EFFECT OF CHILLING ON THE ACTIVITIES OF LIPID DEGRADING ENZYMES IN TOMATO FRUIT MICROSOMAL-MEMBRANES

The effects of chilling temperatures on the activities of four lipid-degrading enzymes, viz., phospholipase D (EC 3.1.4.4), phosphatidate phosphatase (3-sn-phosphatidate phosphohydrolase, EC 3.1.3.4), lipolytic acyl hydrolase and lipoxygenase (EC 1. 1 3.11.12) were studied using microsomal membranes isolated from the pericarp of mature green tomato fruit (Lycopersicon esculentum L. cv. Caruso). The activities of phospholipase D and lipolytic acyl hydrolase remained essentially unchanged with decreasing temperature from 22-degrees-C to 0-degrees-C, as revealed by the liberation of [H-3]choline from L-alpha-dipalmitoyl(choline-methyl-H-3) phosphatidylcholine and of [C-14]arachidonic acid from stearyl-2-(1-C-14)-arachidonyl-sn-glycerol, respectively. In contrast, phosphatidate phosphatase and lipoxygenase where both less active at 0-degrees-C than at 30-degrees-C. There was also a reduction with decreasing temperature in the bulk lipid fluidity of the microsomal membranes, detectable by fluorescence depolarization after labelling with 1,6-diphenyl-1,3,5-hexatriene, that may contribute to this reduction in enzyme activity. The results suggest that an accumulation of phospholipid catabolites in membranes during low temperature storage of tomato fruit, arising from the differential effects of reduced temperature on the activities of lipid-degrading enzymes, may contribute to chilling injury.