containing liposomes in the presence of tBid could be triggered by monomeric Bax, consistent with tBid/Bax cooperation in pore formation. Using CL-null mitochondria from a yeast mutant, we found that tBid binding and cyt c release were dramatically enhanced by transfer acquisition of CLOOH. Additionally, we observed a pre-apoptotic IM-to-OM transfer of oxidized CL in cardiomyocytes treated with the Complex III blocker, antimycin A. These findings provide new mechanistic insights into the role of CL oxidation in the intrinsic pathway of oxidative apoptosis.

Extended cardiolipin anchorage to cytochrome c: a model for protein-mitochondrial membrane binding

Two models have been proposed to explain the interaction of cytochrome c with cardiolipin (CL) vesicles. In one case, an acyl chain of the phospholipid accommodates into a hydrophobic channel of the protein located close the Asn52 residue, whereas the alternative model considers the insertion of the acyl chain in the region of the Met80-containing loop. In an attempt to clarify which proposal offers a more appropriate explanation of cytochrome c-CL binding, we have undertaken a spectroscopic and kinetic study of the wild type and the Asn52Ile mutant of iso-1-cytochrome c from yeast to investigate the interaction of cytochrome c with CL vesicles, considered here a model for the CL-containing mitochondrial membrane. Replacement of Asn52, an invariant residue located in a small helix segment of the protein, may provide data useful to gain novel information on which region of cytochrome c is involved in the binding reaction with CL vesicles. In agreement with our recent results revealing that two distinct transitions take place in the cytochrome c-CL binding reaction, data obtained here support a model in which two (instead of one, as considered so far) adjacent acyl chains of the liposome are inserted, one at each of the hydrophobic sites, into the same cytochrome c molecule to form the cytochrome c-CL complex.

Cardiolipin, a critical determinant of mitochondrial carrier protein assembly and function

The ability of phospholipids to act as determinants of membrane protein structure and function is probably best exemplified by cardiolipin (CL), the signature phospholipid of mitochondria. Early efforts to reconstitute individual respiratory complexes and members of the mitochondrial carrier family, most notably the ADP/ATP carrier (AAC), often demonstrated the importance of CL. Over the past decade, the significance of CL in the organization of components of the electron transport chain into higher order assemblies, termed respiratory supercomplexes, has been established. Another protein required for oxidative phosphorylation, AAC, has received comparatively little attention likely stemming from the fact that AACs were thought to function in isolation as either homodimers or monomers. Recently however, AACs have been demonstrated to interact with the respiratory supercomplex, other members of the mitochondrial carrier family, and the TIM23 translocon. Interestingly, many if not all of these interactions depend on CL. As the paradigm for the mitochondrial carrier family, these discoveries with AAC suggest that other members of this large group of important proteins may be more gregarious than anticipated. Moreover, it is proposed that AAC and perhaps additional members of the mitochondrial carrier family might represent downstream targets of pathological states involving alterations in CL.

Free radical fragmentation of cardiolipin by cytochrome c

The effect of cytochrome c (cyt c) on degradation of cardiolipin in its polar part was investigated in cardiolipin/phosphatidylcholine (CL/PC) liposomes incubated with cyt c/H(2)O(2)/and (or) ascorbate by high-performance thin layer chromatography and MALDI-TOF mass spectrometry. It has been shown that phosphatidic acid (PA) and phosphatidylhydroxyacetone (PHA) were formed in the system under conditions where hydrogen peroxide favours a release of heme iron from cyt c. The formation of PA and PHA occurs via an OH-induced fragmentation taking place in the polar moiety of cardiolipin. Formation of fragmentation products correlated with the loss of CL in CL/PC liposomes incubated with cyt c/H(2)O(2)/ascorbate or with Cu(2+)/H(2)O(2)/ascorbate.

Voltage gating of VDAC is regulated by nonlamellar lipids of mitochondrial membranes

