31P NMR studies of rod outer segment (ROS) and sarcoplasmic reticulum (SR) membranes have been performed under conditions where broad and narrow spectral components can be clearly resolved. Control studies of an anhydrous, solid powder of l,2-dipahnitoyl-sn-glycero-3-phosphocholine (DPPC), as well as aqueous binary mixtures of l,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), demonstrate clearly that broad spectral components can be detected. For the codispersions of DSPC and DOPC in the mixed-phase region at 22 °C, the 31P NMR spectra consist of a superposition of a broad component and a narrow, axially symmetric component, due to coexisting solid and liquid-crystalline domains, which are in slow exchange on the 31P NMR time scale. The 31P NMR spectra of the native ROS and SR membranes, however, consist of only a narrow component, to within experimental error, indicating that most or all of the phospholipids are in the liquid-crystalline (Lα)phase at 22 °C. The above conclusions are in agreement with many, but not all, previous studies [see, e.g., Yeagle, P. L. (1982) Biophys. J. 37, 227–239]. It is estimated that at most 10% of the phospholipids in the ROS and SR membranes could give rise to broad 31P NMR spectral components, similar to those seen for anhydrous or solid-phase lipids, corresponding to ~7 phospholipids/rhodopsin molecule and ~11 phospholipids/ Ca2+-ATPase molecule, respectively.
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