Relationship between lipid saturation and lipid-protein interaction in liver mitochondria modified by catalytic hydrogenation with reference to cardiolipin molecular species

In Search of the Holy Grail: Toward a Unified Hypothesis on Mitochondrial Dysfunction in Age-Related Diseases

Cardiolipin (CL) is a mitochondrial signature phospholipid that plays a pivotal role in mitochondrial dynamics, membrane structure, oxidative phosphorylation, mtDNA bioenergetics, and mitophagy. The depletion or abnormal acyl composition of CL causes mitochondrial dysfunction, which is implicated in the pathogenesis of aging and age-related disorders. However, the molecular mechanisms by which mitochondrial dysfunction causes age-related diseases remain poorly understood. Recent development in the field has identified acyl-CoA:lysocardiolipin acyltransferase 1 (ALCAT1), an acyltransferase upregulated by oxidative stress, as a key enzyme that promotes mitochondrial dysfunction in age-related diseases. ALCAT1 catalyzes CL remodeling with very-long-chain polyunsaturated fatty acids, such as docosahexaenoic acid (DHA). Enrichment of DHA renders CL highly sensitive to oxidative damage by reactive oxygen species (ROS). Oxidized CL becomes a new source of ROS in the form of lipid peroxides, leading to a vicious cycle of oxidative stress, CL depletion, and mitochondrial dysfunction. Consequently, ablation or the pharmacological inhibition of ALCAT1 have been shown to mitigate obesity, type 2 diabetes, heart failure, cardiomyopathy, fatty liver diseases, neurodegenerative diseases, and cancer. The findings suggest that age-related disorders are one disease (aging) manifested by different mitochondrion-sensitive tissues, and therefore should be treated as one disease. This review will discuss a unified hypothesis on CL remodeling by ALCAT1 as the common denominator of mitochondrial dysfunction, linking mitochondrial dysfunction to the development of age-related diseases.

Enhanced hepatic respiratory capacity and altered lipid metabolism support metabolic homeostasis during short-term hypoxic stress

BACKGROUND

Tissue hypoxia is a key feature of several endemic hepatic diseases, including alcoholic and non-alcoholic fatty liver disease, and organ failure. Hypoxia imposes a severe metabolic challenge on the liver, potentially disrupting its capacity to carry out essential functions including fuel storage and the integration of lipid metabolism at the whole-body level. Mitochondrial respiratory function is understood to be critical in mediating the hepatic hypoxic response, yet the time-dependent nature of this response and the role of the respiratory chain in this remain unclear.

RESULTS

Here, we report that hepatic respiratory capacity is enhanced following short-term exposure to hypoxia (2 days, 10% O2) and is associated with increased abundance of the respiratory chain supercomplex III2+IV and increased cardiolipin levels. Suppression of this enhanced respiratory capacity, achieved via mild inhibition of mitochondrial complex III, disrupted metabolic homeostasis. Hypoxic exposure for 2 days led to accumulation of plasma and hepatic long chain acyl-carnitines. This was observed alongside depletion of hepatic triacylglycerol species with total chain lengths of 39-53 carbons, containing palmitic, palmitoleic, stearic, and oleic acids, which are associated with de novo lipogenesis. The changes to hepatic respiratory capacity and lipid metabolism following 2 days hypoxic exposure were transient, becoming resolved after 14 days in line with systemic acclimation to hypoxia and elevated circulating haemoglobin concentrations.

CONCLUSIONS

The liver maintains metabolic homeostasis in response to shorter term hypoxic exposure through transient enhancement of respiratory chain capacity and alterations to lipid metabolism. These findings may have implications in understanding and treating hepatic pathologies associated with hypoxia.

Glycolytic Metabolism, Brain Resilience, and Alzheimer's Disease

Alzheimer's disease (AD) is the most common form of age-related dementia. Despite decades of research, the etiology and pathogenesis of AD are not well understood. Brain glucose hypometabolism has long been recognized as a prominent anomaly that occurs in the preclinical stage of AD. Recent studies suggest that glycolytic metabolism, the cytoplasmic pathway of the breakdown of glucose, may play a critical role in the development of AD. Glycolysis is essential for a variety of neural activities in the brain, including energy production, synaptic transmission, and redox homeostasis. Decreased glycolytic flux has been shown to correlate with the severity of amyloid and tau pathology in both preclinical and clinical AD patients. Moreover, increased glucose accumulation found in the brains of AD patients supports the hypothesis that glycolytic deficit may be a contributor to the development of this phenotype. Brain hyperglycemia also provides a plausible explanation for the well-documented link between AD and diabetes. Humans possess three primary variants of the apolipoprotein E (ApoE) gene - ApoE∗ϵ2, ApoE∗ϵ3, and ApoE∗ϵ4 - that confer differential susceptibility to AD. Recent findings indicate that neuronal glycolysis is significantly affected by human ApoE isoforms and glycolytic robustness may serve as a major mechanism that renders an ApoE2-bearing brain more resistant against the neurodegenerative risks for AD. In addition to AD, glycolytic dysfunction has been observed in other neurodegenerative diseases, including Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis, strengthening the concept of glycolytic dysfunction as a common pathway leading to neurodegeneration. Taken together, these advances highlight a promising translational opportunity that involves targeting glycolysis to bolster brain metabolic resilience and by such to alter the course of brain aging or disease development to prevent or reduce the risks for not only AD but also other neurodegenerative diseases.

System biology analysis reveals the role of voltage-dependent anion channel in mitochondrial dysfunction during non-alcoholic fatty liver disease progression into hepatocellular carcinoma

Non‐alcoholic fatty liver disease (NAFLD) is one of the most common causes of hepatocellular carcinoma (HCC), but the underlying mechanisms behind the correlation of NAFLD with HCC are unclear. We aimed to uncover the genes and potential mechanisms that drive this progression. This study uncovered the genes and potential mechanisms through a multiple ’omics integration approach. Quantitative proteomics combined with phenotype‐association analysis was performed. To investigate the potential mechanisms, a comprehensive transcriptome/lipidome/phenome‐wide association analysis was performed in genetic reference panel BXD mice strains. The quantitative proteomics combined with phenotype‐association results showed that VDAC1 was significantly increased in tumor tissues and correlated with NAFLD‐related traits. Gene co‐expression network analysis indicated that VDAC1 is involved in mitochondria dysfunction in the tumorigenic/tumor progression. The association between VDAC1 and mitochondria dysfunction can be explained by the fact that VDAC1 was associated with mitochondria membrane lipids cardiolipin (CL) composition shift. VDAC1 was correlated with the suppression of mature specie CL(LLLL) and elevation level of nascent CL species. Such profiling shift was supported by the significant positive correlation between VDAC1 and PTPMT1, as well as negative correlation with CL remodeling enzyme Tafazzin (TAZ). This study confirmed that the expression of VADC1 was dysregulated in NAFLD‐driven HCC and associated with NAFLD progression. The VDAC1‐driven mitochondria dysfunction is associated with cardiolipin composition shift, which causes alteration of mitochondria membrane properties.