Evidence is accumulating that lipids play important roles in permeabilization of the mitochondria outer membrane (MOM) at the early stage of apoptosis. Lamellar phosphatidylcholine (PC) and nonlamellar phosphatidylethanolamine (PE) lipids are the major membrane components of the MOM. Cardiolipin (CL), the characteristic lipid from the mitochondrial inner membrane, is another nonlamellar lipid recently shown to play a role in MOM permeabilization. We investigate the effect of these three key lipids on the gating properties of the voltage-dependent anion channel (VDAC), the major channel in MOM. We find that PE induces voltage asymmetry in VDAC current-voltage characteristics by promoting channel closure at cis negative applied potentials. Significant asymmetry is also induced by CL. The observed differences in VDAC behavior in PC and PE membranes cannot be explained by differences in the insertion orientation of VDAC in these membranes. Rather, it is clear that the two nonlamellar lipids affect VDAC gating. Using gramicidin A channels as a tool to probe bilayer mechanics, we show that VDAC channels are much more sensitive to the presence of CL than could be expected from the experiments with gramicidin channels. We suggest that this is due to the preferential insertion of VDAC into CL-rich domains. We propose that the specific lipid composition of the mitochondria outer membrane and/or of contact sites might influence MOM permeability by regulating VDAC gating.

Cloning and characterization of a cDNA encoding human cardiolipin synthase (hCLS1)

Cardiolipin (CL) is a phospholipid localized to the mitochondria, and its biosynthesis is essential for mitochondrial structure and function. We report here the identification and characterization of a cDNA encoding the first mammalian cardiolipin synthase (CLS1) in humans and mice. This cDNA exhibits sequence homology with members of a CLS gene family that share similar domain structure and chemical properties. Expression of the human CLS (hCLS1) cDNA in reticulocyte lysates or insect cells led to a marked increase in CLS activity. The enzyme is specific for CL synthesis, because no significant increase in phosphatidylglycerol phosphate synthase activity was observed. In addition, CL pool size was increased in hCLS1-overexpressing cells compared with controls. Furthermore, the hCLS1 gene was highly expressed in tissues such as heart, skeletal muscle, and liver, which have been shown to have high CLS activities. These results demonstrate that hCLS1 encodes an enzyme that synthesizes CL.

Reactive oxygen species generated by the mitochondrial respiratory chain affect the complex III activity via cardiolipin peroxidation in beef-heart submitochondrial particles

The aim of this study was to investigate the effect of reactive oxygen species (ROS), produced by the mitochondrial respiratory chain, on the activity of complex III and on the cardiolipin content in bovine-heart submitochondrial particles (SMP). ROS were produced by treatment of nicotinamide adenine dinucleotide (NADH) respiring SMP with rotenone. This treatment resulted in a production of superoxide anion, detected by the epinephrine method, which was blocked by superoxide dismutase (SOD). Exposure of SMP to mitochondrial-mediated ROS generation resulted in a marked loss of complex III activity and in a parallel loss of mitochondrial cardiolipin content. Both these effects were completely abolished by SOD + catalase. Exogenous added cardiolipin was able to almost completely prevent the ROS-mediated loss of complex III activity. No effect was obtained with other major phospholipid components of the mitochondrial membrane such as phosphatidylcholine and phosphatidylethanolamine, or with peroxidized cardiolipin. The results demonstrate that mitochondrial-mediated ROS generation affects the activity of complex III via peroxidation of cardiolipin, which is required for the functioning of this multisubunit enzyme complex. These results may prove useful in probing molecular mechanisms of ROS-induced peroxidative damage to mitochondria, which have been proposed to contribute to those physiopathological conditions characterized by an increase in the basal production of ROS such as aging, ischemia/reperfusion and chronic degenerative diseases.

Photoreduction behavior of cytochrome c by zinc porphyrin in lipid media

To elucidate the role of cardiolipin (CL) on redox behavior of cytochrome c (cyt c (III)), the photoreduction of cyt c using the photosensitization of zinc tetraphenylporphyrin in presence of triethanolamine (TEOA) as a sacrificial electron-donating reagent in various lipid media were studied. The initial rate of cyt c (III) photoreduction in various lipid, CL, l-alpha-phosphatidic acid (PA), dimethyldipalmitoylammonium bromide (DMPA) and Triton X-100 media were 1.0, 0.73, 0.80 and 0.67 micromol dm-3 min-1, respectively. The cyt c (III) photoreduction rate slightly increased by the addition of CL.

