Neutral Lipids Rigidify Unsaturated Acyl Chains in Senescing Membranes

Abstract

Senescence in bean cotyledons is accompanied by a progressive increase in the proportion of gel phase lipid in cellular membranes that can be attributed to qualitative changes in the neutral lipids. The resulting mixture of lipid phases leads to impaired membrane function. Insight into the molecular basis for this phenomenon has been gleaned from studies of the effects of these neutral lipids on the phase properties of pure phospholipid membranes. Induction of the gel phase, detectable as a rise in the liquid-crystalline to gel phase transition temperature, was observed when neutral lipid from senescent membranes was introduced into liposomes of unsaturated phospholipids. The transition temperature for phosphatidylcholine rose from −6.5 ° C in control liposomes to 51 ° C when 25% (w/w) neutral lipid was present. A similar rise was obtained for dioleoylphosphatidylcholine. However, for dipalmitoylphosphatidylcholine, which is fully saturated, the rise in transition temperature upon addition of neutral lipids was only 3 ° C. Thus the neutral lipids in senescent membranes appear to selectively rigidify unsaturated acyl chains. At least three types of compounds known to alter the phase properties of lipid bilayers were detectable in the neutral lipid fraction, suggesting that rigidification reflects the concerted action of several neutral lipid components on unsaturated phospholipid.