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Molecules 2014, 19 20097 Among the molecular oxidants, reactions with peroxynitrite, hydrogen peroxide are the most common ones. In several biological models curcumin has been found to protect cells under conditions where there is excessive production of these molecular oxidants. However there are not many studies elucidating the possible chemical reactions and identification of the reaction products. There are few reports in the literature on direct reaction of curcumin with peroxynitrite [40,41]. The reported rate constants and the inhibition concentrations of curcumin to prevent nitrotyrosine formation indicate that curcumin is as powerful antioxidant against peroxynitrite-induced oxidative stress. 5.2. Chemical Degradation and Metabolism Curcumin undergoes chemical degradation in aqueous-organic solutions and the degradation increases as the pH is increased, which is of a serious concern in its applications [6,33,42–45]. Most phenols in solution form polymers over time, but the degradation of curcumin is not through the phenolic group but is rather found to be through the α,β-unsaturated β-diketo moiety. In dilute solutions (i.e., in micromolar solutions) 90% curcumin degrades in 30 minutes. However the percentage degradation will decrease at high concentrations. Several important products have been identified as a result of curcumin degradation. They are trans-6(4'-hydroxy-3'-methoxyphenyl)-2,4-dioxo-5-hexanal, ferulic aldehyde, ferulic acid, feruloylmethane and vanillin (Scheme 2). Although not fully understood, it is believed that the degradation is by hydrolysis through the diketo moiety. However the degradation is significantly decreased when curcumin is attached to lipids, liposomes, albumins, cyclodextrin, cucurbituryl, surfactants, polymers and many other macromolecular and microheterogenous systems [33]. Thus has been found to be of great use that stable curcumin solutions could be prepared in culture medium containing 10% Fetal Bovine Serum (FBS) and also in human blood. Curcumin undergoes much faster degradation when exposed to sunlight [33,46]. It is one common observation that curcumin/turmeric stains can be quickly removed on exposure to sunlight The colorless products identified during photodegradation of curcumin are vanillin, ferullic acid, and other small phenols, indicating a similar product distribution during photochemical degradation as in chemical degradation in solution. This photodegradation involves formation of the excited states of curcumin. Some reports indicate that curcumin generates singlet oxygen and other ROS on photoexcitation and this is actually responsible for the photobiological and photodynamic activity of curcumin [47]. In such case the degradation of curcumin after photoexcitation must proceed though the triplet excited state of curcumin [33]. Photophysical studies reported the lifetime of triplet excited state of curcumin to be in microseconds, suggesting that the degradation may proceed very fast and compete with singlet oxygen formation. The photodegradation is accelerated in presence of TiO2 nanoparticles, and this method can be employed to remove turmeric stains from cotton fabrics [48]. The metabolism of curcumin in rats and humans produces different products [49–53]. Two major pathways have been identified in curcumin metabolism, like O-conjugation and reduction. The O-conjugation products are curcumin glucuronide and curcumin sulfate. The reduction products are tetrahydrocurcumin, hexahydrocurcumin and octahydrocurcumin. Other minor products are dihydrocurcumin glucuronide, tetrahydrocurcumin glucuronide, ferulic acid and dihydroferulic acid. The formation of these products has been confirmed by HPLC and mass spectrometry. Although it has been reported that these processes occur enzymatically, the exact enzymes involved in all these specificPDF Image | Curcumin: From Extraction to Therapeutic Agent
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