Cardiolipin inner membranes

The mitochondrion has an outer and an inner membrane (Figure 1). The outer membrane contains pores formed from a protein, porin, which allow exchange of molecules with molecular weights up to about 2,000 between the cytosol and the intermembrane space. The inner membrane is extensively invaginated to increase its surface area. It has a different lipid composition from the outer membrane and is rich in the acidic phospholipid cardiolipin (diphosphatidyl-glycerol) which is only found in animal cells in mitochondria. Cardiolipin confers good electrical insulating properties on the inner membrane which is impermeable... 

Mitochondria have an outer membrane that is permeable to most metabohtes, an inner membrane that is selectively permeable, and a matrix within (Figure 12-1). The outer membrane is characterized by the presence of various enzymes, including acyl-CoA synthetase and glycerolphosphate acyltransferase. Adenylyl kinase and creatine kinase are found in the intermembrane space. The phospholipid cardiolipin is concentrated in the inner membrane together with the enzymes of the respiratory chain. 

Figure 1. Control of mitochondrial biogenesis by the nuclear genome. Most mitochondrial proteins, including cytochrome c, are nuclear gene products which are subsequently imported into mitochondria. Similarly, most enzymes involved in synthesis of mitochondrial phosphoplipids are encoded in the nuclear genome. Being located in the endoplasmatic reticulum, they synthesize phosphatidylcholine (PtdCho), phosphatidylserine (PtdSer), phosphatidylglycerol (PG) and phosphatidylinositol (Ptdins). The phospholipids are transferred to the outer membrane. The imported lipids then move into the inner membrane at contact sites. Mitochondria then diversify phospholipids. They decarboxylate phosphatidylserine to phosphatidylethanolamine (PtdEtN), but the main reaction is the conversion of imported phosphatidylglycerol to cardiolipin (CL). Cardiolipins localize mainly in the outer leaflet of the inner membrane.

Cardiolipin or diphosphatidyl glycerol is one of the most ancient membrane phospholipids from phylogenic aspects. It is surprising for such a complex molecule as cardiolipin to have evolved as one of the major membrane lipids in prokaryotics, when steroids such as cholesterol and phytosterols did not. In eukaryotic cells, cardiolipin is exclusively localized within the mitochondria where it is particularly emiched in the outer leaflet of the inner membrane. Even though a molecular structure of cardiolipin has been conserved in entire organisms, its biological significance has escaped attention except in the case of anti-cardiolipin auto-antibodies which are clinically associated with the Wasserman reaction. 

In this chapter, we describe a molecular interaction between cytochrome c and cardiolipin which prevents peroxidation of the lipid, implying that cardiolipin may play an important role in determination of the fate of the cell. In other words, a peroxidation of cardiolipin may well allow cytochrome c to discharge from the mitochondrial inner membrane into the cytosoUc space during apoptotic ceU death. 

Mitochondrial cytochrome c is a water-soluble basic protein, residing in the mitochondrial inner membrane where complexes HI and IV of the respiratory chain can reversibly interact with the protein. When the basic protein shuttles between complexes III and IV, cardiolipin facilitates the binding of cytochrome c to cytochrome c oxidase-containing membranes (Salamon and Tollin, 1996). In other words, cytochrome c oxidase in complex IV efficiently receives electrons from cytochrome c only in the presence of cardiolipin. It is well documented that cytochrome c oxidase binds one cardiolipin molecule as an activator and 2 others for anchoring itself to the inner membrane (Awasthi et al, 1971 Suter et al, 2000). 

The small basic protein, cytochrome c, has often been reported to bind strongly to the acidic phospholipid, cardiolipin, rich in outer leaflet of the mitochondrial inner membrane (Gallet et al, 1997). The binding of cytochrome c to cardiolipin is very tight, apparently irreversible, and stoichiometric (Rytomaa and Kinnunen, 1995), suggesting that this unique phospholipid, cardiolipin, may anchor the small heme protein to the inner... 

Hence, in the present chapter, we addressed the important questionas to how cytochrome c is released from mitochondrial inner membrane by apoptotic signals independently of A Fm dissipation. We have proposed that peroxidation of cardiolipin may result in a discharge of cytochrome c due to a weakening of the interaction between the lipid and the protein. 

In this chapter, we have postulated that cardiolipin may participate in determination of cell fate by allowing cytochrome c release from mitochondrial inner membrane through its peroxidative damage during apoptotic ceU death. And anti-oxidant mitochondrial enzyme, PHGPx may protect cardiolipin from its peroxidation as well as apoptotic cell death. 

