Temporal relationships between the loss of vitamin E, protein sulfhydryls and lipid peroxidation in microsomes challenged with different prooxidants.

Microsomal membranes exposed to various prooxidants displayed distinct patterns in the temporal relationships between the loss of antioxidants and the onset of lipid peroxidation. Prooxidants included ADP-chelated iron with reductants (NADPH and ascorbate), or compounds that generate peroxyl radicals (tert-butyl hydroperoxide and 2,2'-azobis(2-amidinopropane)), or free metal ions (Fe2+ and Cu2+). Lipid peroxidation initiated by Fe2+ began simultaneously with the loss of alpha-tocopherol, while about 15% of alpha-tocopherol was lost during the lag phase preceding lipid peroxidation in systems containing chelated iron. A substantial depletion of alpha-tocopherol (49-69%) preceded peroxidation when initiated by peroxyl radicals, while 97% of the antioxidant disappeared preceding peroxidation initiated by copper. The loss of protein sulfhydryl groups also varied between the prooxidants (ranging from 0 to 82% loss prior to the onset of peroxidation) to a degree that roughly paralleled the loss of vitamin E. Therefore, important determinants of peroxidation are prooxidant-dependent, and not only include alpha-tocopherol and sulfhydryl groups, but also membrane integrity, propagation reactions and the availability of catalytic transition metals.