Curcumin: Abilities and Bottlenecks
Phenolic antioxidants have been popular ever since their discovery. This is more so with the increased occurrence of oxidative stress related complications. The modern lifestyle and food habits have further fuelled the need to include antioxidants in the diet so as to get rid of excessive oxidative stress increasing factors. Despite of very excellent potential to serve as antioxidants, phenolic antioxidants suffer from a common problem of low systemic bioavailability. Literature is replete with multiple comprehensive articles and reports highlighting these problems.

Curcumin is the most popular phenolic antioxidant being used by almost all of us in our daily life. Reports and prior studies highlight that even though curcumin intake of 12g/day is safe (in rat models, in humans, it is 8g/day), yet the problem from which this wonderful natural product suffers is that when it is orally consumed, most of it remains segregated I liver and stomach, from where it is excreted gradually. Significantly, conventional attempts have found a very poor absorption of curcumin within the blood tissues and system. The fact which compounds this problem is that curcumin is insoluble in water. Thus, biochemical absorption of curcumin has resulted in a very negative influence on its excellent antioxidant, anti-inflammatory, antidiabetic, anticancerous potential. Curcumin exists in two isomeric forms, namely, the keto form and the enol form.

These isomeric forms show keto-enol tautomerism in their mutual existence. The antioxidant potential of curcumin is due to the presence of three naturally occurring curcuminoids, namely: curcumin (77%), de-methoxy curcumin (17%) and bisdemethoxy curcumin (3%). These are the natural curcuminoid pigments present in curcumin that are responsible for yellow color of curcumin as well as its potent antioxidant effects. The mechanism through which curcumin exerts its antioxidative potential is evident from its structure itself, the hydroxyl groups present on the benzene rings get broken to release their hydrogen ions, which move on to quench the reactive tendencies of oxidative stress causing free radicals. Studies report that the structural aspect contributing to this kind of effect is the presence of electron donating methoxy groups which tend to donate the electron density on the benzene ring. This is combination with the π-conjugation effect of the benzene ring results in the weakening of O-H bond of the hydroxyl groups present. As the free radicals have strong reactivities towards the electron rich species, sop these abstract the generated hydrogen ion.

To address the bioavailability issue of this excellent natural antioxidant (as it has been used by human civilizations since ages), a number of analogues for this curcumin have been designed. Some studies have even reported that when delivered in combination with transition metals such as those of manganese and chromium, curcumin has a more pronounced antioxidant effect. The designed and used analogues can be both natural as well as synthetic. Moreover, it has been found that conjugating phenol moiety imparts better anti-oxidative potential to curcumin rather than that of the non-phenolic moieties being tagged to curcumin. A number of other studies also confirm that bioavailability problem needs to be dealt with only increasing the existence of curcumin in the body tissues, which can be achieved by delivering it through novel carriers, like those of emulsion systems, liposomal complexes, micellar carriers and a number of other strategies. Interestingly as well as surprisingly, the mechanism of anticancer activities of curcumin has yet not been well understood and studies also show that the impact of curcumin prevalence in the body tissues for longer durations as well as its targeted delivery which could be a factor in increasing its concentration, are responsible for shifting its antioxidant capacity to the pro-oxidant ones. It has also been shown that in presence of copper and zinc, curcumin shows the a pro-oxidant activities. So, it remains widely susceptible that the elimination of cancer infected cells by curcumin could be largely due to its pro-oxidant effect, which in turn initiates the apoptotic signals that are responsible for cell death.

A number of interesting aspects still remain to be optimized in order to best characterize and optimize the antioxidant effect of curcumin, to be able to be coupled advantageously.

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