Alteration of cardiolipin biosynthesis and remodeling in single right ventricle congenital heart disease

Despite advances in both medical and surgical therapies, individuals with single ventricle heart disease (SV) remain at high risk for the development of heart failure (HF). However, the molecular mechanisms underlying remodeling and eventual HF in patients with SV are poorly characterized. Cardiolipin (CL), an inner mitochondrial membrane phospholipid, is critical for proper mitochondrial function, and abnormalities in CL content and composition are known in various cardiovascular disease etiologies. The purpose of this study was to investigate myocardial CL content and composition in failing and nonfailing single right ventricle (RV) samples compared with normal control RV samples, to assess mRNA expression of CL biosynthetic and remodeling enzymes, and to quantitate relative mitochondrial copy number. A cross-sectional analysis of RV myocardial tissue from 22 failing SV (SVHF), 9 nonfailing SV (SVNF), and 10 biventricular control samples (BVNF) was performed. Expression of enzymes involved in CL biosynthesis and remodeling were analyzed using RT-qPCR and relative mitochondrial DNA copy number determined by qPCR. Normal phase high-pressure liquid chromatography coupled to electrospray ionization mass spectrometry was used to quantitate total and specific CL species. While mitochondrial copy number was not significantly different between groups, total CL content was significantly lower in SVHF myocardium compared with BVNF controls. Despite having lower total CL content however, the relative percentage of the major tetralinoleoyl CL species is preserved in SVHF samples relative to BVNF controls. Correspondingly, expression of enzymes involved in CL biosynthesis and remodeling were upregulated in SVHF samples when compared with both SVNF samples and BVNF controls.NEW & NOTEWORTHY The mechanisms underlying heart failure in the single ventricle (SV) congenital heart disease population are largely unknown. In this study we identify alterations in cardiac cardiolipin metabolism, composition, and content in children with SV heart disease. These findings suggest that cardiolipin could be a novel therapeutic target in this unique population of patients.

High Cholesterol/Low Cholesterol: Effects in Biological Membranes: A Review

Lipid composition determines membrane properties, and cholesterol plays a major role in this determination as it regulates membrane fluidity and permeability, as well as induces the formation of coexisting phases and domains in the membrane. Biological membranes display a very diverse lipid composition, the lateral organization of which plays a crucial role in regulating a variety of membrane functions. We hypothesize that, during biological evolution, membranes with a particular cholesterol content were selected to perform certain functions in the cells of eukaryotic organisms. In this review, we discuss the major membrane properties induced by cholesterol, and their relationship to certain membrane functions.

CLD1 Reverses the Ubiquinone Insufficiency of Mutant cat5/coq7 in a Saccharomyces cerevisiae Model System

Ubiquinone (Q_n) functions as a mobile electron carrier in mitochondria. In humans, Q biosynthetic pathway mutations lead to Q₁₀ deficiency, a life threatening disorder. We have used a Saccharomyces cerevisiae model of Q₆ deficiency to screen for new modulators of ubiquinone biosynthesis. We generated several hypomorphic alleles of coq7/cat5 (clk-1 in Caenorhabditis elegans) encoding the penultimate enzyme in Q biosynthesis which converts 5-demethoxy Q₆ (DMQ₆) to 5-demethyl Q₆, and screened for genes that, when overexpressed, suppressed their inability to grow on non-fermentable ethanol—implying recovery of lost mitochondrial function. Through this approach we identified Cardiolipin-specific Deacylase 1 (CLD1), a gene encoding a phospholipase A₂ required for cardiolipin acyl remodeling. Interestingly, not all coq7 mutants were suppressed by Cld1p overexpression, and molecular modeling of the mutant Coq7p proteins that were suppressed showed they all contained disruptions in a hydrophobic α-helix that is predicted to mediate membrane-binding. CLD1 overexpression in the suppressible coq7 mutants restored the ratio of DMQ₆ to Q₆ toward wild type levels, suggesting recovery of lost Coq7p function. Identification of a spontaneous Cld1p loss-of-function mutation illustrated that Cld1p activity was required for coq7 suppression. This observation was further supported by HPLC-ESI-MS/MS profiling of monolysocardiolipin, the product of Cld1p. In summary, our results present a novel example of a lipid remodeling enzyme reversing a mitochondrial ubiquinone insufficiency by facilitating recovery of hypomorphic enzymatic function.

Biosynthesis, remodeling and turnover of mitochondrial cardiolipin

Among mitochondrial lipids, cardiolipin occupies a unique place. It is the only phospholipid that is specific to mitochondria and although it is merely a minor component, accounting for 10-20% of the total phospholipid content, cardiolipin plays an important role in the molecular organization, and thus the function of the cristae. This review covers the formation of cardiolipin, a phospholipid dimer containing two phosphatidyl residues, and its assembly into mitochondrial membranes. While a large body of literature exists on this topic, the review focuses on papers that appeared in the past three years. This article is part of a Special Issue entitled: Lipids of Mitochondria edited by Guenther Daum.

Novel super-resolution capable mitochondrial probe, MitoRed AIE, enables assessment of real-time molecular mitochondrial dynamics

Mitochondria and mitochondrial dynamics play vital roles in health and disease. With the intricate nanometer-scale structure and rapid dynamics of mitochondria, super-resolution microscopy techniques possess great un-tapped potential to significantly contribute to understanding mitochondrial biology and kinetics. Here we present a novel mitochondrial probe (MitoRed AIE) suitable for live mitochondrial dynamics imaging and single particle tracking (SPT), together with a multi-dimensional data analysis approach to assess local mitochondrial (membrane) fluidity. The MitoRed AIE probe localizes primarily to mitochondrial membranes, with 95 ms fluorophore on-time delivering 106 photons/ms, characteristics which we exploit to demonstrate live cell 100 fps 3D time-lapse tracking of mitochondria. Combining our experimental and analytical approaches, we uncover mitochondrial dynamics at unprecedented time scales. This approach opens up a new regime into high spatio-temporal resolution dynamics in many areas of mitochondrial biology.

