Metals inactivators/chelators/scavengers

Flavonoids have the ability to act as antioxidants by a free radical scavenging mechanism with the formation of less reactive flavonoid phenoxyl radicals [Eq. (1) and (2)]. On the other hand, through then-known chelating ability these compounds may inactivate transition metals ions (iron, copper), thereby suppressing the superoxide-driven Fenton Reaction, Eqs. (3) and (4), which is currently believed to be the most important route to activate oxygen species [51]. 

We will not go in depth into the subject of antioxidants (12), which is more a part of preformulation than a stress test, but the autoxidation mechanism does suggest that oxidation can be inhibited by peroxy radical scavengers (chain-breaking antioxidants) like phenol antioxidants, by heavy metal chelating agents, and by peroxide inactivating substances (preventive antioxidants). 

Complete removal of zinc and thus inactivation of the enzyme can be accomplished in these systems at low D-PEN concentrations if a secondary scavenger chelator is added to the system. Such chelators bind metal that has been released from the enzyme but do not participate in the release.In the case of carboxypeptidase A, aM thionein (apo-metallothionen see Metallothioneins) inhibits catalysis by only about 10% over a 15-min period consistent with its action as a secondary chelator. However, in the presence of 250 aM D-PEN and aM thionein total inhibition is achieved in less than 15 min. D-PEN accelerates zinc equilibration between carboxypeptidase A and thionein (Scheme 1). This is accomplished by D-PEN catalyzing the release of Zn from the enzyme. Since D-PEN is in vast excess over both the enzyme and thionein, the enzyme-released zinc would be expected to bind to D-PEN first. However, since thionein binds zinc more tightly than D-penicillamine and can accept 7 moles of zinc per mole of thionein, it should be the ultimate acceptor of the released zinc. 

Metal-chelate complexes are ubiquitous in biology. Bacteria such as Escherichia coli and Salmonella enterica in your gut excrete the chelator enterobactin (Figure 13-4) to scavenge iron that is essential for their growth. Chelates excreted by microbes to gather iron are called siderophores. The iron-enterobactin complex binds to the bacterial cell surface and is taken into the cell. Iron is then released by enzymatic disassembly of the chelate. To fight bacterial infection, your immune system produces a protein called siderocalin to sequester and inactivate enterobactin. ... 

Stabilizers inhibit the chemical reaction between two or more other chemicals, and inhibit the separation of suspensions, emulsions, or foams. Stabilizers include (1) antioxidants that prevent unwanted oxidation of food materials. (2) UV stabilizers that protect food materials from harmful effects of UV radiation, being (a) UV absorbers which absorb UV radiation and prevent it from penetrating the materials, as sunscreens, (b) Quenchers which dissipate the radiation energy as heat instead of letting it break chemical bonds, (c) Scavengers that eliminate the free radicals formed by UV radiation, as hindered-amine light stabilizers. (3) Sequestrants that inactivate traces of metal ions that would otherwise act as catalysts by forming chelate complexes. (4) Emulsifiers and surfactants that stabilize emulsions. 

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