TANNINS AS ANTIOXIDANTS

TANNINS AS ANTIOXIDANTS

Although dietary tannins are often perceived as detremental because of their potential to affect protein digestibility or metal ion availability, it is also possible that tannins are beneficial. It is likely, based on our knowledge of tannin chemistry, that tannins are potential biological antioxidants.

Antioxidants are widely believed to be an important line of defense against oxidative damage, which has been implicated in a range of diseases including cancer, cardiovascular disease, arthritis, and aging (Kehrer, J.P. Crit. Rev. Toxicol. 1993, 23, 21). Biological antioxidants are generally divided into three groups: enzymes, such as superoxide dismutase; inhibitors of radical formation, such as Fenton reaction inhibitors; and free radical quenching agents, such as alpha-tocopherol (Vitmain E). Phenolics are good candidates as antioxidants because of their favorable redox potentials and the relative stability of the aryloxy radical (Simic, M.G. and Jovanovic, S.V. 1994. In: Ho, C-T., Osawa, T., Huand, M-T., & Rosen, R.T. (Editors), Food Phytochemicals for Cancer Prevention II. Teas, Spices and Herbs. Page 20, American Chemical Society: Washington DC).

Many low molecular weight, naturally occurring phenolics scavenge radicals as effectively as the antioxidant vitamins E and A when tested in vitro (Rice-Evans, C.A.; Miller, N.J.; Paganga, G. Free Rad. Biol. Med. 1996, 20, 933). Our recent work suggests that free or protein-complexed condensed and hydrolyzable tannins are more effective than small phenolics

Some low molecular weight phenolics are pro-oxidants in Fenton-driven systems, apparently because the phenolic is able to redox cycle the metal ion required for radical formation (Aruoma, O.I.; Murcia, A.M.;

Butler, J.; Halliwell, B. J. Agric. Food Chem. 1993, 41, 1880). The tannins we have tested do not act as pro-oxidants in Fenton systems, and in fact react very rapidly to quench the hydroxyl radical.

We have used the deoxyribose method and the metmyoglobin (Randox) method to characterize the antioxidant capabilities of the tannins (Hagerman, A.E.; Riedl, K.M.; Jones, G.A.; Sovik, K.N.; Ritchard, N.T.; Hartzfeld, P.W.; Riechel, T.L. J. Agric. Food. Chem. 1998, 46, 1887-1892). We have also