Ypgom5tnteribratoaeattereoaoatonche 。·一 字 eneganeacemgngeatathionean 65rgeasaa2urbeywaem AUE bfecoehodR3soh6nedeemeReoeyMwwater ehRhkpienpoednteertonotoatdani,such very high doses of acet hen exhibits such toxidty CH Bothndnctionken 11 11 Both propanedial and the α,β-unsaturated aldehydes are extremely toxic because they are capable of cross-linking nucleophilic amino and mercapto groups from two different parts of one protein molecule, or from two different protein molecules. Naturally occurring antioxidant systems defend lipid molecules inside cell membranes from oxidative destruction. The most important of these is vitamin E, a reducing agent possessing a long hydrocarbon chain, which makes it lipid￾soluble. Vitamin E functions by its ability to break the propagation chain of lipid oxidation by the reduction of radical species. The vitamin-E radical formed is relatively unreactive because of extensive delocalization of the electron and the steric hindrance of the methyl substituents. Vitamin E is regenerated at the membrane surface by water￾soluble reducing agents such as vitamin c. The semidehydroascorbic acid decomposes to lower MW water￾soluble compounds, which are excreted by the body. Benzoquinones consume glutathione, an intracellular reducing agent. Glutathione is a small peptide that incorporates a mercapto functional group. The mercapto group serves to reduce disulfide linkages in proteins to –SH groups and to maintain the iron in hemoglobin in the +2 state. Glutathione also is involved in the reduction of oxidants, such as H2O2, which may be present in the cell: Extensive glutathione depletion may result in cell death. Benzoquinones and related compounds react irreversibly with glutathione in the liver by conjugate addition. Very high doses of acetaminophen exhibits such toxicity: Synthetic analogs of vitamin E are preservatives. Several food additives function as antioxidants: BHA and BHT. Addition of BHA to butter increases its storage life from months to years. Both BHA and BHT function like vitamin E, reducing oxygen radicals and interrupting the propagation of oxidative processes