Distribution of lipids in subcellular particles of guinea-pig brain.

It is becoming increasingly apparent that lipids act as essential structural elements in the multienzyme systems associated with the cell organelles. Many of the enzymes of the electron-transport chain in mitochondria can be isolated as discrete lipoproteins (Green, 1959), and the phospholipid contained in these is essential for enzymic activity (Reich & Wainio, 1961). Moreover, lipids play a fundamental role in the microstructure of cell meimbranes and may even participate metabolically in the transport of cations (Hokin & Hokin, 1964). Clearly, therefore, the distribution of individual lipids within the cell becomes of fundamental importance. The technique of differential centrifugation has enabled various subcellular fractions to be prepared and numerous lipid analyses of these fractions have been carried out. With liver tissue, consisting predominantly of hepatic cells, the relative cellular homogeneity allows these analyses to apply to defined morphological entities of the cell such as the nuclei or mitochondria (Spiro & McKibbin, 1956; Macfarlane, Gray & Wheeldon, 1960; Getz & Bartley, 1961). However, in tissues like the brain with its complicated cytological differentiation, the application of the same centrifugation technique produces fractions that are grossly heterogeneous. Lipid analyses of such fractions (Peterson & Schou, 1955; Biran & Bartley, 1961) therefore, although ofgreat value, must be regarded as preliminary. In recent years, the use of density-gradient centrifugation (Hebb & Whittaker, 1958; Whittaker, 1959, 1961; Whittaker, Michaelson & Kirkland, 1963, 1964), coupled with morphological characterization in the electron microscope (Gray & Whittaker, 1960, 1962; Whittaker, 1960; Whittaker etal. 1963, 1964), has permitted the isolation of certain morphologically defined structures in relatively pure form. These include pinched-off nerve endings or 'synaptosomes', and synaptic vesicles which are contained within the synaptosomes and can be released from them by suitable disruptive procedures. The present paper describes detailed analyses of the individual lipids present in such subfractions by using, among other techniques, a recently developed method for the determination of all known phospholipids (Dawson, Hemington & Davenport, 1962). It provides new data on the lipid composition of brain mitochondria, nuclei and microsomes, as well as the subcellular particles peculiar to brain: myelin fragments, synaptosomes and synaptic vesicles.