Loss of protein association causes cardiolipin degradation in Barth syndrome

Cardiolipin is a specific mitochondrial phospholipid that has a high affinity for proteins and that stabilizes the assembly of supercomplexes involved in oxidative phosphorylation. We found that sequestration of cardiolipin in protein complexes is critical to protect it from degradation. The turnover of cardiolipin is slower by almost an order of magnitude than the turnover of other phospholipids. However, in subjects with Barth syndrome, cardiolipin is rapidly degraded via the intermediate monolyso-cardiolipin. Treatments that induce supercomplex assembly decrease the turnover of cardiolipin and the concentration of monolyso-cardiolipin, whereas dissociation of supercomplexes has the opposite effect. Our data suggest that cardiolipin is uniquely protected from normal lipid turnover by its association with proteins, but this association is compromised in subjects with Barth syndrome, leading cardiolipin to become unstable, which in turn causes the accumulation of monolyso-cardiolipin.

Chronic Insulin Exposure Induces ER Stress and Lipid Body Accumulation in Mast Cells at the Expense of Their Secretory Degranulation Response

Lipid bodies (LB) are reservoirs of precursors to inflammatory lipid mediators in immunocytes, including mast cells. LB numbers are dynamic, increasing dramatically under conditions of immunological challenge. We have previously shown in vitro that insulin-influenced lipogenic pathways induce LB biogenesis in mast cells, with their numbers attaining steatosis-like levels. Here, we demonstrate that in vivo hyperinsulinemia resulting from high fat diet is associated with LB accumulation in murine mast cells and basophils. We characterize the lipidome of purified insulin-induced LB, and the shifts in the whole cell lipid landscape in LB that are associated with their accumulation, in both model (RBL2H3) and primary mast cells. Lipidomic analysis suggests a gain of function associated with LB accumulation, in terms of elevated levels of eicosanoid precursors that translate to enhanced antigen-induced LTC4 release. Loss-of-function in terms of a suppressed degranulation response was also associated with LB accumulation, as were ER reprogramming and ER stress, analogous to observations in the obese hepatocyte and adipocyte. Taken together, these data suggest that chronic insulin elevation drives mast cell LB enrichment in vitro and in vivo, with associated effects on the cellular lipidome, ER status and pro-inflammatory responses.

Imaging mass spectrometry of diversified cardiolipin molecular species in the brain

MALDI imaging mass spectrometry (MALDI-IMS) has been used successfully in mapping different lipids in tissue sections, yet existing protocols fail to detect the diverse species of mitochondria-unique cardiolipins (CLs) in the brain which are essential for cellular and mitochondrial physiology. We have developed methods enabling the imaging of individual CLs in brain tissue. This was achieved by eliminating ion suppressive effects by (i) cross-linking carboxyl/amino containing molecules on tissue with 1-ethyl-3-[3-(dimethylamino)propyl]-carbodiimide hydrochloride and (ii) removing highly abundant phosphatidylcholine head groups via phospholipase C treatment. These treatments allowed the detection of CL species at 100 μm resolution and did not affect the amount or molecular species distribution of brain tissue CLs. When combined with augmented matrix application, these modifications allowed the visualization and mapping of multiple CL species in various regions of the brain including the thalamus, hippocampus, and cortex. Areas such as the dentate and stratum radiatum exhibited higher CL signals than other areas within the hippocampal formation. The habenular nuclear (Hb)/dorsal third ventricle (D3 V) and lateral ventricle (LV) areas were identified as CL "hot spots". Our method also allowed structural MS/MS fragmentation and mapping of CLs with identified fatty acid residues and demonstrated a nonrandom distribution of individual oxidizable (polyunsaturated fatty acid containing) and nonoxidizable (nonpolyunsaturated containing) CLs in different anatomical areas of the brain. To our knowledge, this method is the first label-free approach for molecular mapping of diversified CLs in brain tissue.

Genetic variability of respiratory complex abundance, organization and activity in mouse brain

Mitochondrial dysfunction is implicated in the etiology and pathogenesis of numerous human disorders involving tissues with high energy demand. Murine models are widely used to elucidate genetic determinants of phenotypes relevant to human disease, with recent studies of C57BL/6J (B6), DBA/2J (D2) and B6xD2 populations implicating naturally occurring genetic variation in mitochondrial function/dysfunction. Using blue native polyacrylamide gel electrophoresis, immunoblots and in-gel activity analyses of complexes I, II, III, IV and V, our studies are the first to assess abundance, organization and catalytic activity of mitochondrial respiratory complexes and supercomplexes in mouse brain. Remarkable strain differences in supercomplex assembly and associated activity are evident, without differences in individual complexes I, II, III or IV. Supercomplexes I1 III2 IV2-3 exhibit robust complex III immunoreactivity and activities of complexes I and IV in D2, but with little detected in B6 for I1 III2 IV2 , and I1 III2 IV3 is not detected in B6. I1 III2 IV1 and I1 III2 are abundant and catalytically active in both strains, but significantly more so in B6. Furthermore, while supercomplex III2 IV1 is abundant in D2, none is detected in B6. In aggregate, these results indicate a shift toward more highly assembled supercomplexes in D2. Respiratory supercomplexes are thought to increase electron flow efficiency and individual complex stability, and to reduce electron leak and generation of reactive oxygen species. Our results provide a framework to begin assessing the role of respiratory complex suprastructure in genetic vulnerability and treatment for a wide variety of mitochondrial-related disorders.

Emerging roles of cardiolipin remodeling in mitochondrial dysfunction associated with diabetes, obesity, and cardiovascular diseases

Cardiolipin (CL) is a phospholipid exclusively localized in inner mitochondrial membrane where it is required for oxidative phosphorylation, ATP synthesis, and mitochondrial bioenergetics. The biological functions of CL are thought to depend on its acyl chain composition which is dominated by linoleic acids in metabolically active tissues. This unique feature is not derived from the de novo biosynthesis of CL, rather from a remodeling process that involves in phospholipases and transacylase/acyltransferase. The remodeling process is also believed to be responsible for generation of CL species that causes oxidative stress and mitochondrial dysfunction. CL is highly sensitive to oxidative damages by reactive oxygen species (ROS) due to its high content in polyunsaturated fatty acids and location near the site of ROS production. Consequently, pathological remodeling of CL has been implicated in the etiology of mitochondrial dysfunction commonly associated with diabetes, obesity, heart failure, neurodegeneration, and aging that are characterized by oxidative stress, CL deficiency, and abnormal CL species. This review summarizes recent progresses in molecular, enzymatic, lipidomic, and metabolic studies that support a critical regulatory role of pathological CL remodeling as a missing link between oxidative stress and mitochondrial dysfunction in metabolic diseases and aging.

