Sarcoplasmic membrane lipids

The calcium-independent ATPase of the lipid modified preparations is not only different from the calcium-dependent ATPase but also from the calcium-independent ATPase of native preparations — the basic ATPase — which has a lower nucleotide specificity126. The experiments in which the lipid matrix of the sarcoplasmic membranes has been replaced by lipid compounds not present in native membranes reveal a high degree of functional flexibility of the enzyme. On the other hand, a few residual lipids in the protein are sufficient to prevent these changes in the structure of the enzyme and to preserve its calcium sensitivity. 

Li, Y., Ge, M., Ciani, L., Kuriakose, G., Westover, E.J., Dura, M., Covey, D.F., Freed, J.H., Maxfield, F.R., Lytton, J., and Tabas, I., 2004, Enrichment of endoplasmic reticulum with cholesterol inhibits sarcoplasmic-endoplasmic reticulum calcium ATPase-2b activity in parallel with increased order of membrane lipids implications for depletion of endoplasmic reticulum calcium stores and apoptosis in cholesterol-loaded macrophages../. Biol. Chem. 279, 37030—37039 Lin, P., Yao, Y., Hofmeister, R., Tsien, R.Y., and Farquhar, M.G., 1999, Overexpression of CALNUC (nucleobindin) increases agonist and thapsigargin releasable Ca2+ storage in the Golgi. J. Cell Biol. 145, 279-289... 

Fig. 3.45 Effect of drug concentration on the partition coefficients of tamoxifen (open symbols) and 4-hydroxytamoxifen (closed symbols) in DMPC bilayers (A) and in liposomes of sarcoplasmic reticulum lipids and native membranes (B). Note that the linear correlation between Kp and drug concentration is...

The effect of hydrogenation on membrane protein function has been examined using the Ca2+ pump of sarcoplasmic reticulum of rabbit hind-leg muscle [36, 37]. Up to 35% of the unsaturated bonds of the membrane lipids could be saturated in the presence of Wilkinson s catalyst. ATPase activity was completely inhibited on adding catalyst but this could be prevented by preserving the catalyst in its hydride form. When the effect of hydrogenation of sarcoplasmic reticulum on calcium pump activity was assayed in buffers saturated with H2, it was found that removal of 25% of cis double bonds did not affect the activity of Ca2+-ATPase. 

Researchers have studied whether acyl-chain order could be responsible for the preferred sterol interaction with SMs. Acyl-chain order was deduced from diphenylhexatriene anisotropy and from the deuterium order parameter obtained by H-NMR on bilayers made from either 14 0/14 0(d27)-PC, or 14 0(d27)-SM7 Some researcher analyzed the ground and excited states of phospholamban (PLN), a membrane protein that regulates sarcoplasmic reticulum (SR) calcium ATPase (SERCA), in dilferent membrane mimetic environments. Gustavsson et al. have previously proposed that the conformational equilibrium of PLN are central to SERCA regulation. They have now shown that these equilibrium detected in micelles and bicelles are also present in native sarcoplasmic reticulum lipid membranes as probed by MAS solid-state NMR. ... 

Li Y, Ge M, Ciani L et al. (2004) Enrichment of endoplasmic reticulum with cholesterol inhibits sarcoplasmic-endoplasmic reticulum calcium AT-Pase-2b activity in parallel with increased order of membrane lipids implications for depletion of endoplasmic reticulum calcium stores and apoptosis in cholesterol-loaded macrophages. J Biol Chem 279, 37030-37039. 

Correlations between critical temperatures for membrane lipid phases determined with EPR techniques and discontinuities in Arrhenius plots have also been shown for the ATPase from sarcoplasmic reticulum (Eletr and Inesi, 1972), for UDP-glucuronyltransferase, and for G-6-Pase (Eletr et al., 1973) from liver microsomes. In the case of the microsomal membranes, perturbation of the membrane lipids by treatment with detergents or phospholipase A leads to linear Arrhenius plots for both enzyme activities and Tq between 5° and 30 C. For UDP-glucuronyltransferase, the phase change in the lipids also results in a loss of substrate specificity and a loss of sensitivity to an allosteric effector (Vessey and Zakim, 1974). 

Purified membrane proteins or enzymes can be incorporated into these vesicles in order to assess what factors (eg, specific lipids or ancillary proteins) the proteins require to reconstitute their function. Investigations of purified proteins, eg, the Ca " ATPase of the sarcoplasmic reticulum, have in certain cases suggested that only a single protein and a single lipid are required to reconstitute an ion pump. 

Blasie and his colleagues have determined the separate profile structures of the lipid bilayer and of the Ca transport ATPase molecule within the sarcoplasmic reticulum membrane to 11 A resolution by a combination of X-ray and neutron diffraction techniques [128,140,187-199]. 

Table 5.4. Fluorescence Parameters of DPH, DPH-PC, and TMA-DPH in Intact Sarcoplasmic Reticulum Membranes and Liposomes Made from Total Lipid Extracts of the Membranes at 37°Cab...

C. D. Stubbs, K. Kinosita, Jr., F. Munkonge, P. J. Quinn, and A. Ikegami, The dynamics of lipid motion in sarcoplasmic reticulum membranes determined by steady-state and time-resolved fluorescence measurements on l,6-diphenyl-l,3,5-hexatriene and related molecules, Biochim. Biophys. Acta 775, 374-380 (1984). 

E. London and G. W. Feigenson, Fluorescence quenching in model membranes. 2. Determination of the local lipid environment of the calcium adenosinetriphosphatase from sarcoplasmic reticulum, Biochemistry 20, 1939-1948 (1981). 

