Lipid peroxidation: a novel enzymatic mechanism of protective effect of α-tocopherol in biological membranes

Shortage of lipid-radical cycles in membranes as a possible prime cause of energetic failure in aging and Alzheimer disease

Polyunsaturated fatty acids (PUFA) and alpha-tocopherol (alpha-TOH) are the most oxygen-sensitive constituents of cells. alpha-TOH is a member of the vitamin E family that is considered the most important lipophilic antioxidant in cell membranes. Its importance is emphasized by the involvement of oxidative stress in injury to the central nervous system and neurodegenerative diseases. Currently, alpha-TOH transfer protein (TTP), is believed to play a significant role in maintaining the vitamin status but the presence of alpha-TOH in membranes is required but not sufficient to protect the membranes against lipid hydroperoxides (LOOH) formation. The lipid-radical theory presented in this review considers the role of two membrane factors--alpha-tocopherol and cytochrome b5; these factors secure the functioning of lipid-radical cycles and the participation of lipid-radical reactions in the key membrane processes. The prominent intermembrane reaction realized via a protein-lipid interaction, during which electron transport from cytochrome b5--located in the outer membrane--to peroxyl radical (LOO*)--located in inner membrane--causes reduction of the peroxyl radical: cyt.b5red + LOO* --> cyt.b5ox + LOO(-). This secures an interaction of alpha-TOH with other intermediate, LOO(- )excepting the LOOH formation. The discussion will be focused on the consequences of ineffective electron transfer to LOO* and excessive oxidative pathway of metabolism of the PUFA (LOO* --> LOOH). Assuming the operation of cytochrome b5/alpha-tocopherol-controlled lipid-radical cycles and considering the role of the cycles in membrane bioenergetics we arrive at a model for effective function of adenine nucleotide translocator and ATP synthesis in mitochondria. This paper summarizes our experimental evidence that the oxidative and non-oxidative pathways of metabolism of PUFA via their respective intermediates occur in the cells. While this fact is not widely appreciated it may be relevant to elucidation of new mechanisms of neurodegenerative diseases.

A new role of phosphoglucose isomerase. Involvement of the glycolytic enzyme in aldehyde metabolism

Lipid peroxidation in biological membranes is accompanied by malonic dialdehyde (MDA) formation, but the problem of its further metabolism in cytoplasm remains unsolved. The experimental data obtained in this work showed that the liver fraction prepared by centrifugation at 10,000g contained phosphoglucose isomerase and enzymes of the glyoxalase system. In this fraction in the presence of GSH there is an aggregate of reactions taking place both in membranes (lipid peroxidation) and outside membranes (MDA conversion to methylglyoxal and further to neutral D-lactate). This means that MDA is slowly accumulated because it is a substrate of aldehyde isomerase (MDA <--> methylglyoxal). Most probably, phosphoglucose isomerase serves as this enzyme. We concluded that D-lactate should be regarded as the end product of two different parametabolic reactions: lipid peroxidation or protein glycation.

Bacterial luminescence: luminescence mechanism with cyclic peroxide participation and dependence on reactive oxygen species (a hypothesis)

Chemically initiated exchange (CIEE) luminescence reactions were reviewed and a new mechanism of luminescence with peracid as an intermediate is proposed; bacterial luminescence is generally considered to be a case of dioxetane luminescence, or, to be more precise, CIEE-luminescence which includes the generation of a cyclic peroxide. In the hypothesis the monooxygenase reaction (aldehyde -->fatty acid) should not be coupled with emitter generation as is usually believed, but only with the generation of peracid. As to the generation of the emitter, excited flavin, it is likely to occur later, during the interaction of flavin with cyclic peroxide. Its consequence is the breaking of two chemical bonds (O-O and C-C) in the cyclic peroxide and simultaneous generation of 4alpha-hydroxyflavin in exited state. In general, the generation of light includes three stages: 1) the monooxygenase reaction and the concurrent production of peracid; 2) the conversion of peracid to cyclic peroxide; and 3) the interaction of cyclic peroxide with flavin (through the CIEE mechanism).