Myocardial regulation of lipidomic flux by cardiolipin synthase: setting the beat for bioenergetic efficiency

Lipidomic regulation of mitochondrial cardiolipin content and molecular species composition is a prominent regulator of bioenergetic efficiency. However, the mechanisms controlling cardiolipin metabolism during health or disease progression have remained elusive. Herein, we demonstrate that cardiac myocyte-specific transgenic expression of cardiolipin synthase results in accelerated cardiolipin lipidomic flux that impacts multiple aspects of mitochondrial bioenergetics and signaling. During the postnatal period, cardiolipin synthase transgene expression results in marked changes in the temporal maturation of cardiolipin molecular species during development. In adult myocardium, cardiolipin synthase transgene expression leads to a marked increase in symmetric tetra-18:2 molecular species without a change in total cardiolipin content. Mechanistic analysis demonstrated that these alterations result from increased cardiolipin remodeling by sequential phospholipase and transacylase/acyltransferase activities in conjunction with a decrease in phosphatidylglycerol content. Moreover, cardiolipin synthase transgene expression results in alterations in signaling metabolites, including a marked increase in the cardioprotective eicosanoid 14,15-epoxyeicosatrienoic acid. Examination of mitochondrial bioenergetic function by high resolution respirometry demonstrated that cardiolipin synthase transgene expression resulted in improved mitochondrial bioenergetic efficiency as evidenced by enhanced electron transport chain coupling using multiple substrates as well as by salutary changes in Complex III and IV activities. Furthermore, transgenic expression of cardiolipin synthase attenuated maladaptive cardiolipin remodeling and bioenergetic inefficiency in myocardium rendered diabetic by streptozotocin treatment. Collectively, these results demonstrate the unanticipated role of cardiolipin synthase in maintaining physiologic membrane structure and function even under metabolic stress, thereby identifying cardiolipin synthase as a novel therapeutic target to attenuate mitochondrial dysfunction in diabetic myocardium.

Age-related decline in mitochondrial bioenergetics: does supercomplex destabilization determine lower oxidative capacity and higher superoxide production?

Mitochondrial decay plays a central role in the aging process. Although certainly multifactorial in nature, defective operation of the electron transport chain (ETC) constitutes a key mechanism involved in the age-associated loss of mitochondrial energy metabolism. Primarily, mitochondrial dysfunction affects the aging animal by limiting bioenergetic reserve capacity and/or increasing oxidative stress via enhanced electron leakage from the ETC. Even though the important aging characteristics of mitochondrial decay are known, the molecular events underlying inefficient electron flux that ultimately leads to higher superoxide appearance and impaired respiration are not completely understood. This review focuses on the potential role(s) that age-associated destabilization of the macromolecular organization of the ETC (i.e. supercomplexes) may be important for development of the mitochondrial aging phenotype, particularly in post-mitotic tissues.

Cardiolipin content in mitochondria from cultured skin fibroblasts harboring mutations in the mitochondrial ATP6 gene

The role of phospholipids in normal assembly and organization of the membrane proteins has been well documented. Cardiolipin, a unique tetra-acyl phospholipid localized in the inner mitochondrial membrane, is implicated in the stability of many inner-membrane protein complexes. Loss of cardiolipin content, alterations in its acyl chain composition and/or cardiolipin peroxidation have been associated with dysfunction in multiple tissues in a variety of pathological conditions. The aim of this study was to analyze the phospholipid composition of the mitochondrial membrane in the four most frequent mutations in the ATP6 gene: L156R, L217R, L156P and L217P but, more importantly, to investigate the possible changes in the cardiolipin profile. Mitochondrial membranes from fibroblasts with mutations at codon 217 of the ATP6 gene, showed a different cardiolipin content compared to controls. Conversely, results similar to controls were obtained for mutations at codon 156. These findings may be attributed to differences in the biosynthesis and remodeling of cardiolipin at the level of the inner mitochondrial transmembrane related to some mutations of the ATP6 gene.

The potassium channel KcsA: a model protein in studying membrane protein oligomerization and stability of oligomeric assembly?

Many membrane proteins are functional as stable oligomers. An understanding of the conditions that elicit and enhance oligomerization is important in many therapeutics. In this regard, protein-protein and protein-lipid interactions play crucial roles in the assembly and stability of oligomeric complexes. Recent years have seen a rapid increase in the mechanistic information on the importance of cytoplasmic termini in determining subunit assembly and stability of oligomeric complexes. In addition, the role of specific protein-lipid interaction between anionic phospholipids and "hot spots" on the protein surface has also become evident in stabilizing oligomeric assemblies. This review focuses on several contemporary developments of membrane proteins that stabilize oligomers by taking the potassium channel KcsA as an exemplary ion channel.

The role of phosphatidic acid and cardiolipin in stability of the tetrameric assembly of potassium channel KcsA

In this study, the roles of two anionic phospholipids-phosphatidic acid (PA), which is an important signaling molecule, and cardiolipin (CL), which plays a crucial role in the bioenergetics of the cell-in stabilizing the oligomeric structure of potassium channel KcsA were determined. The stability of KcsA was drastically increased as a function of PA or CL content (mol%) in phosphatidylcholine (PC) bilayers. Deletion of the membrane-associated N terminus significantly reduced channel stability at high levels of PA content; however, the intrinsic stability of this protein was marginally affected in the presence of CL. These studies indicate that the electrostatic-hydrogen bond switch between PA and N terminus, involving basic residues, is much stronger than the stabilizing effect of CL. Furthermore, the unique properties of the PA headgroup alter protein assembly and folding properties differently from the CL headgroup, and both lipids stabilize the tetrameric assembly via their specific interaction on the extra- or the intracellular side of KcsA.

Membrane protein dynamics: limited lipid control

Correlation of lipid disorder with membrane protein dynamics has been studied with infrared spectroscopy, by combining data characterizing lipid phase, protein structure and, via hydrogen-deuterium (H/D) exchange, protein dynamics. The key element was a new measuring scheme, by which the combined effects of time and temperature on the H/D exchange could be separated. Cyanobacterial and plant thylakoid membranes, mammalian mitochondria membranes, and for comparison, lysozyme were investigated. In dissolved lysozyme, as a function of temperature, H/D exchange involved only reversible movements (the secondary structure did not change considerably); heat-denaturing was a separate event at much higher temperature. Around the low-temperature functioning limit of the biomembranes, lipids affected protein dynamics since changes in fatty acyl chain disorders and H/D exchange exhibited certain correlation. H/D exchange remained low in all membranes over physiological temperatures. Around the high-temperature functioning limit of the membranes, the exchange rates became higher. When temperature was further increased, H/D exchange rates went over a maximum and afterwards decreased (due to full H/D exchange and/or protein denaturing). Maximal H/D exchange rate temperatures correlated neither with the disorder nor with the unsaturation of lipids. In membrane proteins, in contrast to lysozyme, the onsets of sizable H/D exchange rates were the onsets of irreversible denaturing as well. Seemingly, at temperatures where protein self-dynamics allows large-scale H/D exchange, lipid-protein coupling is so weak that proteins prefer aggregating to limit the exposure of their hydrophobic surface regions to water. In all membranes studied, dynamics seemed to be governed by lipids around the low-temperature limit, and by proteins around the high-temperature limit of membrane functionality.PACS codes: 87.14.ep, 87.14.cc, 87.16.D.