Effect of aluminium phosphide exposure on kinetic properties of cytochrome oxidase and mitochondrial energy metabolism in rat brain

This study involves the effect of aluminium phosphide exposure on the kinetic characteristics of cytochrome oxidase and the mitochondrial respiratory chain function in rat brain. Mitochondrial preparations from both control and aluminium phosphide-treated rats demonstrated significant decrease in the maximal activity of cytochrome oxidase (approximately 50%) when expressed per unit membrane protein and on a turnover number basis (nmol/min/nmol haem a). The results indicated that there was a decrease in the catalytic efficiency of the active oxidase molecules on aluminium phosphide treatment. Arrhenius plot characteristics differ for cytochrome oxidase activity in mitochondria isolated from treated and control rats, in the break point of the biphasic plot which was shifted to a higher temperature. The decreased activity of cytochrome oxidase along with altered NADH and succinic dehydrogenase activities might have contributed towards a significant decline in state 3 and state 4 respiration. These alterations in the electron transport chain complexes in turn affected the ATP synthesis rate adversely in the mitochondria, isolated from treated rats. The data reflect the interaction of aluminium phosphide with redox chain components leading to the impairment of the electron transfer along the respiratory chain.

Mass spectrometric detection of protein, lipid and heme components of cytochrome c oxidase from R. sphaeroides and the stabilization of non-covalent complexes from the enzyme

The cytochrome c oxidase enzyme from the Rhodobacter sphaeroides bacteria exists as a complex of four peptide subunits, two hemes, and a variety of lipids and metal ions held together by non-covalent forces. While the native enzyme functions as an associated unit, this complex usually dissociates during MALDI- TOF analysis. Through the use of matrix additives such as sucrose, the complete complex and partial complexes can be stabilized in the MALDI-TOF experiment. The dissociation of the complex allows for the detection of the components of the enzyme. The direct detection of associated lipids from an aqueous solution of the intact enzyme may eliminate the need for enzyme disruption and lipid extraction. The partial dissociation of multisubunit enzymes in such experiments may allow for the determination of subunit-subunit and subunit-lipid interactions

Regulation of plasma membrane H+-ATPase activity by the membrane environment

The plant plasma membrane H(+)-ATPase is a proton pump which plays a central role in physiological functions such as nutrient uptake and intracellular pH regulation. This pump belongs to the P(3)-type ATPase family and creates an electrochemical gradient across the plasma membrane. The generation of this gradient has a major role in providing the energy for secondary active transport across the plasma membrane. The activity of the proton pump is regulated by the transcriptional and post-translational levels and by membrane environmental factors such as membrane lipids. Several reviews have appeared during the last few years concerning the regulatory mechanism at transcriptional and post-translational levels. The plasma membrane H(+)-ATPase requires lipids for activity. This lipid dependency suggests a possible mode of regulation of the H(+)-ATPase via modification of its lipid environment. This review focuses on the regulation of plasma membrane H(+)-ATPase by membrane lipids surrounding H(+)-ATPase molecules.

Purification and characterization of monolysocardiolipin acyltransferase from pig liver mitochondria

In mammalian tissues cardiolipin is rapidly remodeled by monolysocardiolipin acyltransferase subsequent to its de novo biosynthesis (Ma, B. J., Taylor, W. A, Dolinsky, V. W., and Hatch, G. M. (1999) J. Lipid Res. 40, 1837-1845). We report here the purification and characterization of a monolysocardiolipin acyltransferase activity from pig liver mitochondria. Monolysocardiolipin acyltransferase activity was purified over 1000-fold by butanol extraction, hydroxyapatite chromatography, and preparative SDS-PAGE. The purified 74-kDa protein catalyzed acylation of monolysocardiolipin to cardiolipin with [(14)C]linoleoyl coenzyme A. Photoaffinity labeling of the protein with 12-[(4-[(125)I]azidosalicyl)amino]dodecanoyl coenzyme A indicated coenzyme A was bound at its active site and photoaffinity cross-linking of 12-[(4-azidosalicyl)amino]dodecanoyl coenzyme A to the enzyme inhibited enzyme activity. Enzyme activity was optimum at pH 7.0, and the enzyme did not utilize other lysophospholipids as substrate. The purified enzyme was heat-labile and exhibited an isoelectric point of pH 5.4. To determine the enzymes kinetic mechanism the effect of varying concentrations of linoleoyl coenzyme A and monolysocardiolipin on initial velocity were determined. Double-reciprocal plots revealed parallel lines consistent with a ping pong kinetic mechanism. When the enzyme was incubated in the absence of monolysocardiolipin, coenzyme A was produced from linoleoyl coenzyme A at a rate consistent with the formation of an enzyme-linoleate intermediate. The true K(m) value for linoleoyl coenzyme A and true K(m) value for monolysocardiolipin were 100 and 44 microM, respectively. The calculated V(max) was 6802 pmol/min per mg of protein. A polyclonal antibody, raised in rabbits to the purified protein, cross-reacted with the protein in crude pig liver mitochondrial fractions. In liver mitochondria prepared from thyroxine-treated rats, the level of the protein was elevated compared with euthyroid controls indicating that expression of monolysocardiolipin acyltransferase is regulated by thyroid hormone. The study represents the first purification and characterization of a monolysocardiolipin acyltransferase activity from any organism.

