Lipids signal separation

Many cells have an asymmetric structure because of the necessity for function (Drubin and Nelson, 1996). For example, (the outer surface of) the plasma membrane of epithelial cells is fenced by a tight junction so that the lipids are separated between the apical part and the basolateral part (Fig. 9) (Eaton and Simons, 1995). Therefore, some molecular mechanisms must exist to sort the plasma membrane proteins into these two parts. Some signals related to the secretory/endocytic pathways have been found important (Matter and Mellman, 1994). Their details are not described here because the area is too specific for predictive purposes. 

Fig. 15. Comparison of a water suppressed muscle spectrum and a spectrum from yellow bone marrow containing almost pure fat (triglycerides). Measurement parameters STEAM sequence, TE=10 ms, TM=15 ms, TR = 2 s, 40 acq., VOI (11 X 11 X 20) mm. (a) Spectrum from TA muscle recorded after careful positioning of the VOI, avoiding inclusion of macroscopic fatty septa allows separation of extramyocellular (EMCL, broken lines) and intramyocellular lipid signals (IMCL, dotted lines) based on susceptibility differences. For this reason characteristic signals from fatty acids occur double. Signals of creatine (methyl, Crs, and methylene, Cr2) show triplet and doublet structure, respectively, due to dipolar coupling effects. Further signals of TMA (including carnitine and choline compartments), Taurine (Tau), esters, unsaturated fatty acids (-HC=CH-), and residual water are indicated, (b) Spectrum from yellow fatty bone marrow of the tibia with identical measuring parameters, but different amplitude scale.

Cortical astrocytes, cerebellar neurons and 0-2A progenitor cells have been investigated with MASS H NMR. All cells contained creatine and significant amounts of lipid resonances even when a T2-edited CPMG pulse sequence was used to selectively attenuated resonances from macromolecules. Principle component analysis readily differentiated the spectra, however the lipid content of the cells contributed most to the separation of cell types. The nature of the NMR-visible mobile lipid signal that is observed under certain conditions in... 

The result of a typical diffusion measurement is shown in Figure 2. In the 1H-NMR spectrum of a cubic phase of monoolein and 2H20 with 10% Desmopressin, the signals from the aromatic residues (Tyr and Phe) in Desmopressin, appear in a spectral region which does not contain any signals from the lipid. Therefore, the peptide and lipid diffusion coefficients could be determined separately (Table II), and in Figure 2 the spectra from such an experiment are shown. The lipid diffusion coefficient was also determined in a cubic phase in the absence of Desmopressin. 

Animal cells are separated from each other by lipid membranes. During signal transduction this barrier has to be passed, which can be realized by permanently or temporarily opened channels or by an indirect mechanism without material flux between the extra- and intracellular lumen (Fig. 1). 

Fig. 17. Principle of separation of IMCL and EMCL signal contributions by deconvolution and analysis of the field distribution, (a) Right cut-off spectrum out of the SOL muscle. Left MFD of the lipids calculated by deconvolution with aid of a reference lipid spectrum out of yellow tibial bone marrow, (b) Left fitted IMCL part of the MFD. Right corresponding convolution with the characteristic lipid pattern A. (c) Left resulting EMCL part of the MFD. Right corresponding convolution with the characteristic lipid pattern A. (d) Left residual of fitting the MFD.

Spectra recorded from smaller VOI (down to 0.25 ml) with careful positioning in regions without fatty septa provide a clearly decreasing EMCL signal contamination, resulting in improved visibility of IMCL in SOL at 3.0 T (see Fig. 36). Separate depiction of IMCL improves quantification of this lipid compartment, as inaccuracies due to the inhomogenous lineshape and resulting contaminations of EMCL can be avoided. 

Not all NMR-active nuclei may therefore be suitable for the study of phase separation in phospholipids. Limited information can be gained on phospholipid phase transformation from 1H- and UC-NMR because of problems in resolution. Only some signals, for example the choline methyl groups of phospholipids in the outer and inner leaflets of unilamellar bilayers, can be identified by 1H- and 13C-NMR when chemical shift reagents are used. However, 13C-NMR can be applied to the study of phospholipid phase transition when the lipid is specifically enriched at the sn-2, carbonyl position [91]. 

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