THE LAG PHASE

Peroxidation of lipids in membranes and lipoproteins proceeds through the classical free radical sequence encompassing initiation, propagati on, and termination phases which are expressed by a lag phase in which Little oxidation occurs, followed by a rapid increase in autocatalysi s by chain-propagating intermediates and, finally, a decrease in the r ate of oxidation. The lag phase is lengthened by preventive or chain-b reaking antioxidants, which scavenge the initiation reaction or interc ept the chain-carrying species. Hence, the lag phase in lipid peroxida tion processes reflects the antioxidant status of membranes and lipopr oteins and, as a corollary, their resistance to oxidation. A large num ber of lipid peroxidation studies with different membranes attest to t he complex free radical network underlying this process. The type of i nitiator and the steady-state level of oxygen are important factors th at affect differently the rates of the individual steps of peroxidatio n. Equally complex are the factors that influence the lag phase preced ing the oxidation of LDL. Lipid peroxyl radicals play a key role in th e dynamics of lipid peroxidation: on the one hand, the lag phase is be st defined for chain-breaking compounds able to reduce peroxyl radical s; on the other hand, the overall time course of lipid peroxidation is largely influenced by the rate constants for propagation reactions an d termination involving peroxyl radical recombination.