Etomoxir mediates differential metabolic channeling of fatty acid and glycerol precursors into cardiolipin in H9c2 cells

We examined the effect of etomoxir treatment on de novo cardiolipin (CL) biosynthesis in H9c2 cardiac myoblast cells. Etomoxir treatment did not affect the activities of the CL biosynthetic and remodeling enzymes but caused a reduction in [1-14C]palmitic acid or [1-14C]oleic acid incorporation into CL. The mechanism was a decrease in fatty acid flux through the de novo pathway of CL biosynthesis via a redirection of lipid synthesis toward 1,2-diacyl-sn-glycerol utilizing reactions mediated by a 35% increase (P < 0.05) in membrane phosphatidate phosphohydrolase activity. In contrast, etomoxir treatment increased [1,3-3H]glycerol incorporation into CL. The mechanism was a 33% increase (P < 0.05) in glycerol kinase activity, which produced an increased glycerol flux through the de novo pathway of CL biosynthesis. Etomoxir treatment inhibited 1,2-diacyl-sn-glycerol acyltransferase activity by 81% (P < 0.05), thereby channeling both glycerol and fatty acid away from 1,2,3-triacyl-sn-glycerol utilization toward phosphatidylcholine and phosphatidylethanolamine biosynthesis. In contrast, etomoxir inhibited myo-[3H]inositol incorporation into phosphatidylinositol and the mechanism was an inhibition in inositol uptake. Etomoxir did not affect [3H]serine uptake but resulted in an increased formation of phosphatidylethanolamine derived from phosphatidylserine. The results indicate that etomoxir treatment has diverse effects on de novo glycerolipid biosynthesis from various metabolic precursors. In addition, etomoxir mediates a distinct and differential metabolic channeling of glycerol and fatty acid precursors into CL.

Phospholipid-cytochrome c interaction: evidence for the extended lipid anchorage

Binding of cytochrome c (cyt c) to fatty acids and acidic phospholipid membranes produces pronounced and essentially identical changes in the spectral properties of cyt c, revealing conformational changes in the protein. The exact mechanism of the interaction of fatty acids and acidic phospholipids with cyt c is unknown. Binding of cyt c to liposomes with high contents (mole fraction X > 0.7) of acidic phospholipids caused spectral changes identical to those due to binding of oleic acid. Fluorescence spectroscopy of a cyt c analog containing a Zn(2+) substituted heme moiety and brominated lipid derivatives (9,10)-dibromostearate and 1-palmitoyl-2-(9,10)-dibromo-sn-glycero-3-phospho-rac-glycerol demonstrated a direct contact between the fluorescent [Zn(2+)-heme] group and the brominated acyl chain. These data constitute direct evidence for interaction between an acyl chain of a membrane phospholipid and the inside of the protein containing the heme moiety and provide direct evidence for the so-called extended-lipid anchorage of cyt c to phospholipid membranes. In this mechanism, one of the phospholipid acyl chains protrudes out of the membrane and intercalates into a hydrophobic channel in cyt c while the other chain remains in the bilayer.

Reactive oxygen species affect mitochondrial electron transport complex I activity through oxidative cardiolipin damage

The aim of this study was to investigate the influence of reactive oxygen species (ROS) on the activity of complex I and on the cardiolipin content in bovine heart submitochondrial particles (SMP). ROS were generated through the use of xanthine/xanthine oxidase (X/XO) system. Treatment of SMP with X/XO resulted in a large production of superoxide anion, detected by acetylated cytochrome c method, which was blocked by superoxide dismutase (SOD). Exposure of SMP to ROS generation resulted in a marked loss of complex I activity and to parallel loss of mitochondrial cardiolipin content. Both these effects were completely abolished by SOD+catalase. Exogenous added cardiolipin was able to almost completely restore the ROS-induced loss of complex I activity. No restoration was obtained with other major phospholipid components of the mitochondrial membrane such as phosphatidylcholine and phosphatidylethanolamine, nor with peroxidized cardiolipin. These results demonstrate that ROS affect the mitochondrial complex I activity via oxidative damage of cardiolipin which is required for the functioning of this multisubunit enzyme complex. These results may prove useful in probing molecular mechanisms of ROS-induced peroxidative damage to mitochondria, which have been proposed to contribute to those pathophysiological conditions characterized by an increase in the basal production of reactive oxygen species such as aging, ischemia/reperfusion and chronic degenerative diseases.

