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Related Experiment Videos

Modulation of sarcoplasmic reticulum Ca2+-pump activity by membrane fluidity.

L M Almeida, W L Vaz, K A Zachariasse

    Biochemistry
    |September 25, 1984
    PubMed
    Summary

    Fluidity of sarcoplasmic reticulum membranes was assessed using a pyrene probe. Cholesterol affects membrane fluidity and Ca2+-ATPase activity, with significant impacts observed at high cholesterol concentrations.

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    Area of Science:

    • Biochemistry
    • Membrane Biophysics
    • Lipid Bilayer Dynamics

    Background:

    • Sarcoplasmic reticulum (SR) membranes are crucial for muscle contraction.
    • Membrane fluidity influences protein function, including Ca2+-ATPase activity.
    • Cholesterol is a key regulator of membrane properties.

    Purpose of the Study:

    • To investigate the fluidity of SR membranes using a pyrene-based probe.
    • To determine the effects of cholesterol on SR membrane fluidity and lipid phase transitions.
    • To assess the impact of cholesterol on Ca2+-ATPase activity within SR membranes.

    Main Methods:

    • Utilized intramolecular excimerization of 1,3-di-1-pyrenylpropane [Py(3)Py] as a fluidity probe.
    • Analyzed spectral data to determine probe incorporation and membrane fluidity.

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  • Investigated lipid phase transitions in model bilayers (DMPC, DPPC) and SR lipid extracts.
  • Incorporated cholesterol into liposomes and native SR membranes via exchange techniques.
  • Measured Ca2+-ATPase activity at varying cholesterol/phospholipid molar ratios.

    • Main Results:

    • The Py(3)Py probe localizes near polar head groups, not the hydrophobic core.
    • Excimerization rate is sensitive to lipid phase transitions; cholesterol broadens and abolishes transitions.
    • SR lipid extracts show fluidity discontinuities around 20°C, affected by cholesterol.
    • Cholesterol enrichment in native SR membranes broadens these discontinuities.
    • Ca2+-ATPase activity is unaffected until a cholesterol/phospholipid ratio of 0.4; significant decrease (~20%) at 0.6.

    Conclusions:

    • Cholesterol significantly modulates SR membrane fluidity and lipid phase behavior.
    • SR membrane fluidity exhibits distinct characteristics compared to simple phospholipid bilayers.
    • Ca2+-ATPase activity is relatively resistant to cholesterol-induced changes at physiological levels but is impaired at high concentrations.
    • The vesicle structure remains intact even at high cholesterol enrichment levels.