Sarcoplasmic membrane lipid functions

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 The experiments in which the lipid matrix of the sarcoplasmic membranes has been replaced by lipid compounds not present in native membranes reveal a high degree of functional flexibility of the enzyme.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. 

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. 

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. 

THE LIPID COMPOSITION AND Ca TRANSPORT FUNCTION OF SARCOPLASMIC RETICULUM(SR) MEMBRANES DURING DEVELOPMENT IN VIVO AND IN VITRO... 

Lipids play an essential role in the structure and function of biological membranes. The concept that the lipid enviroment influences the activity of membrane bound enzymes is now generally accepted. The sarcoplasmic reticulum(SR) of skeletal muscle is an intracellular membranous system with an important physiological role in contraction and relaxation (Hasselbach, 1964 Martonosi, 1971). The Ca2+ transport and (Ca2++ Mg2+)-ATPase activities which exhibit isolated SR preparations have been shown to be dependent on membrane phospholipids (Martonosi, 1968). An involvement of the fatty acid component of phospholipids in the functioning of the Ca2+ pump has also been suggested (Seiler et al., 1970 Seiler Hasselbach,... 

During development of chicken skeletal muscle cells, the marked increase in Ca transport of sarcoplasmic reticulum, observed both in vivo and in vitro systems, can be interpreted mainly as the result of an increase in the concentration of Ca transport ATPase. Changes in the fatty acid composition of muscle membranes developed in vivo occur with a balance between chain length and unsaturation, without affecting significantly their Ca permeability. In cultured muscle cells their fatty acid composition can be manipulated to a great extent by lipid supplementation of the culture medium. The effects of these in vitro modifications in lipid composition on the calcium transport function of muscle membranes should be investigated. 

There is abundant evidence for the view that calcium translocation, as well as ATP hydrolysis, is mediated by a Ca " -ATPase situated in the sarcoplasmic reticulum membranes (MacLennan and Holland, 1975). Lipid dependency of both of these functions has also been known for some time. Treatment of membrane preparations or of the Ca -ATPase with phospholipases results in parallel loss of both functions, whereas the restoration of both follows phospholipid addition. In early studies, lecithin alone has been effective in restoring ATPase activity of the delipidated enzyme while, for calcium translocation, phosphatidyl-ethanolamine was required as well (MacLennan and Holland, 1976). 

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