MODULATION OF BILAYER FLUIDITY AFFECTS LIPID CATABOLISM IN MICROSOMAL-MEMBRANES OF TOMATO FRUIT

As tomato fruit ripen, microsomal lipid phosphate levels expressed on a fresh weight of tissue basis decline by approximately 80%. Analysis of the water soluble products formed during the metabolism of choline-labelled phosphatidylcholine by microsomal membranes from mature green fruit indicates that this loss of phospholipids is attributable to the sequential actions of microsomal phospholipase D and phosphatidic acid phosphatase. Direct measurements of microsomal phospholipase D indicated that the activity of the enzyme is stimulated by approximately 10-fold in the presence of 5 mM Ca2+ and by approximately 2.5-fold in the presence of 5 mM spermine. These same concentrations of Ca2+ and spermine also strongly promoted the release of phospholipids from microsomal membranes of mature green tomato fruit during incubation in buffer. H-1-nuclear magnetic resonance measurements showed that 5 mM Ca2+ and 5 mM spermine both cause a decrease in bilayer fluidity of the microsomes indicating that the increase in phospholipase D activity may be attributable to a biophysical effect of the cations on the membranes. These observations further suggest that the decrease in plasma and microsomal membrane fluidity known to accompany ripening of tomato fruit may result in promotion of phospholipase D activity and thus facilitate phospholipid metabolism.