Preclinical Efficacy Studies of Green and Black Tea Extracts

Abstract

Tea (Camellia sinensis) is one of the most popular beverages consumed worldwide. Tea leaves are primarily manufactured as green, black, or oolong, with black tea representing approximately 80% of tea products consumed. Green tea is the nonoxidized/nonfermented product and contains several polyphenols components such as epicatechin, epicatechin gallate, epigallocatechin, and epigallocatechin gallate (EGCG). EGCG is the major green tea polyphenol (GTP) (>40% dry weight). The major components of black tea (the fermented product) are theaflavins (1%-2% dry weight) and thearubigins (10%-20% dry weight). Theaflavins, which determine the quality and flavor of the tea, are formed by oxidation of quinones derived from the epicatechins. Thearubigins are highly colored flavonol oxidation products that are often bound to peptides or proteins. Oolong tea is the partially oxidized/fermented product that retains a considerable amount of the original poly phenolic material (1).

In the National Cancer Institute (NCI) testing effort, tea extracts, tea polyphenol-enriched fractions, isolated catechin fractions, and a theaflavin-rich fraction were used (Table I). The black and green tea extracts were tested in both caffeinated and decaffeinated forms. The percentages of polyphenols in both green and black teas are approximately the same; however, levels of EGCG and caffeine in green tea are 4- to 5-fold that of black tea. Theaflavins are unique to black tea due to the oxidation process.

In an effort to determine the cancer chemopreventive activity of tea compounds, the NCI Chemoprevention Branch initially conducted in vitro mechanistic assays and in vitro cell transformation assays to screen for efficacy of these compounds. The mechanistic assays measure: a) inhibition of DNA adduct formation; b) inhibition of free radical formation; and c) enhancement of glutathione (GSH), glutathione-S-transferase (GST), ornithine decarboxylase (ODC), and NAD(P)H:quinone reductase (QR) activity.