Cardiolipin synthesis for the assembly of bacterial and mitochondrial membranes

In this article, the formation of prokaryotic and eukaryotic cardiolipin is reviewed in light of its biological function. I begin with a detailed account of the structure of cardiolipin, its stereochemistry, and the resulting physical properties, and I present structural analogs of cardiolipin that occur in some organisms. Then I continue to discuss i) the de novo formation of cardiolipin, ii) its acyl remodeling, iii) the assembly of cardiolipin into biological membranes, and iv) the degradation of cardiolipin, which may be involved in apoptosis and mitochondrial fusion. Thus, this article covers the entire metabolic cycle of this unique phospholipid. It is shown that mitochondria produce cardiolipin species with a high degree of structural uniformity and molecular symmetry, among which there is often a dominant form with four identical acyl chains. The subsequent assembly of cardiolipin into functional membranes is largely unknown, but the analysis of crystal structures of membrane proteins has revealed a first glimpse into the underlying principles of cardiolipin-protein interactions. Disturbances of cardiolipin metabolism are crucial in the pathophysiology of human Barth syndrome and perhaps also play a role in diabetes and ischemic heart disease.

Role of cardiolipin alterations in mitochondrial dysfunction and disease

Cardiolipin (CL) is a structurally unique dimeric phospholipid localized in the inner mitochondrial membrane where it is required for optimal mitochondrial function. In addition to its role in maintaining membrane potential and architecture, CL is known to provide essential structural and functional support to several proteins involved in mitochondrial bioenergetics. A loss of CL content, alterations in its acyl chain composition, and/or CL peroxidation have been associated with mitochondrial dysfunction in multiple tissues in a variety of pathological conditions, including ischemia, hypothyroidism, aging, and heart failure. Recently, aberrations in CL metabolism have been implicated as a primary causative factor in the cardioskeletal myopathy known as Barth syndrome, underscoring an important role of CL in human health and disease. The purpose of this review is to provide an overview of evidence that has linked changes in the CL profile to mitochondrial dysfunction in various pathological conditions. In addition, a brief overview of CL function and biosynthesis, and a discussion of methods used to examine CL in biological tissues are provided.

A novel cardiolipin-remodeling pathway revealed by a gene encoding an endoplasmic reticulum-associated acyl-CoA:lysocardiolipin acyltransferase (ALCAT1) in mouse

Cardiolipin is a major membrane polyglycerophospholipid that is required for the reconstituted activity of a number of key mitochondrial enzymes involved in energy metabolism. Cardiolipin is subjected to remodeling subsequent to its de novo biosynthesis to attain appropriate acyl composition for its biological functions. Yet, the enzyme(s) involved in the remodeling process have not been identified. We report here the identification and characterization of a murine gene that encodes an acyl-CoA:lysocardiolipin acyltransferase 1 (ALCAT1). Expression of the ALCAT1 cDNA in either insect or mammalian cells led to a significant increase in acyl-CoA:monolysocardiolipin acyltransferase and acyl-CoA: dilysocardiolipin acyltransferase activities that exhibited a dependence upon ALCAT1 enzyme levels. The recombinant ALCAT1 enzyme recognizes both monolysocardiolipin and dilysocardiolipin as substrates with a preference for linoleoyl-CoA and oleoyl-CoA as acyl donors. In contrast, no significant increases in acyltransferase activities by the recombinant ALCAT1 were detected against either glycerol-3-phosphate or a variety of other lysophospholipids as substrates, including lysophosphatidylcholine, lysophosphatidylethanolamine, and lysophosphatidylserine. Immunocytohistochemical analysis showed that the ALCAT1 enzyme is localized in the endoplasmic reticulum, which is supported by a significant ALCAT activity in isolated liver and heart microsomes. Northern blot analysis indicates that the mouse ALCAT1 is widely distributed, with the highest expression in heart and liver. In support of a role for ALCAT1 in maintaining heart function, the ALCAT1 gene is conserved among different species of vertebrates, but not in non-atrium organisms. ALCAT1 represents the first identified cardiolipin-remodeling enzyme from any living organism; its identification implies a novel role for the endoplasmic reticulum in cardiolipin metabolism.

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.

Heat shock protein coinducers with no effect on protein denaturation specifically modulate the membrane lipid phase

The hydroxylamine derivative bimoclomol (BM) has been shown to activate natural cytoprotective homeostatic responses by enhancing the capability of cells to cope with various pathophysiological conditions. It exerts its effect in synergy with low levels of stress to induce the synthesis of members of major stress protein families. We show here that the presence of BM does not influence protein denaturation in the cells. BM and its derivatives selectively interact with acidic lipids and modulate their thermal and dynamic properties. BM acts as a membrane fluidizer at normal temperature, but it is a highly efficient membrane stabilizer, inhibiting the bilayer-nonbilayer phase transitions during severe heat shock. We suggest that BM and the related compounds modify those domains of membrane lipids where the thermally or chemically induced perturbation of lipid phase is sensed and transduced into a cellular signal, leading to enhanced activation of heat shock genes. BM may be a prototype for clinically safe membrane-interacting drug candidates that rebalance the level and composition of heat shock proteins.

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.

Biological significance of phospholipid hydroperoxide glutathione peroxidase (PHGPx, GPx4) in mammalian cells

Reactive oxygen species (ROS) are known mediators of intracellular signal cascades. Excessive production of ROS may lead to oxidative stress, loss of cell function, and cell death by apoptosis or necrosis. Lipid hydroperoxides are one type of ROS whose biological function has not yet been clarified. Phospholipid hydroperoxide glutathione peroxidase (PHGPx, GPx4) is a unique antioxidant enzyme that can directly reduce phospholipid hydroperoxide in mammalian cells. This contrasts with most antioxidant enzymes, which cannot reduce intracellular phospholipid hydroperoxides directly. In this review, we focus on the structure and biological functions of PHGPx in mammalian cells. Recently, molecular techniques have allowed overexpression of PHGPx in mammalian cell lines, from which it has become clear that lipid hydroperoxides also have an important function as activators of lipoxygenase and cyclooxygenase, participate in inflammation, and act as signal molecules for apoptotic cell death and receptor-mediated signal transduction at the cellular level.