Changes in mitochondrial stmcture are very relevant during X-ray induced apoptosis. A few hours after irradiation, a hyperpolarisation of A /m is noticed. This likely represents the attempt to restore the depleted ATP levels, stimulating the oxidative burst of surviving mitochondria. If this secondary oxidative stress overcomes the threshold given by mitochondrial thiols, mitochondrial cardiolipin is oxidized and mitochondrial inner membrane allows the leakage of A /m with the consequent initiation of the execution phase. 

The drug interferes with the mitochondrial electron transport chain. It seems that this is due to a high affinity of the drug for lipids such as cardiolipin, a component of the mitochondrial inner membrane. It therefore accumulates there. 

Five kinds of phospholipid predominate phosphatidylcholine, phosphatidylethanolamine, phosphatidyl-serine, phosphatidylglycerols, and sphingomyelin. Usually there are also small amounts of phosphatidyli-nositol. The major phospholipid in animal cells is phosphatidylcholine, but in bacteria it is phosphatidylethanolamine. The phospholipids of E. coli consist of 80% phosphatidylethanolamine, 15% phosphati-dylglycerol, and 5% diphosphatidylglycerol (cardio-lipin). Significant amounts of cardiolipin are found only in bacteria and in the inner membrane of mitochondria. Sphingomyelin is almost absent from mitochondria, endoplasmic reticulum, or nuclear membranes. 

The outer membranes of mitochondria can be removed from the inner membranes by osmotic rupture.13 Analyses on separated membrane fractions show that the outer membrane is less dense (density — 1.1 g / cm3) than the inner (density 1.2 g / cm3). It is highly permeable to most substances of molecular mass 10 kDa or less because of the presence of pores of 2 nm diameter. These are formed by mitochondrial porins,14-17 which are similar to the outer membrane porins of gram-negative bacteria (Fig. 8-20). The ratio of phospholipid to protein ( 0.82 on a weight basis) is much higher than in the inner membrane. Extraction of the phospholipids by acetone destroys the membrane. Of the lipids present, there is a low content of cardiolipin, a high content of phosphatidylinositol and cholesterol, and no ubiquinone. 

The inner membrane is impermeable to many substances. Neutral molecules of <150 Da can penetrate die membranes, but the permeability for all other materials including small ions such as H+, K+, Na+, and Cl- is tightly controlled. The ratio of phospholipid to protein in the inner membrane is 0.27, and cardiolipin makes up 20% of the phospholipid present. Cholesterol is absent. Ubiquinone and other components of die respiratory chain are all found in die inner membrane. Proteins account for 75% of the mass of die membrane. 

The outer membrane of the mitochondrion contains a large number of pores, so that molecules with molecular weights less than 1,000 can pass from the cytoplasm into the intermembrane space without any specialized transport mechanisms. This means, for example, that NADH, ADP, and inorganic phosphate can reach the intermembrane space from the cytoplasm while NAD and ATP can reach the cytoplasm. The inner membrane is much less permeable, in part due to the presence of a specialized membrane lipid known as cardiolipin (meaning heart lipid—cardiac cells have a large number of mitochondria). The inner membrane of the mitochondrion is highly folded into cristae, so that it has an interleaved appearance in the electron microscope. 

The method of estimating the amount of inner membrane in mitochondria has varied from investigation to investigation. In a few cases morphometric analysis of the amount of membrane per square micrometer of mitochondrial profile has been made in some, inner membrane is measured in terms of the phospholipid or cardiolipin content of the mitochondrion, the latter being particularly suitable since cardiolipin is fairly well localized in the inner membrane of mitochondria in others, mitochondria have been disrupted by osmotic shock, detergents, and sonication, and the protein content of the insoluble fraction has then been... 

Mitochondria have been prepared from plant tissues by a number of different techniques. Unfortunately, the variations in technique may also be reflected in the somewhat different compositions reported. Mitochondria, in general, have membranes containing a high concentration of phospholipids. Phosphatidylcholine and phosphatidylethanolamine are major components and are fairly evenly apportioned between the inner and outer membranes. Cardiolipin is found in high amounts in the inner membrane, while phospha-tidylinositol is mainly in the outer membrane. Phosphatidylglycerol is a minor component. These results are illustrated in Table XV. 

Bisphosphatidylglycerol (BPG, cardiolipin) is concentrated in the mitochondrial inner membrane in plants (Table 1), as in animals. The pathway of biosynthesis is... 

Mitochondria have two membranes (Figure 1S.9). The outer membrane is freely permeable to molecules as large as cytochrome c (molecular weight 13 000) the function of this membrane is not yet clear. The inner membrane contains folds or cristae and has a number of distinctive properties. The protein/lipid ratio of this membrane is high. It contains the phospholipid cardiolipin which is found nowhere else in mammalian organisms. The unique composition and structure of this membrane give it a very low permeability even to small charged molecules. 

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