Chronic hypoxia alters fatty acid composition of phospholipids in right and left ventricular myocardium

Adult male Wistar rats were exposed to intermittent high altitude hypoxia of 7000 m simulated in a hypobaric chamber for 8 h/day, 5 days a week; the total number of exposures was 25. The concentration of individual phospholipids and their fatty acid (FA) profile was determined in right (RV) and left (LV) ventricles. Adaptation to hypoxia decreased the concentration of diphosphatidytglycerol (DPG) in hypertrophied RV by 19% and in non-hypertrophied LV by 12% in comparison with normoxic controls. Chronically hypoxic hearts exhibited lower phospholipid n-6 polyunsaturated FA(PUFA) content mainly due to decreased linoleic acid (18:2n-6), which was opposed by increased n-3 PUFA mainly due to docosahexaenoic acid (22:6n-3) in phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylinositol (PI). The content of arachidonic acid (20:4n-6) was unchanged in total phospholipids, but in PC it was increased in both ventricles (by 22%) and in PE decreased in LV only (by 20%). Chronic hypoxia increased the un-saturation index of PC and PE in both ventricles. The content of monounsaturated FA (MUFA) was increased and 18:2n-6 decreased in DPG. The proportion of saturated FA was increased in PC and PI of hypoxic RV but not LV. The FA composition of phosphatidylserine was not altered in hypoxic ventricles. It is concluded that chronic hypoxia led to only minor changes in individual phospholipid concentration in rat ventricular myocardium, but markedly altered their FA profile. These changes, in particular the greater incorporation of n-3 PUFA into phospholipids and increased un-saturation index, may lead to a better preservation of membrane integrity and thereby contribute to improved ischemic tolerance of chronically hypoxic hearts.

ATP induces a conformational change in lipid-bound cytochrome c

Resonance energy transfer studies using a pyrene-labeled phospholipid derivative 1-palmitoyl-2-[10-(pyren-1-yl)decanoyl]-sn-glycero-3-phosphoglycerol (donor) and the heme (acceptor) of cytochrome c (cyt c) have indicated that ATP causes changes in the conformation of the lipid-bound protein (Rytömaa, M., Mustonen, P., and Kinnunen, P. K. J. (1992) J. Biol. Chem. 267, 22243-22248). Accordingly, after binding cyt c via its so called C-site to neat phosphatidylglycerol liposomes (mole fraction of PG = 1.0) has commenced, further quenching of donor fluorescence is caused by ATP, saturating at 2 mm nucleotide. ATP-induced conformational changes in liposome-associated cyt c could be directly demonstrated by CD in the Soret band region (380-460 nm). The latter data were further supported by time-resolved spectroscopy using the fluorescent cyt c analog with a Zn(2+)-substituted heme moiety. A high affinity ATP-binding site has been demonstrated in cyt c (Craig, D. B., and Wallace, C. J. A. (1993) Protein Sci. 2, 966-976) that is compromised by replacing the invariant Arg(91) to norleucine. Although no major effects on conformation and function of cyt c were concluded due to the modification, a significantly reduced effect by ATP on the lipid-bound [Nle(91)]cyt c was evident, implying that this modulation is mediated via the Arg(91)-containing binding site.

Effect of heme iron valence state on the conformation of cytochrome c and its association with membrane interfaces. A CD and EPR investigation

Recently cytochrome c has been mentioned as an important mediator in the events of cellular oxidative stress and apoptosis. To investigate the influence of charged interfaces on the conformation of cytochrome c, the CD and magnetic circular dichroic behavior of ferric and ferrous cytochrome c in homogeneous medium and in phosphatidylcholine/phosphatidylethanolamine/cardiolipin and dicetylphosphate liposomes was studied in the 300-600 and 200-320 nm wavelength region. EPR spectra demonstrate that the association of cytochrome c with membranes promotes alterations of the crystal field symmetry and spin state of the heme Fe(3+). The studies also include the effect of P(i), NaCl, and CaCl(2). Magnetic circular dichroism and CD results show that the interaction of both ferrous and ferric cytochrome c with charged interfaces promotes conformational changes in the alpha-helix content, tertiary structure, and heme iron spin state. Moreover, the association of cytochrome c with different liposomes is sensitive to the heme iron valence state. The more effective association with membranes occurs with ferrous cytochrome c. Dicetylphosphate liposomes, as a negatively charged membrane model, promoted a more pronounced conformational modification in the cytochrome c structure. A decrease in the lipid/protein association is detected in the presence of increasing amounts of CaCl(2), NaCl, and P(i), in response to the increase of the ionic strength.