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.

The adipocyte fatty acid-binding protein binds to membranes by electrostatic interactions

The adipocyte fatty acid-binding protein (AFABP) is believed to transfer unesterified fatty acids (FA) to phospholipid membranes via a collisional mechanism that involves ionic interactions between lysine residues on the protein surface and phospholipid headgroups. This hypothesis is derived largely from kinetic analysis of FA transfer from AFABP to membranes. In this study, we examined directly the binding of AFABP to large unilamellar vesicles (LUV) of differing phospholipid compositions. AFABP bound LUV containing either cardiolipin or phosphatidic acid, and the amount of protein bound depended upon the mol % anionic phospholipid. The K(a) for CL or PA in LUV containing 25 mol % of these anionic phospholipids was approximately 2 x 10(3) M(-1). No detectable binding occurred when AFABP was mixed with zwitterionic membranes, nor when acetylated AFABP in which surface lysines had been chemically neutralized was mixed with anionic membranes. The binding of AFABP to acidic membranes depended upon the ionic strength of the incubation buffer: >/=200 mM NaCl reduced protein-lipid complex formation in parallel with a decrease in the rate of FA transfer from AFABP to negatively charged membranes. It was further found that AFABP, but not acetylated AFABP, prevented cytochrome c, a well characterized peripheral membrane protein, from binding to membranes. These results directly demonstrate that AFABP binds to anionic phospholipid membranes and suggest that, although generally described as a cytosolic protein, AFABP may behave as a peripheral membrane protein to help target fatty acids to and/or from intracellular sites of utilization.

Remodeling of phospholipid fatty acids in mitochondrial membranes of estivating snails

The effects of estivation on the phospholipid-specific fatty acid composition of mitochondrial membranes in the hepatopancreas of the terrestrial snail Cepaea nemoralis were investigated. The fatty acid composition of all phospholipids was significantly altered in snails estivating for 6 wk, indicating that substantial remodeling occurs. The most profound changes occurred in cardiolipin (CL). CL of estivating snails was 13-fold more saturated, contained 9-fold more monoenes, and had 45% fewer polyenes than in active snails. These differences were due, in part, to a reduction in linoleic acid (1 8:2n-6) content of CL from estivators. As in mammals, CL of active snails appears to preferentially incorporate 18:2n-6, which accounts for 60% of the acyl chains in this phospholipid. This proportion was reduced by 50% in estivators. Changes in the fatty acyl content of other phospholipids of estivating snails included increased monoenes in phosphatidylethanolamine (PE) and phosphatidylinositol, reduced ratios of n-3/n-6 polyenes in PE and phosphatidylcholine (PC), and an increased n-3/n-6 ratio in phosphatidylserine (PS). Arachidonic acid (20:4n-6) levels were reduced in PS but increased in CL and PC. Taken together, these alterations to fatty acid composition are consistent with decreased biological activity of membrane-related processes which occur in conjunction with the reduction of mitochondrial aerobic metabolism observed during estivation.

Membrane physical state controls the signaling mechanism of the heat shock response in Synechocystis PCC 6803: identification of hsp17 as a "fluidity gene"

The fluidity of Synechocystis membranes was adjusted in vivo by temperature acclimation, addition of fluidizer agent benzyl alcohol, or catalytic lipid hydrogenation specific to plasma membranes. The reduced membrane physical order in thylakoids obtained by either downshifting growth temperature or administration of benzyl alcohol was paralleled with enhanced thermosensitivity of the photosynthetic membrane. Simultaneously, the stress-sensing system leading to the cellular heat shock (HS) response also has been altered. There was a close correlation between thylakoid fluidity levels, monitored by steady-state 1,6-diphenyl-1,3,5-hexatriene anisotropy, and threshold temperatures required for maximal activation of all of the HS-inducible genes investigated, including dnaK, groESL, cpn60, and hsp17. The causal relationship between the pre-existing thylakoid physical order and temperature set point of both the transcriptional activation and the de novo protein synthesis was the most striking for the 17-kDa HS protein (HSP17) associated mostly with the thylakoid membranes. These findings together with the fact that the in vivo modulation of lipid saturation within cytoplasmic membrane had no effect on HS response suggest that thylakoid acts as a cellular thermometer where thermal stress is sensed and transduced into a cellular signal leading to the activation of HS genes.

Cardiolipin hydrolysis by human phospholipases A2. The multiple enzymatic activities of human cytosolic phospholipase A2

The ability of mammalian phospholipases A2 (PLA2) to hydrolyse cardiolipin (diphosphatidylglycerol) was monitored with a fluorescent displacement assay which allows the use of natural phospholipid substrates. The mammalian enzymes used were porcine pancreatic (Group I) secretory PLA2 (sPLA2), human non-pancreatic (Group II) sPLA2 and human cytosolic PLA2 (cPLA2). High activity was observed with porcine pancreas sPLA2 whereas the human sPLA2 demonstrated only minimal activity with this substrate. In comparison, sPLA2 from Naja naja venom (Group I) also showed only modest activity with this substrate. Since many lipases possess PLA1 activity, a representative enzyme from Rhizopus arrhizus was also assessed for its ability to hydrolyse cardiolipin which proved to be a good substrate for this fungal lipase. In all cases dilysocardiolipin was the major product while some monolyso intermediate was detected after chromatographic separation. Human cPLA2 was unable to hydrolyse cardiolipin at a significant rate, however, both monolysocardiolipin and dilysocardiolipin, which are prepared by the PLA2-catalysed hydrolysis of cardiolipin, were good substrates providing a further example of the extensive lysophospholipase activity of this enzyme. Moreover, cardiolipin that was initially hydrolysed in situ with either excess porcine pancreatic PLA2 or R. arrhizus lipase (PLA1) was subsequently hydrolysed by human cPLA2. One explanation of this result is that human cPLA2 is able to hydrolyse both 1-acyl and 2-acyl-lysophospholipids. (c) 1998 Elsevier Science B.V.

