Impact of Oxidized Phospholipids on Membrane Organization

Abstract

Lipids have often been seen as basic structural membrane subunits with proteins doing the actual work. This view has changed in recent years where it has been shown that lipids are also directly involved in numerous physiological processes and are often required for specific membrane protein functions. However, how a membrane and its function becomes modified under intracellular oxidative conditions, which e.g. trigger mitochondria-mediated apoptotic cell death, is still not really known. Oxidative stress can generate oxidized phospholipids (OxPls), which have a great impact on mitochondrial membrane integrity. Therefore, we studied the impact of OxPls on DMPC based bilayers membranes by differential scanning calorimetry (DSC) and solid state nuclear magnetic resonance (NMR) spectroscopy. Incorporation of OxPls with functional groups (carboxyl or aldehyde) at their truncated sn-2-chain ends generated information about the effect which OxPl species exert on the basic structural and physico-chemical properties of DMPC bilayers. DSC experiments revealed significant changes in the thermotropic phase behavior of these vesicles in the presence of OxPls as a function of their concentration. In addition, solid state 31P NMR provided molecular information of the behavior of the DMPC headgroups when OxPls were present. In addition changes could also be monitored during temperature induced phase transitions, where OxPls induced a very complex phase behavior. Between 293 K (onset of Lα-phase) and 298 K two overlapping NMR subspectra occured which indicated the co-existence of two liquid-crystalline lamellar phases. Most likely one phase reflected an OxPls poor domain and the other an OxPl-rich domain. In summary, the presence of OxPls seems to alter the mitochondrial membrane organization, which has serious implication for the role of this membrane and its Bcl-2 proteins involved in mitochondrial apoptosis.