Direct evidence for the cooperative unfolding of cytochrome c in lipid membranes from H-(2)H exchange kinetics

The interaction of cytochrome c (cyt c) with anionic lipid membranes is known to disrupt the tightly packed native structure of the protein. This process leads to a lipid-inserted denatured state, which retains a native-like alpha-helical structure but lacks any specific tertiary interactions. The structural and dynamic properties of cyt c bound to vesicles containing an anionic phospholipid (DOPS) were investigated by amide H-(2)H exchange using two-dimensional NMR spectroscopy and electrospray ionisation mass spectrometry. The H-(2)H exchange kinetics of the core amide protons in cyt c, which in the native protein undergo exchange via an uncorrelated EX2 mechanism, exchange in the lipid vesicles via a highly concerted global transition that exposes these protected amide groups to solvent. The lack of pH dependence and the observation of distinct populations of deuterated and protonated species by mass spectrometry confirms that exchange occurs via an EX1 mechanism with a common rate of 1(+/-0.5) h(-1), which reflects the rate of transition from the lipid-inserted state, H(l), to an unprotected conformation, D(i), associated with the lipid interface.

Mitochondrial phospholipid hydroperoxide glutathione peroxidase inhibits the release of cytochrome c from mitochondria by suppressing the peroxidation of cardiolipin in hypoglycaemia-induced apoptosis

Cytochrome c (cyt. c) is a proapoptotic factor that binds preferentially to cardiolipin (CL), a mitochondrial lipid, but not to cardiolipin hydroperoxide (CL-OOH). Cyt. c that had bound to CL liposomes was liberated on peroxidation of the liposomes by a radical. The generation of CL-OOH in mitochondria occurred before the release of cyt. c in rat basophile leukaemia (RBL)2H3 cells that had been induced to undergo apoptosis by exposure to hypoglycaemia with 2-deoxyglucose (2DG). The amount of cyt. c bound to CL prepared from the mitochondria of 2DG-treated cells was lower than that of untreated cells. The release of cyt. c was completely suppressed when the production of CL-OOH in mitochondria was inhibited by the overexpression of mitochondrial phospholipid hydroperoxide glutathione peroxidase (PHGPx). The fluorescence from CL-labelling dye (10-N-nonyl Acridine Orange) decreased on the induction of apoptosis by 2DG. However, no decrease in fluorescence was observed in PHGPx-overexpressing cells. Cyt. c was released from mitochondria that had been isolated from control cells on peroxidation by t-butylhydroperoxide, but no similar liberation of cyt. c from mitochondria isolated from mitochondrial PHGPx-overexpressing cells was observed. These findings suggest that the generation of CL-OOH in mitochondria might be a primary event that triggers the release of cyt. c from mitochondria in the apoptotic process in which mitochondrial PHGPx participates as an anti-apoptotic factor by preventing the formation of CL-OOH.

Liposomes as carriers in electrokinetic capillary chromatography

Liposomes are small membrane-enclosed vesicles composed of either natural or synthetic lipids. Their size can be adjusted on a wide scale and they can be made with well-defined compositions. While liposomes have been extensively used as model biomembranes they have also gained a considerable degree of attention as carriers for drugs as well as for genetic material. The physical properties of liposomes are critically dependent on their chemical composition. In this study liposomes were applied as pseudostationary phases in electrokinetic capillary chromatography. Various negatively charged liposomes, consisting of mixtures of zwitterionic and anionic lipids, were investigated. Major emphasis was put on clarifying the effects of the total lipid concentration, the lipid molar ratio, the lipid head group, and the buffer on the capillary electrophoretic separation of neutral analytes. In addition, the influence of the physical state of the membrane, ie., gel vs. fluid, on the separation was investigated. Corticosteroids were applied as model analytes.