Catalytic hydrogenation of polyunsaturated biological membranes: effects on membrane fatty acid composition and physical properties

The relationship between phospholipid saturation and membrane physical structure in a complex, highly polyunsaturated biological membrane (trout liver microsomes) has been studied by the graded and specific hydrogenation of polyunsaturated fatty acids. The homogeneous catalyst Pd(QS)2 caused rapid and effective hydrogenation, increasing the proportion of saturated fatty acids from 20-30% up to 60%, without loss or fragmentation. Long chain, polyunsaturated fatty acids (20:5 omega 3, 22:6 omega 3) were rapidly converted to a large number of partially hydrogenated isomers, and ultimately to the fully saturated C20 or C22 fatty acids. C18 mono- and di-unsaturates showed slower rates of hydrogenation. Increased saturation was closely associated with an increased membrane physical order as determined by the fluorescence anisotropy probe, 1,6-diphenyl-1,3,5-hexatriene. However, extensive hydrogenation led to highly ordered membranes exhibiting a gel-liquid crystalline phase transition between 30 and 60 degrees C. Polyunsaturated membranes can thus be converted into partially or substantially saturated membranes with measurable phase structure without direct alteration of other membrane components. This offers a less equivocal means of assessing the influence of polyunsaturation upon membrane structure and function.

The acylation of lysophosphatidylglycerol in rat heart: evidence for both in vitro and in vivo activities

The reacylation of lysophospholipids back to their parent molecules is important for attaining the appropriate fatty acyl composition in many phospholipids and for preventing the accumulation of arrhythmia generating lysophospholipids in the heart. In this study, we report the presence of an active acyltransferase activity for lysophosphatidylglycerol reacylation to phosphatidylglycerol in rat heart membrane preparations. The activity of acyl-Coenzyme A:1-acylglycerophosphorylglycerol acyltransferase in rat heart subcellular fractions was in the order of microsomal > mitochondrial > cytosol. The activity in the membrane fractions were characterized and found to have a pH optimum in the alkaline range. However, significant enzyme activity was observed at physiological pH. With oleoyl-Coenzyme A as substrate, the microsomal activity had a preference for lysophosphatidylglycerol substrates in the order of myristoyl > palmitoyl > oleoyl > stearoyl. The apparent K(m) values for 1-palmitoylglycerophosphorylglycerol and oleoyl-Coenzyme A were 9.4 and 7.1 microM, respectively. In contrast, the mitochondrial activity had a preference for lysophosphatidylglycerol substrates in the order of oleoyl > myristoyl = stearoyl = palmitoyl. The apparent K(m) values for 1-oleoylglycerophosphorylglycerol and oleoyl-Coenzyme A were 17.8 and 18.0 microM, respectively. Both membrane activities were heat labile as pre-incubation at 55 degrees C for 1 min completely abolished the activity. However, pre-incubation at 50 degrees C resulted in different profiles of inactivation in both microsomal and mitochondrial fractions. Both membrane activities were inhibited by high concentrations of lysophosphatidylglycerol and affected to a similar extent by various detergents. To demonstrate whether reacylation of lysophosphatidylglycerol to phosphatidylglycerol occurred in vivo, isolated rat hearts were perfused for 60 min in the Langendorff mode with 0.1 microM 1-palmitoylglycerophosphoryl[14C]glycerol bound to albumin. 1-Palmitoylglycerophosphoryl[14C]glycerol was readily taken up by the isolated perfused rat heart and significant synthesis of phosphatidyl[14C]glycerol was observed. The findings indicate the presence of an acyl-Coenzyme A:1-acylglycerophosphorylglycerol acyltransferase activity in the rat heart subcellular membranes which is capable of catalyzing lysophosphatidylglycerol acylation to phosphatidylglycerol in vitro and in vivo.

Analysis by fast-atom bombardment tandem mass spectrometry of phosphatidylcholine isolated from heart mitochondrial fractions: Evidence of incorporation of monohydroxylated fatty acyl moieties

Phosphatidylcholine (PC) is one of the main phospholipids present in mitochondrial membranes. According to current knowledge, the predominant fatty acyl moieties in this phospholipid are 16, 18, 20, or 22 carbon atoms long with chains that contain only carbon and hydrogen atoms. We have conducted a detailed analysis of the fatty acid substituents of the phospholipids present in mitochondrial fractions by using fast-atom bombardment tandem PC extracted from mitochondrial fractions of rat heart. The structure of one of these monohydroxylated fatty acids has been elucidated and corresponded to 12-hydroxy 9-octadecenoic acid. Indications that concern the structure of the five other monohydroxylated fatty acids are presented. These monohydroxylated fatty acyl groups are preferentially associated in the PC molecule with C-18 and C-20 fatty acyl moieties. We present arguments to suggest that the formation of these compounds is probably not due to a free-radical initiated mechanism. The potential implication of these monohydroxylated fatty acids in several physiological functions is suggested by the fact that free hydroxylated fatty acids that are identical or closely related to those found in the mitochondrial fractions possess various biological activities.

Reversibility of the binding of cytochrome c to liposomes. Implications for lipid-protein interactions

Studies on the membrane binding of cytochrome c revealed liposome-associated and soluble cytochrome c not to be in rapid equilibrium. In brief, cytochrome c attached to pyrene phospholipid-labeled, fluorescent liposomes containing either 17.6 mol % cardiolipin (CL) or 30 mol % egg phosphatidylglycerol (PG) is practically not at all or very slowly, respectively, detached by a subsequently added excess (up to 20-fold) of nonlabeled liposomes containing these acidic lipids. Cytochrome c was fully dissociated from PG-containing liposomes by increasing the ionic strength by NaCl, whereas dissociation from CL-containing membranes was less complete, presumably because of the scavenging of the protein within inverted intramembrane micelles. Importantly, the apparent irreversibility of the binding of cytochrome c to liposomes is strongly dependent on the structure of the acidic phospholipid. Cytochrome c bound to lyso-PG/PC liposomes could be dissociated with an excess of nonlabeled PG-containing liposomes. Cytochrome c was also efficiently bound to membranes containing the negatively charged dicetylphosphate yet could be readily dissociated by nonlabeled PG-containing liposomes. We conclude both proper geometry of the phosphate group and the presence of two acyl chains to be required for the tight binding of cytochrome c to acidic phospholipids. These data provide evidence for the membrane association of cytochrome c by an acidic phospholipid in the extended conformation (Kinnunen, P. K. J., Köiv, A., Lehtonen, J. Y. A., Rytömaa, M., and Mustonen, P. (1994) Chem. Phys. Lipids 73, 181-207) in which one of the acyl chains of the lipid becomes accommodated within a hydrophobic cavity of the protein. Based on the crystal structure of cytochrome c we putatively assign the invariant Asn-52 (horse heart cytochrome c) as the site liganding the protonated phosphate of the lipid, whereas Lys-72 and -73 should bind the deprotonated form.

