Dissipation of the electrochemical proton gradient in phospholipase-induced degradation of plant mitochondria and microsomes

Phospholipase A 2 (PLA2), added to isolated pea (Pisum sativum L., cv. Alaska) microsomal membranes to simulate a condition of induction of senescence, prevented the formation of the ATP- or PPi-dependent proton gradient. In addition, the NADH- dependent electrical potential of mitochondria was decreased by PLA 2 and unaffected by phospholipase D (PLD). The effect of PLA 2 can be correlated to a physical damage of membranes due to phospholipid breakdown and also to liberated fatty acids which can act as protonophores, dissipating the electrochemical proton gradient. The addition of PLA 2 to both mitochondria and microsomes, also induced a lipoxygenase-like activity triggering a lipoperoxidative degradation of membrane phospholipids. Mitochondria were less sensitive than microsomes to this degradation. Bovine serum albumin (BSA), known to protect mitochondria from free fatty acid-induced uncoupling, preserved microsomal ATP- or PPi-dependent proton gradients from the PLA2-induced dissipation more efficiently than the antioxidant BHT. This suggests that the protonophore effect of free fatty acids may constitute an important event that precedes the activation of lipoxygenase in plant senescence.