Cytosolic lipid-protein particles

Several observations support the contention that cytosolic lipid-protein particles originate from membranes. In particular, they contain phospholipid with the same fatty acids, albeit in different proportions, found in phospholipid of corresponding microsomal membranes [3]. In addition, lipid-protein particles with essentially similar properties can be generated in vitro from isolated microsomal membranes under conditions in which phospholipid catabolism has been activated by the addition of Ca [2,3]. The finding that the particles contain high concentrations of free fatly acids and steryl and wax esters suggests that they are formed at specific... 

Figure L Electron micrograph of cytosolic lipid-protein particles isolated from carnation petals and stained with uranyl acetate. Bar = 100 nm.

Figure 2. Thin layer chromatogram of total lipids extracted from microsomal membranes (lane 1) and cytosolic lipid-protein particles (lane 2) isolated from cotyledon tissue of Phaseolus vulgaris. PC, phosphatidylcholine PE, phosphatidylethanolamine PG, phosphatidylglycerol PI, phsophatylinositol FFA, free fatty acids TAG, triacylglycerol SE/WE, steryl and wax esters.

Figure 3. Wide angle X-ray diffraction pattern recorded at 25°C from a hydrated mixture of free fatty acids and steryl and wax esters isolated from cytosolic lipid-protein particles of Phaseolus vulgaris cotyledon tissue. Arrows denote X-ray reflections derived from gel phase packing.

Osmiophilic lipid-protein particles enriched in membrane lipid catabolites have been isolated from the cytosol of carnation petals and of cotyledons (Phaseolus vulgaris) by ultrafiltration or flotation centrifugation [2,3]. The particles are spherical in nature and range from 150 to 300 nm in diameter [Fig. 1]. Osmiophilic particles of similar size and shape are also detectable in the cell cytoplasm [3]. The particles contain phospholipid, but are enriched (10- to 100- fold) by comparison with membranes in free fatty acids, triacylglycerols, and steiyl and wax esters [Fig. 2]. 

Hudak, K.A., and Thompson, J.E. Flotation of lipid-protein particles containing triacylglycerol and phospholipid from the cytosol of carnation petals. Physiol Plant, (in press). 

In adipose tissue, TAG is stored in the cytosol of the cells in a nearly anhydrous form. It serves as "depot fat," ready for mobilization when the body requires it for fuel. Little TAG is stored in the liver. Instead, most is exported, packaged with cholesteryl esters, cholesterol, phospholipid, and protein (apolipoprotein B-100, see p. 229) to form lipoprotein particles called very low density lipoproteins (VLDL). Nascent VLDL are secreted into the blood where they mature and function to deliver the endogenously-derived lipids to the peripheral tissues. [Note Recall that chylomicrons deliver primarily dietary (exogenously-derived) lipids.] Plasma lipoproteins are discussed in Chapter 18, p. 225. 

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