Direct analysis and significance of cardiolipin transverse distribution in mitochondrial inner membranes

The distribution of cardiolipin across the inner mitochondrial membrane was directly determined by using the ability of the fluorescent dye 10-N-nonyl-3,6-bis(dimethylamino)acridine (10-N-nonyl acridine orange) to form dimers when it interacts with the diacidic phospholipid. Two independent methods were employed: (a) a spectrophotometric measurement of 10-N-nonyl acridine orange binding to isolated rat liver mitochondria, mitoplasts and inside-out submitochondrial particles, and (b) a flow-cytometric analysis of specific red fluorescence, emitted when two dye molecules are bound to one membrane cardiolipin; the stoichiometry of 10-N-nonyl acridine orange binding to phosphatidylserine and phosphatidylinositol, 1 mol dye/mol phospholipid, prevented dye dimerisation and subsequent red-fluorescence appearance. 57% total cardiolipin was present in the outer leaflets of inner membranes of isolated organelles, a distribution confirmed by saturation measurements for mitoplasts and inside-out submitochondrial particles. The same asymmetry was directly observed in situ with mitochondrial membranes of quiescent L1210 cells, and with mitochondrial membranes of respiring yeasts. Nevertheless, alterations in ATP synthesis and inhibition of mitochondrial protein synthesis revealed that cardiolipin distribution was apparently tightly correlated with mitochondrial membrane assembly and activity.

Mitochondrial cardiolipin in diverse eukaryotes. Comparison of biosynthetic reactions and molecular acyl species

Cardiolipin, a unique dimeric phospholipid of bacteria and mitochondria, can be synthesized by two alternative pathways discovered in rat and Escherichia coli, respectively. In mitochondrial preparations from fungi (Saccharomyces cerevisiae, Neurospora crassa), higher plants (Phaseolus aureus), molluscs (Mytilus edulis) and mammals (rat liver, bovine adrenal gland), cardiolipin was synthesized from CDP-diacylglycerol and phosphatidylglycerol, suggesting a common eukaryotic mechanism of cardiolipin formation which is in contrast to the prokaryotic biosynthesis from two molecules of phosphatidylglycerol. All mitochondrial cardiolipin synthases were inhibited by lysophosphatidylglycerol, were insensitive to N-ethylmaleimide and required divalent cations, although they had different cation specificities. The molecular species of cardiolipin from rat liver, bovine heart, S. cerevisiae and N. crassa were analysed by high-performance liquid chromatography of the derivative 1,3-bis[3'-sn-phosphatidyl]-2-benzoyl-sn-glycerol dimethyl ester. Cardiolipins from these organisms contained mainly monounsaturated or diunsaturated chains with 16 or 18 carbon atoms, resulting in a relatively homogeneous distribution of double bonds and carbon numbers among the four acyl positions. About half of the molecular species were symmetrical, i.e. they combined two identical diacylglycerol moieties. In N. crassa, the same species pattern was found at growth temperatures of 25 degrees C and 37 degrees C. Tentative molecular models were created for the most abundant molecular species and subjected to energy minimization. Geometric data, derived from these models, suggested similarities in the gross structure of the major cardiolipin species from different sources.

Effect of the unsaturation of phospholipid acyl chains on leucine transport of Lactococcus lactis and membrane permeability

The effect of the degree of unsaturation of the phospholipid acyl chains on the branched-chain amino acid transport system of Lactococcus lactis was investigated by the use of a membrane fusion technique. Transport activity was analyzed in hybrid membranes composed of equimolar mixtures of synthetic unsaturated phosphatidylethanolamine (PE) and phosphatidylcholine (PC) in which the number of cis double bonds in the 18-carbon acyl chains was varied. The accumulation level and initial rate of both counterflow and protonmotive-force driven transport of leucine decreased with increasing number of double bonds. The reduction in transport activity with increasing number of double bonds correlated with an increase in the passive permeability of the membranes to leucine. The membrane fluidity was hardly affected by the double bond content. It is concluded that the degree of lipid acyl chain unsaturation is a minor determinant of the activity of the branched chain amino acid transport system, but effects strongly the passive permeability of the membrane.

Cardiolipins and biomembrane function

Evidence is discussed for roles of cardiolipins in oxidative phosphorylation mechanisms that regulate State 4 respiration by returning ejected protons across and over bacterial and mitochondrial membrane phospholipids, and that regulate State 3 respiration through the relative contributions of proteins that transport protons, electrons and/or metabolites. The barrier properties of phospholipid bilayers support and regulate the slow proton leak that is the basis for State 4 respiration. Proton permeability is in the range 10(-3)-10(-4) cm s-1 in mitochondria and in protein-free membranes formed from extracted mitochondrial phospholipids or from stable synthetic phosphatidylcholines or phosphatidylethanolamines. The roles of cardiolipins in proton conductance in model phospholipid membrane systems need to be assessed in view of new findings by Hübner et al. [313]: saturated cardiolipins form bilayers whilst natural highly unsaturated cardiolipins form nonlamellar phases. Mitochondrial cardiolipins apparently participate in bilayers formed by phosphatidylcholines and phosphatidylethanolamines. It is not yet clear if cardiolipins themselves conduct protons back across the membrane according to their degree of fatty acyl saturation, and/or modulate proton conductance by phosphatidylcholines and phosphatidylethanolamines. Mitochondrial cardiolipins, especially those with high 18:2 acyl contents, strongly bind many carrier and enzyme proteins that are involved in oxidative phosphorylation, some of which contribute to regulation of State 3 respiration. The role of cardiolipins in biomembrane protein function has been examined by measuring retained phospholipids and phospholipid binding in purified proteins, and by reconstituting delipidated proteins. The reconstitution criterion for the significance of cardiolipin-protein interactions has been catalytical activity; proton-pumping and multiprotein interactions have yet to be correlated. Some proteins, e.g., cytochrome c oxidase are catalytically active when dimyristoylphosphatidylcholine replaces retained cardiolipins. Cardiolipin-protein interactions orient membrane proteins, matrix proteins, and on the outerface receptors, enzymes, and some leader peptides for import; activate enzymes or keep them inactive unless the inner membrane is disrupted; and modulate formation of nonbilayer HII-phases. The capacity of the proton-exchanging uncoupling protein to accelerate thermogenic respiration in brown adipose tissue mitochondria of cold-adapted animals is not apparently affected by the increased cardiolipin unsaturation; this protein seems to take over the protonophoric role of cardiolipins in other mitochondria. Many in vivo influences that affect proton leakage and carrier rates selectively alter cardiolipins in amount per mitochondrial phospholipids, in fatty acyl composition and perhaps in sidedness; other mitochondrial membrane phospholipids respond less or not at all.(ABSTRACT TRUNCATED AT 400 WORDS)