Antioxidant and prooxidant modulation of lipid peroxidation by integral membrane proteins.

Lipid peroxidation is a biochemically adverse phenomenon with key involvement in many different diseases including premature infant blindness, nonalcoholic steatohepatitis, or Parkinson's disease. Moreover, lipid peroxidation may be the most important universal driver of the biological aging process. Canonic lipid peroxidation is a free radical chain reaction consisting of three kinetically independent steps, initiation, propagation, and termination. During the bulk propagation phase, only lipids and oxygen are consumed as substrates and maintain the chain reaction. In native biological membranes, however, lipid peroxidation takes place in direct vicinity to high concentrations of inserted membrane proteins with their exposed hydrophobic amino acid side chains. In the following, we review the evidence that redox-active intramembrane amino acid residues have a profound impact on the course and extent of lipid peroxidation . Specifically, tyrosine and tryptophan are concluded to be chain-breaking antioxidants that effectuate termination, whereas cysteine is a chain-transfer catalyst that accelerates propagation and thereby promotes lipid peroxidation. Methionine, in turn, is highly accumulated in mitochondrial membrane proteins of animal species with high metabolic rates and imminent danger of lipid peroxidation, though its specific role has not been entirely defined. Potentially, it interferes with initiation on the membrane protein surface. Nevertheless, all four residues are distinguished by their clear relevance to lipid peroxidation as deduced from either experimental or genetic and comparative data. The latter have uncovered distinct evolutionary pressures in favor or against each residue in lipid membranes and have shed light on formerly unacknowledged chemical mechanisms.