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Molecules 2014, 19 20100 Figure 1. (a) Structure of 2:1 curcumin:metal complex; (b) Mixed ligand curcumin:metal complex. HO H3CO (a) OH OCH3 (b) OH OCH3 OO M2+ O OO HOOL1 OCH3 H3CO M2+ L4 L2 H3CO HO OH L3 Curcumin-metal complexes not only modify the physico-chemical properties of curcumin but they also affect the biological reactivity of the metals. In general it has been observed that complexation with curcumin reduces the toxicity of the metals and some curcumin complexes with metals like Cu2+, Mn2+, act as new metal-based antioxidants [67–74]. Due to the reversible electron transfer reactions with superoxide ions, Cu2+ and Mn2+ complexes of curcumin act as superoxide dismutase enzyme mimics. Metal complexes of curcumin have greater significance in view of the pathology of Alzheimer’s disease, where it has been found that due to its lipophilic nature, curcumin can cross the blood brain barrier and chelate metal ions that are toxic to the neurons. It has also been observed that the incidence of Alzheimer’s disease is significantly reduced among people that are known to regularly consume turmeric in their diet. Curcumin forms stable complexes with all the metals involved in Alzheimer’s disease [74–76]. The interaction of curcumin with Al3+, earlier considered to be responsible for development of Alzheimer’s disease, has been studied extensively. Curcumin forms three different types of complexes with Al3+, depending on the stoichiometry of the reaction. The 1:1 Al3+-curcumin complex showed less affinity to DNA binding than free Al3+, which has been attributed to its ability to reduce development of Al3+ induced Alzheimer’s disease [75,76]. Many other applications of curcumin metal complexes have been reported. Ga3+ curcumin complexes are being developed as innovative bioceramics [77]. Zn2+-curcumin complexes showed anti-cancer, gastroprotective and antidepressant effects in rats [78,79]. In vivo antiarthritic activity was reported for five co-ordinated curcumin-gold (Au3+) complexes [80]. Vanadyl-curcumin (VO(Cur)2)2+ complexes show antioxidant and anti-rheumatic activity [81]. Through metal co-ordination, curcumin reduces the toxicity of heavy metals like Hg2+, Cd2+, Pb2+ where significant reduction in heavy metal-induced oxidative stress is reported through complex formation [82–86]. Owing to their positive charges, most of the metal complexes of curcumin bind to DNA [66,78]. Some reports indicate that due to binding, the curcumin-metal complexes induce DNA damage and thereby exhibit pro-oxidant behavior. Accordingly, curcumin-metal complexes are also being explored as better anti-tumor agents than curcumin itself [87,88]. Therefore there is still some confusion about the role of metal-curcumin complexes, in biology whether they act as antioxidants or pro-oxidants. It appears that some metal complexes act as antioxidants, while some others may be pro-oxidants. This antioxidant/pro-oxidant activity of a metal complex depends on several factors such as nature of metal ion, co-ordination number, structure, stability and electrochemical potential of the complex. More systematic classification and chemistry of metal-curcumin complexes is necessary in the future. Biological evaluation of these complexes is more difficult in in vivo systems, as the bioavailabilty of mostPDF Image | Curcumin: From Extraction to Therapeutic Agent
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