Arrangement of Lipids and Proteins in Sarcoplasmic Reticulum Membranes... 

The low lipid-protein ratio of 0.5 together with the size of the sarcoplasmic vesicles implies that only approximately 30% of their membranes can be occupied by a regular lipid bilayer structure. Consequently, a large fraction of the membrane protein must interrupt the lipid bilayer and reach throughlt. The fact that only one polypeptide chain constitutes the structural unit of the calcium transport protein strong-... 

Fig. 11. Reactivation of calcium dependent ATPase activity of lipid deprived sarcoplasmic reticulum membranes by single chain lipid...

The fluorescence probe l-anilinonaphthalene-8-sulfonate was used to examine the binding of spin-labeled local anesthetics to membranes of human blood cells and rabbit sarcoplasmic reticulum. Two distinct fluorescence lifetimes are exhibited by this probe the shorter life time represents the probe associated with pure lipid, the... 

A similar difference has been observed for Kp M values of the anticancer drags tamoxifen and 4-hydroxytamoxifen determined in mitochondria, sarcoplasmic reticulum, and in liposomes made from lipids extracted from the corresponding native membranes [85]. The largest Kp M was observed in mitochondria, followed by sarcoplasmic reticulum and liposomes. The authors introduced for these experiments derivative spectroscopy, a reliable and rapid procedure to estimate drag partitioning into biomembranes. The method used the shift in the absorption spectra of the drug when removed from the aqueous phase to a hydrophobic environment (see Section 3.10). 

Radicals generated during peroxidation of lipids and proteins show reactivity similar to that of the hydroxyl radical however, their oxidative potentials are lower. It is assumed that the reactive alkoxyl radicals rather than the peroxyl radicals play a part in protein fragmentation secondary to lipid peroxidation process, or protein exposure to organic hydroperoxides (DIO). Reaction of lipid radicals produces protein-lipid covalent bonds and dityrosyl cross-links. Such cross-links were, for example, found in dimerization of Ca2+-ATPase from skeletal muscle sarcoplasmic reticulum. The reaction was carried out in vitro by treatment of sarcoplasmic reticulum membranes with an azo-initiator, 2,2/-azobis(2-amidinopropane) dihydrochloride (AAPH), which generated peroxyl and alkoxyl radicals (V9). 

A biologic reason for the abundance of nonlamellar lipids in membranes is that they possess the ability to modulate the activities of membrane proteins (15, 16). It has been recognized that membranes exist in a state of curvature frustration, which may be sufficiently large to have significant effect on certain protein conformations (17). Many examples show that the lipid bilayer elastic curvature stress indeed couples to conformational changes of membrane proteins (15, 18, 19). Protein kinase C is one such example of an enzyme activated by lipids that exhibit a propensity for nonlamellar phase formation (20). The activity of Ca " -ATPase from sarcoplasmic reticulum membranes also strongly correlates with the occurrence of nonbilayer lipids in the membrane and increases with the increase of their amount. It is noteworthy that the protein activity does not depend on the chemical structure of the lipids but only on their phase propensity thus specific binding interactions are ruled out. The list of proteins with activities that depend on the phase properties... 

The sarcolemmal membrane has invaginations which run parallel to the z bands. These invaginations are also known as the T system and are involved in the release of calcium into the cell. The release of calcium leads to a contraction of the myofibrils. Sarcoplasm accounts for 40% of the volume of the fiber and contains glycogen, mitochondria, and lipid vacuoles. 

Other experiments carried out with sarcoplasmic reticulum membranes gave results that do not support the idea that cholesterol is excluded by a lipid annulus. This work showed that as the cholesterol content of the biomembrane was raised, the enzyme activity decreased [77,78]. Spin-labelled cholesterol has also been used to study this problem with the conclusion that cholesterol does contact the surface of the protein, but with a lower probabihty than does the lipid [79]. 

Biochemical alterations have been found in fragmented sarcoplasmic reticulum isolated from dystrophic human, mouse and chicken muscle. Alterations in calcium transport, ATP hydrolysis and phosphoenzyme formation have been reported. Some of these biochemical alterations in the dystrophic sarcoplasmic reticulum are suggested to be due to alterations of the lipid environment of these membranes it has been suggested that the cholesterol content of dystrophic sarcoplasmic reticulum is elevated [182-187]. 

One example, which demonstrates the feasibility of this approach, is a time-resolved study on sarcoplasmic reticulum membranes during active transport this will be reviewed in some detail in the following paragraphs. Other important applications are the studies on the kinetics of lipid phase transitions, where only optical turbidity methods and certain spectroscopic relaxation techniques are able to provide useful information on the rates, however not on the actual structural processes and possible intermediates. 

What studies have been performed One of the more interesting of the few studies that exist was the examination of the activity of the Ca2+ adenosinetriphosphatase (ATPase) of sarcoplasmic reticulum membranes that were reconstituted into bilayer vesicles of different, defined lipid compositions (38). This survey found that the activity, which may be defined as the number of Ca2+ pumped across the bilayer per adenosine 5 -triphosphate (ATP) hydrolyzed, correlated strongly with the Hu tendencies of the imbedding lipid bilayer. For example, activities were high in compositions rich in either DOPE or MGDG, both of which readily form low-temperature Hu phases. By contrast, activities were low in DOPC or DGDG, as well as in other lipids that do not readily form Hu phases. Decreasing the amount of... 

Two examples of membrane proteins physically associated to lipids will be discussed bovine erythrocyte acetylcholinesterase and skeletal muscle sarcoplasmic reticulum Ca -Mg " ATPase. 

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