Free radical metal-binding proteins

As an indirect effect of increased metal uptake, the physiological state of the cell can alter and defence mechanisms can be induced. Phytochelatin (metal binding proteins) synthesis and induction of free radical quenching enzymes and metabolites were frequently observed. Especially the latter can protect membranes against oxidative breakdown. 

Although both GPx and Cat are very efficient in removing H202, HO can still be formed in abundance (Fenton and Haber-Weiss chemistry). To partially offset the influence of transition metal ions on free radical production, there are numerous metal-binding proteins which prevent these reactions from taking place these include, among others, ferritin, transferrin, ceruloplasmin, and metallotheinein (Table 2). 

In spite of the protective effect of several antioxidant enzymes and metal-binding proteins, free radicals are still widely prevalent. Thus, Ames et al. (A 10) estimated that in each rat cell there are 100,000 radical hits each day, while in every human cells there are 10,000/day. Importantly, there are numerous natural free radical scavengers/chain breakers, the most notable being vitamins C andE, various carotenoids (beta-carotene, lycopene, etc.), flavonoids (rutin, quercetin, catechin, etc.), uric acid, and bilirubin, among others (Table 2). 

Antioxidants. Excessive concentrations of ROS can have serious effects on membranes, nucleic acid bases and proteins (Section 3.1.3). If uncontrolled, mutations and membrane damage could lead to cell death. To minimize damage, defensive control systems exist. Besides enzymes, there are hydrophilic- and lipophilic-soluble molecules called antioxidants , scavenging free radicals to prevent destruction of cellular biomolecules crucial for cell viability. Non-enzymatic biological antioxidants include tocopherols, carotenoids, qui-nones, bilirubin, steroids, ascorbate, uric acid, GSH, cysteine and metal-binding proteins, such as ferritin (Krinsky, 1992). 

Although LDL is well protected against oxidation in blood plasma by an ample supply of endogenous antioxidants and metal-binding proteins, this protection may not be adequate in the arterial wall where the oxidative modification of LDL is induced by endothelial cells. One hypothesis proposed is that natural antioxidants may be depleted within the arterial sub-endothelial space where oxidation takes place. The oxidative modification of LDL by endothelial cells can be completely inhibited in the presence of sufficient vitamin E or other antioxidants, such as butylated hydroxytoluene. These antioxidants can inhibit two steps in the free radical oxidation pathway, by reacting with the peroxyl... 

Figure 1.9 Examples of functionally important intrinsic metal atoms in proteins, (a) The di-iron center of the enzyme ribonucleotide reductase. Two iron atoms form a redox center that produces a free radical in a nearby tyrosine side chain. The iron atoms are bridged by a glutamic acid residue and a negatively charged oxygen atom called a p-oxo bridge. The coordination of the iron atoms is completed by histidine, aspartic acid, and glutamic acid side chains as well as water molecules, (b) The catalytically active zinc atom in the enzyme alcohol dehydrogenase. The zinc atom is coordinated to the protein by one histidine and two cysteine side chains. During catalysis zinc binds an alcohol molecule in a suitable position for hydride transfer to the coenzyme moiety, a nicotinamide, [(a) Adapted from P. Nordlund et al., Nature 345 593-598, 1990.)...

Copper is an essential trace element. It is required in the diet because it is the metal cofactor for a variety of enzymes (see Table 50—5). Copper accepts and donates electrons and is involved in reactions involving dismu-tation, hydroxylation, and oxygenation. However, excess copper can cause problems because it can oxidize proteins and hpids, bind to nucleic acids, and enhance the production of free radicals. It is thus important to have mechanisms that will maintain the amount of copper in the body within normal hmits. The body of the normal adult contains about 100 mg of copper, located mostly in bone, liver, kidney, and muscle. The daily intake of copper is about 2—A mg, with about 50% being absorbed in the stomach and upper small intestine and the remainder excreted in the feces. Copper is carried to the liver bound to albumin, taken up by liver cells, and part of it is excreted in the bile. Copper also leaves the liver attached to ceruloplasmin, which is synthesized in that organ. 

Metallothioneins are a group of small proteins (about 6.5 kDa), found in the cytosol of cells, particularly of liver, kidney, and intestine. They have a high content of cysteine and can bind copper, zinc, cadmium, and mercury. The SH groups of cysteine are involved in binding the metals. Acute intake (eg, by injection) of copper and of certain other metals increases the amount (induction) of these proteins in tissues, as does administration of certain hormones or cytokines. These proteins may function to store the above metals in a nontoxic form and are involved in their overall metaboHsm in the body. Sequestration of copper also diminishes the amount of this metal available to generate free radicals. 

Other potential health benefits of dietary flavonoids are too numerous to mention here. Suffice it to say that our understanding of the importance of flavonoids in the human diet is continuing to advance rapidly. One suspects that much of the physiological activity associated with flavonoids can be attributed to (i) their proven effectiveness as antioxidants and free radical scavengers, (ii) to their metal complexing capabilities (a capability that drove early advances in absorption spectroscopy and NMR studies), and (iii) to their ability to bind with a high degree of specificity to proteins. 

One form of antioxidant defense may be the binding of excess Fe3+ and other transition metal ions, preventing Fe3+, and other transition metal pro-oxidants from catalyzing free radical reactions. Most intracellular Fe3+ is stored in ferritin. Mammalian ferritins consist of a hollow protein shell 12-13 nm outside diameter... 

Monosaccharides can oxidise when catalysed by trace amounts of transition metals, generating free radicals, hydrogen peroxide and reactive dicarbonyls directly [27]. The process of glucose oxidation can lead to protein damage by free radicals and by covalent binding of the carbonyl products of the process to protein components (see below) (Fig. 5). 

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