Characterization of the broad resonance in 31P NMR spectra of excised rat brain.

31P NMR spectra of excised rat brain showed a broad resonance between-12 and -13 ppm. Subcellular fractions of brain, rich in membranes, exhibited the broad resonance and it was also present in isolated myelin, the major membrane component of brain. However, it was absent in brain cytosol (161,100 X g supernatant). Raising the temperature of the brain above 50 degrees C caused a gradual downfield chemical shift of the broad resonance, to about -1 ppm at 90 degrees C. An even larger downfield shift was produced by halothane or deoxycholate with concomitant narrowing of the line width of this resonance. Vesicles prepared from the phospholipids of excised brain or isolated myelin showed the broad resonance, and halothane produced the same downfield shift and peak sharpening in brain phospholipid vesicles as that in the intact brain. The chemical shift anisotropy was estimated to be 45 ppm for both myelin and the brain, as characteristic for biological membranes. The T1 and T2 relaxation times of the perpendicular 31P chemical shift tensor component of the broad resonance were 0.66 sec and 1.6 msec, respectively, in the same range as those for other biological membranes. Halothane-treatment of the brain increased both the T1 and T2 times considerably, as expected from the disruption of the phospholipid bilayer in a membrane. These data indicate that the broad resonance in the 31P NMR spectrum of excised rat brain originates exclusively from the phosphate head group of membrane bound phospholipids. Similar broad resonances were found in autopsied human brain and porcine spinal cord and to a lesser extent in excised rat liver and kidney.