Effect of Adriamycin on the boundary lipid structure of cytochrome c oxidase: pico-second time-resolved fluorescence depolarization studies

The fluorescence dynamics of the dye 3,3'-diethyloxadicarbocyanine iodide (DODCI) was used to probe the microenvironment of cytochrome c oxidase (CcO) and cardiolipin. The dye was partitioned between an aqueous and a hydrophobic phase. The 'bound' and 'free' populations of DODCI could be separated by analysis of the time-resolved fluorescence decay of the dye. The anisotropy decay of the DODCI bound to CcO showed a unique 'dip and rise' shape that was analyzed by a combination of rotational correlation times with time-dependent weight factors for each lifetime component. Rotational dynamics studies revealed the existence of a restricted motion of the dye bound at the enzyme surface. Adriamycin, an anticancer, albeit cardiotoxic drug, was previously proposed to affect the surface structure of CcO, most likely by causing a disorder to the surface lipid arrangement. A drastic change in the rotational correlation time of the dye bound to the enzyme surface was observed, which suggested a depletion of cardiolipin layer due to complexation with the drug. The effect of Adriamycin on cardiolipin was drastic, leading to its phase separation. The present study suggests that the effect of Adriamycin on CcO is primarily a segregation of the cardiolipins.

Unfolding and refolding of cytochrome c driven by the interaction with lipid micelles

Binding of native cyt c to L-PG micelles leads to a partially unfolded conformation of cyt c. This micelle-bound state has no stable tertiary structure, but remains as alpha-helical as native cyt c in solution. In contrast, binding of the acid-unfolded cyt c to L-PG micelles induces folding of the polypeptide, resulting in a similar helical state to that originated from the binding of native cyt c to L-PG micelles. Far-ultraviolet (UV) circular dichroism (CD) spectra showed that this common micelle-associated helical state (HL) has a native-like alpha-helix content, but is highly expanded without a tightly packed hydrophobic core, as revealed by tryptophan fluorescence, near-UV, and Soret CD spectroscopy. The kinetics of the interaction of native and acid-unfolded cyt c was investigated by stopped-flow tryptophan fluorescence. Formation of H(L) from the native state requires the disruption of the tightly packed hydrophobic core in the native protein. This micelle-induced unfolding of cyt c occurs at a rate approximately 0.1 s(-1), which is remarkably faster in the lipid environment compared with the expected rate of unfolding in solution. Refolding of acid-unfolded cyt c with L-PG micelles involves an early highly helical collapsed state formed during the burst phase (<3 ms), and the observed main kinetic event reports on the opening of this early compact intermediate prior to insertion into the lipid micelle.

Lipid peroxidation and alterations to oxidative metabolism in mitochondria isolated from rat heart subjected to ischemia and reperfusion

Ischemia-reperfusion injury to cardiac myocytes involves membrane damage mediated by oxygen free radicals. Lipid peroxidation is considered a major mechanism of oxygen free radical toxicity in reperfused heart. Mitochondrial respiration is an important source of these reactive oxygen species and hence a potential contributor to reperfusion injury. We have examined the effects of ischemia (30 min) and ischemia followed by reperfusion (15 min) of rat hearts, on the kinetic parameters of cytochrome c oxidase, on the respiratory activities and on the phospholipid composition in isolated mitochondria. Mitochondrial content of malonyldialdheyde (MDA), an index of lipid peroxidation, was also measured. Reperfusion was accompanied by a significant increase in MDA production. Mitochondrial preparations from control, ischemic and reperfused rat heart had equivalent Km values for cytochrome c, although the maximal activity of the oxidase was 25 and 51% less in ischemic and reperfused mitochondria than that of controls. These changes in the cytochrome c oxidase activity were associated to parallel changes in state 3 mitochondrial respiration. The cytochrome aa3 content was practically the same in these three types of mitochondria. Alterations were found in the mitochondrial content of the major phospholipid classes, the most pronounced change occurring in the cardiolipin, the level that decreased by 28 and by 50% as function of ischemia and reperfusion, respectively. The lower cytochrome c oxidase activity in mitochondria from reperfused rat hearts could be almost completely restored to the level of control hearts by exogenously added cardiolipin, but not by other phospholipids nor by peroxidized cardiolipin. It is proposed that the reperfusion-induced decline in the mitochondrial cytochrome c oxidase activity can be ascribed, at least in part, to a loss of cardiolipin content, due to peroxidative attack of its unsaturated fatty acids by oxygen free radicals. These findings may provide an explanation for some of the factors that lead to myocardial reperfusion injury.

