Sarcoplasmic reticulum cholesterol

The effect of organophosphorous insecticides such as methylbromfenvinfos on membrane fluidity has been studied using the fluorescence anisotropy of 1,6-diphenyl-l,3,5-hexatriene (DPH), a probe known to be located in the hydrophobic core of the bilayer, and 1,3-bis (1-pyrene) propane (Py(3)Py), a probe distributed in the outer layer region [54]. DPH revealed a broadening of the transition profile and a solidifying effect in the fluid phase of DMPC and DPPC in the presence of 50 pM insecticide. An ordering effect of the insecticide in the fluid state was revealed by Py(3)Py. In addition, the pretransition in DPPC and DMPC vesicles was abolished by the insecticide. The addition of cholesterol decreased the influence of the insecticide. It was also observed that the influence on native membranes (erythrocytes, lymphocytes, brain microsomes, and sarcoplasmic reticulum) depended on the cholesterol content of the membranes. 

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]. 

Cholesterol, in various proportions, is a natural constituent of mammalian plasma membranes but, if the proportion is allowed to increase, membrane function is usually diminished. Thus the membrane that surrounds the sarcoplasmic reticulum vesicles in muscle progressively loses the calcium-transporting function of its ATPase when cholesterol starts replacing the phospholipids. Similarly, ox-heart mitochondria, when exposed to cholesterol, progressively lose the activity of ATPase, succinate dehydrogenase, and /3-hydroxybutyrate dehydrogenase (Warren et aL, 1975). 

Fig. 15. Proton-decoupled P-NMR spectra (at 60.7 MHz) of pure DMPC and of protein-or cholesterol-containing complexes at 32 2 C in excess water. (A) Pure DMPC. (B) DMPC sample containing 80 wt % cytochrome c oxidase. (Q DMPC sample containing — 70 wt % sarcoplasmic reticulum ATPase. (D) DMPC sample containing 70 wt % human lipophilin (N2 protein). (E) DMPC system containing 25 wt % cholesterol. Spectral conditions were typically a 50-kHz spectral width, 1-s recycle time, 4-/is 90° pulse, SO data-acquisition delay time, 8192 data points, and 50-Hz line broadening. The number of scans varied between 4000 and 16,000. Sample volume was 250 /tl. Gated proton decoupling with pulses of50-100 ms, 40 W, were used. From Rajan et al. (1981).

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... 

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. 

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