New aspects in the biological role of zinc: a stabilizer of macromolecules and biological membranes.

The direct oxidation of PUFA by triplet oxygen is spin forbidden. The data reviewed indicate that lipid peroxidation is initiated by nonenzymatic and enzymatic reactions. One of the first steps in the initiation of lipid peroxidation in animal tissues is by the generation of a superoxide radical (see Figure 16), or its protonated molecule, the perhydroxyl radical. The latter could directly initiate PUFA peroxidation. Hydrogen peroxide which is produced by superoxide dismutation or by direct enzymatic production (amine oxidase, glucose oxidase, etc.) has a very crucial role in the initiation of lipid peroxidation. Hydrogen peroxide reduction by reduced transition metal generates hydroxyl radicals which oxidize every biological molecule. Hydrogen peroxide also activates myoglobin, hemoglobin, and other heme proteins to a compound containing iron at a higher oxidation state, Fe(IV) or Fe(V), which initiates lipid peroxidation even on membranes. Complexed iron could also be activated by O2- or by H2O2 to ferryl iron compound, which is supposed to initiate PUFA peroxidation. The presence of hydrogen peroxide, especially hydroperoxides, activates enzymes such as cyclooxygenase and lipoxygenase. These enzymes produce hydroperoxides and other physiological active compounds known as eicosanoids. Lipid peroxidation could also be initiated by other free radicals. The control of superoxide and perhydroxyl radical is done by SOD (a) (see Figure 16). Hydrogen peroxide is controlled in tissues by glutathione-peroxidase, which also affects the level of hydroperoxides (b). Hydrogen peroxide is decomposed also by catalase (b). Caeruloplasmin in extracellular fluids prevents the formation of free reduced iron ions which could decompose hydrogen peroxide to hydroxyl radical (c). Hydroxyl radical attacks on target lipid molecules could be prevented by hydroxyl radical scavengers, such as mannitol, glucose, and formate (d). Reduced compounds and antioxidants (ascorbic acid, alpha-tocopherol, polyphenols, etc.) (e) prevent initiation of lipid peroxidation by activated heme proteins, ferryl ion, and cyclo- and lipoxygenase. In addition, cyclooxygenase is inhibited by aspirin and nonsteroid drugs, such as indomethacin (f). The classical soybean lipoxygenase inhibitors are antioxidants, such as nordihydroguaiaretic acid (NDGA) and others, and the substrate analog 5,8,11,14 eicosatetraynoic acid (ETYA), which also inhibit cyclooxygenase (g). In food, lipoxygenase is inhibited by blanching. Initiation of lipid peroxidation was derived also by free radicals, such as NO2. or CCl3OO. This process could be controlled by antioxidants (e).(ABSTRACT TRUNCATED AT 400 WORDS)

Initiation of lipid peroxidation in biological systems

Superoxide dismutase plus catalase enhances the efficacy of hypothermic cardioplegia to protect the globally ischemic, reperfused heart.

SUMMARY 1. Fresh ox-brain homogenate prepared under standard conditions at 4°C and stored at - 20°C autoxidizes spontaneously and reproducibly when re-heated to 37°C. 2. The preparation can be used for assaying the antioxidant activity of serum and of other biological fluids. The preparatory and assay procedures are described. 3. The antioxidant activity of normal serum, cerebrospinal fluid, synovial fluid and of various known antioxidants have been compared.

Assay using brain homogenate for measuring the antioxidant activity of biological fluids

Protective role of the glutathione redox cycle against adriamycin-mediated toxicity in isolated hepatocytes.

Alterations of microsomal membrane integrity were examined during lipid peroxidation. Loss of membrane-bound NADPH cytochromec reductase activity and protein release into the aqueous phase were related to disruption of the lipophilic region of the membrane. Formation of fluorescent products in the lipid phase of the membrane occurred only in the presence of peroxidation products. Changes in the membrane lipid phase during peroxidation included a decrease in the reactive amino groups of the phospholipids and a decrease in detectable phosphatidyl ethanolamine. In an ultrafiltration continuous flow chamber, peroxidized microsomal membranes were hydrolyzed to a lesser degree by solubilized lysosomal cathepsins and to a greater degree by lysosomal nucleases than were nonperoxidized membranes.

Damage to microsomal membrane by lipid peroxidation.

Sir: A TPNH lipid peroxidation system was found in rat liver microsomes and first described by Hochstein and Ernster (CIBA Cellular Injury 1964: 123). More recent publications from the laboratories of Wills (Biochem. J. 113:315, 325, 333 [1969]) and McCay (May and McCay, J. Biol. Chem. 243:2288, 2296 [1968]; Tam and McCay, Ibid. 245:2295 [1970]; Poyer and McCay, Ibid. 246:263 [1971]; Pfeifer and McCay, Ibid. 246:6401 [1971]; and McCay et al., Lipids 6:297 [ 1971 ]) have confirmed the presence of a TPNH oxidase (cytochrome c reductase) and have described extensively the requirements necessary for it to be involved in lipid peroxidation. These requirements include the presence of TPNH, iron (Fe +2 or Fe +3) and oxygen. In addition, this TPNH oxidase is involved in the initial steps of the cytochrome P450 hydroxylating system (Wills, Biochem. J. 1 I3:333 [ 1969]; Cohen and Estrabrook, Arch. Bioch. Biophys. 143:54 [ 1971 ] ). However the mechanism by which the oxidase functions in lipid peroxidation has not been explained. A study of the evidence available leads us to propose that the TPNH oxidase is catalyzing the reduction of the iron (Fe +3 to Fe +2) required in the reaction mixture. The ferrous form of the iron would then cause cleavage of any hydroperoxides (LOOH) present and initiate further peroxidation via the alkoxy radical (LO.).

W. R. Bidlack

Papers

Damage to microsomal membrane by lipid peroxidation.

A proposed mechanism for the TPNH enzymatic lipid peroxidizing system of rat liver microsomes.