The helical domain of intestinal fatty acid binding protein is critical for collisional transfer of fatty acids to phospholipid membranes

Fatty acid binding proteins (FABPs) exhibit a beta-barrel topology, comprising 10 antiparallel beta-sheets capped by two short alpha-helical segments. Previous studies suggested that fatty acid transfer from several FABPs occurs during interaction between the protein and the acceptor membrane, and that the helical domain of the FABPs plays an important role in this process. In this study, we employed a helix-less variant of intestinal FABP (IFABP-HL) and examined the rate and mechanism of transfer of fluorescent anthroyloxy fatty acids (AOFA) from this protein to model membranes in comparison to the wild type (wIFABP). In marked contrast to wIFABP, IFABP-HL does not show significant modification of the AOFA transfer rate as a function of either the concentration or the composition of the acceptor membranes. These results suggest that the transfer of fatty acids from IFABP-HL occurs by an aqueous diffusion-mediated process, i.e., in the absence of the helical domain, effective collisional transfer of fatty acids to membranes does not occur. Binding of wIFABP and IFABP-HL to membranes was directly analyzed by using a cytochrome c competition assay, and it was shown that IFABP-HL was 80% less efficient in preventing cytochrome c from binding to membranes than the native IFABP. Collectively, these results indicate that the alpha-helical region of IFABP is involved in membrane interactions and thus plays a critical role in the collisional mechanism of fatty acid transfer from IFABP to phospholipid membranes.

Liposome effect on the cytochrome c-catalyzed peroxidation of carbonyl substrates to triplet species

Cytochrome c exhibits peroxidase activity on diphenylacetaldehyde (DPAA) and 3-methylacetoacetone (MAA), which is greatly affected by the presence and nature of charged liposome or micelle interfaces interacting with the enzyme. The ferricytochrome c reaction with DPAA is accelerated when the enzyme is attached to negatively charged interfaces. Whatever the medium, bulk solution or negatively charged dicetylphosphate (DCP), phosphatidylcholine/phosphatidylethanolamine/cardiolipin (PC/PE/CL) liposomes, this chemiluminescent reaction is accompanied by reduction of cytochrome c to its ferrous form. In turn, MAA is oxidized by cytochrome c exclusively when bound to DCP liposomes. Contrary to DPAA oxidation, the MAA reaction is followed by bleaching of cytochrome c, reflecting damage to the hemeprotein chromophore. The cytochrome-c-catalyzed oxidation of either DPAA or MAA leads to concomitant disappearance of the enzyme charge transfer absorption band at 695 nm. This suggests that the peroxidase activity of cytochrome c involves substrate-induced loss of the methionine ligand at the iron sixth coordination position, which is favored by interaction of cytochrome c with negatively charged interfaces. Accordingly, a decrease and blue shift of the charge transfer band could be observed in cytochrome-c-containing negatively charged DCP, PC/PE/CL liposomes or lysophosphatidylethanolamine micelles in the presence of DPAA or MAA.

Cytochrome-c oxidase isolated from the brain of swayback-diseased sheep displays unusual structure and uncharacteristic kinetics

Swayback disease, a neurodegenerative disorder of lambs, and Menkes disease, the human equivalent, are caused by a deficiency of dietary copper. Reports of low enzymic activity suggest that several copper-containing enzymes, including cytochrome-c oxidase (COX), may influence the progress of these diseases. To investigate its role in the development of neurodegenerative disorders, in particular swayback disease, we isolated COX from the brains and livers of swayback-diseased lambs. Comparative sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) combined with densitometric analysis revealed that whereas the structure of COX from the liver of diseased animals was normal, the corresponding brain enzyme was subunits II-, III-, and IV-deficient; the deficiency was 55, 30, and 65% respectively. The activities of liver and brain COX from normal and diseased lambs were compared by polarographic assay at low ionic strength. Whereas the enzyme from normal brains and both forms of the liver enzyme yielded characteristic biphasic Eadie-Hofstee plots, the brain enzyme from diseased animals displayed a single phase with a K(m) of 4.7 +/- 2.4 x 10(-6) M: the K(m) values of COX from the normal brain were 12 +/- 2.5 x 10(-6) and 5.5 +/- 0.5 x 10(-7) M. We conclude that the altered enzyme structure accounts for the uncharacteristic kinetics and low activity we have observed for the isolated brain enzyme. We also conclude that the altered enzyme structure partly accounts for the low oxidase activity and decreased ATP synthesis that has been widely reported for brain tissue from swayback-diseased animals. We postulate that the subunit deficiency probably results from incomplete crosslinking between the subunits and the membrane, and predict that similar structural and kinetic factors may also account for low COX activity in Menkes disease.