Thermal Behavior and Lipid Composition of Cauliflower Plasma Membranes in Relation to ATPase Activity and Chilling Sensitivity

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Abstract
A plasma-membrane fraction rich in ion-stimulated ATPase activity was isolated from cauliflower (Brassica oleracea L.) buds. The activity of the ATPase was dependent on Mg2+ and stimulated 4-fold by K+. The lipids of the membrane fraction contained 57% by weight of phospholipid, 16% glycolipid including sterol glycosides, and 27% neutral lipids. Sterols and sterol esters comprised 9% by weight of the total lipid fraction, and the m ratio of total sterol to phospholipid was 0.5. Fatty acid unsaturation of the membrane lipids was 75%. Arrhenius plots of the Mg2+ and Mg2+ + K+ stimulated ATPase activity were biphasic with an increase in activation energy occurring below about 12°C, a response typical of some membrane-associated enzymes of chilling-sensitive plants. No thermal transitions were detected in the membranes or membrane lipids between 0 and 30°C using differential scanning calorimetry and electron spin resonance spectroscopy. This type of thermal behavior is typical of membranes of chilling-resistant plants. It was concluded that the low temperature increase in activation energy of the ion-stimulated, membrane-associated ATPase is an intrinsic property of the enzyme system and not the result of a transition in the bulk membrane lipid.