Yarandi, Niousha (2016) Evaluation of the antioxidant composition of teas via chemical and biological profiling. (PhD thesis), Kingston University, .
Abstract
The health benefits of tea, one of the most consumed beverages in the world, are largely attributed to phenolic constituents and their antioxidant properties. Teas have been shown to neutralise free radicals, prevent oxidative stress, and act as inhibitors of proteinases involved in cellular degradation, which has led to the platform for this study. The aim of this study was to investigate the antioxidant activities of phenolics and other components of major types of tea (white, green and black) using various assays, for screening components of these three types, for their antioxidant activity and for their anti-inflammatory abilities. A new functional High Performance Thin Layer Chromatography approach was adapted and employed to (i) separate and identify antioxidant components of tea, (ii) compare the antioxidant profiles of major types of tea, and (iii) screen the antioxidant activities of identified compounds in tea via treatment with redox-active metal ions. Twelve phenolic compounds were identified which were grouped according to their reactivity towards redox-active metal ions. In addition, non-phenolic constituents were identified as antioxidants and assigned as chlorophyll analogues by comparison to reference compounds. A further key finding was that the coutner ion to the metal salt could play a marked role in antioxidant assays used with chloride, but not sulfate, enhancing antioxidant depletion. The HPTLC observations were supported by HPLC profiling to compare the constituents of tea types and phenolic loss upon challenge with oxidants. The predominant phenolics were (-)-EGCG, caffeine, (-)-ECG and quercetin - the latter two being the most active against oxidant challenge. Metal ion analysis by ICP-MS revealed the teas contained several redox-active metal ions and black tea had significantly higher levels of redox-active Mn ions. The antioxidant enzyme activity (catalase and superoxide dismutase) of the teas and selected compounds was also investigated, a variety of activity across the tea samples and the selected compounds with regards to their Catalase activity and SOD inhibitory/activity capabilities was observed. Chlorophyll showed the highest catalase activity, and black and green teas showed the highest superoxide dismutase activity. The effect of tea extracts and selected compounds on immortalised human keratinocyte cells (HaCaT) and human cervical carcinoma cells (CaSki), and their abilities to guard against oxidative damage in cell lines was also investigated. White tea (Wt), green tea (Gt), Black tea (Bt), EC, ECG, EGCG, quercetin and chlorophyll a and b showed powerful antioxidant activities in HaCaT cells both H[sub]2O[sub]2- and paraquat-induced cell death and for CaSki cells H[sub]2O[sub]2- induced cell death. These promising data demonstrate that teas and the individual components of tea have the potential to act as antioxidants and anti-inflamatory agents both in vitro and in vivo, and therefore further investigation of their bioactive effects may prove beneficial.
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