Oxidative stress, a result of an overproduction and accumulation of free radicals, is the leading cause of several degenerative diseases such as cancer, atherosclerosis, cardiovascular diseases, ageing and inflammatory diseases. Polyphenols form an important class of naturally occurring antioxidants, having innumerable biological activities such as anticancer, antifungal, antibacterial, antiviral, antiulcer and anticholesterol, to name a few. Among various polyphenols, gallic acid (3,4,5-trihydroxybenzoic acid), a naturally occurring low molecular weight triphenolic compound, has emerged as a strong antioxidant and an efficient apoptosis inducing agent. Starting from the bioavailability and the biosynthetic pathway of gallic acid, this review includes various in vitro, in vivo and in silico studies providing the mode of action, radical scavenging activity, ability to inhibit lipid peroxidation, maintenance of endogenous defense systems and metal ion chelation by this triphenolic molecule, along with a comprehensive overview of factors responsible for its high antioxidant activity. Gallic acid derivatives have also been found in a number of phytomedicines with diverse biological and pharmacological activities, including radical scavenging, interfering with the cell signaling pathways and apoptosis of cancer cells. The diverse range of applications of this simple polyphenol is due to a fine amalgam between its antioxidant and prooxidant potential. The existing literature on this dual behavior of gallic acid and its derivatives is reviewed here. This is followed by an account of their potential clinical and industrial applications.

Gallic acid: a versatile antioxidant with promising therapeutic and industrial applications

USDA Database for the Flavonoid Content of Selected Foods Release 3.1

Levels of essential elements with antioxidant activity, as well as catechins, gallic acid, and caffeine levels, in a total of 45 samples of different teas commercialized in Spain have been evaluated. Chromium, manganese, selenium, and zinc were determined in the samples mineralized with HNO(3) and V(2)O(5), using ETAAS as the analytical technique. The reliability of the procedure was checked by analysis of a certified reference material. Large variations in the trace element composition of teas were observed. The levels ranged from 50.6 to 371.4 ng/g for Cr, from 76.1 to 987.6 microg/g for Mn, from 48.5 to 114.6 ng/g for Se, and from 56.3 to 78.6 ng/g for Zn. The four major catechins [(-)-epigallocatechin gallate (EGCG), (-)-epigallocatechin (EGC), (-)-epicatechin gallate (ECG), and (-)-epicatechin (EC)], gallic acid (GA), and caffeine were simultaneously determined by a simple and fast HPLC method using a photodiode array detector. In all analyzed samples, EGCG ranged from 1.4 to 103.5 mg/g, EGC from 3.9 to 45.3 mg/g, ECG from 0.2 to 45.6 mg/g, and EC ranged from 0.6 to 21.2 mg/g. These results indicated that green tea has a higher content of catechins than both oolong and fermented teas (red and black teas); the fermentation process during tea manufacturing reduces the levels of catechins significantly. Gallic acid content ranged from 0.039 to 6.7 mg/g; the fermentation process also elevated remarkably gallic acid levels in black teas (mean level of 3.9 +/- 1.5 mg/g). The amount of caffeine in the analyzed samples ranged from 7.5 to 86.6 mg/g, and the lower values were detected in green and oolong teas. This study will be useful for the appraisal of trace elements and antioxidant components in various teas, and it will also be of interest for people who like drinking this beverage.

Determination of Tea Components with Antioxidant Activity

Free radicals are well documented for playing a dual role in our body as both deleterious and beneficial species. In low/moderate concentrations free radicals are involved in normal physiological functions but excess production of free radicals or decrease in antioxidant level leads to oxidative stress. It is a harmful process that can be mediates damage to cell structures, including lipids, proteins, RNA and DNA which leads to number of diseases. A variety of synthetic medicine employed in the treatment of different diseases also capable to generate free radicals in body which may causes another disease. The plant sources are rich of antioxidants, phyto-constituents are capable to terminate free radical reactions and prevent our body from oxidative damage. Vegetables and fruits are also important sources of antioxidant substances. Different phytoconstituents and herbal product which are safer then synthetic medicine and beneficial in the treatment of diseases caused by free radicals, it also protect the body by prevent the free radicals to cause tissue injury. Phytoconstituents are conferring less side effect and compatible to body physiology. Therefore it is demand of modern era to use such phytoconstituents or phytomedicines.

Free radicals, antioxidants, diseases and phytomedicines: current status and future prospect

Methanol extract of whole plant of Oxalis corniculata Linn (Family: Oxalidaceae) was assessed for its antioxidant and anti‐inflammatory activity by in­vitro methods. Antioxidant activity was studied using 1, 1‐Diphenyl‐2‐Picrylhydrazyl (DPPH) and nitric oxide radical scavenging activity. Inhibition of lipid peroxidation was studied by thiobarbituric acid reactive substances (TBARS) method on isolated rat liver tissues. Quantitative analysis of antioxidative components like total amount of phenolics, flavonoids and flavonols were estimated using spectrophotometric method. In­vitro anti‐inflammatory activity was evaluated using albumin denaturation assay, membrane stabilization assay and proteinase inhibitory activity at different concentrations. Aspirin was used as a sta ndard drug for the study of anti‐ inflammatory activity.  Linear regression analysis was used to calculate IC50 value. Results showed that, the extract exhibited significant DPPH and nitric oxide radical scavenging activity with IC50 v alue of 302.93±4.17 and 73.07±8.28µg/ml respectively. Lipid peroxidation induced by the Fe2+, was inhibited by the extract with IC50 value 58.71±2.55µg/ml. Total phenol content was estimated as 25.62±0.10mg of gallic acid equivalents of dry extract. Total flavonoids and flavonols were found to be 150.88±12.61 and 150.16±2.16 mg of rutin equivalents per gram of dry extract respectively. Extract also showed in­vitro anti‐inflammatory activity by inhibiting the heat induced albumin denaturation and Red Blood Cells membrane stabilization with the IC50 values of 288.04±2.78 and 467.14±9.56µg/ml respectively. Proteinase activity was also significantly inhibited by the extract (IC50=435.28±5.82µg/ml). From the results, it is concluded that flavonoids and related polyphenols present in the O. corniculata extract may be responsible for the activity.

In­vitro antioxidant and anti­inflammatory activity of methanol extract of oxalis corniculata linn

Anthocyanin antioxidants from edible fruits

Chrysophyllum cainito L. (Sapotaceae), known commonly as star apple or caimito, is a tropical tree that bears edible fruits. The fruits are grown commercially in certain tropical and subtropical areas, such as southern Florida. In this study, the fresh fruits were extracted with methanol and partitioned with hexane and ethyl acetate sequentially. The ethyl acetate soluble fraction displayed high antioxidant activity in the 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay (IC50 = 22 microg/mL). Activity-guided fractionation of the ethyl acetate soluble fraction was performed to identify the antioxidant constituents. Nine known polyphenolic antioxidants, (+)-catechin (1), (-)-epicatechin (2), (+)-gallocatechin (3), (-)-epigallocatechin (4), quercetin (5), quercitrin (6), isoquercitrin (7), myricitrin (8), and gallic acid, have been identified from the fruits. Of these nine antioxidants, 2 is present in the highest concentration in star apple fruits (7.3 mg/kg fresh weight), and 5 showed the highest antioxidant activity (IC50 = 40 microM) in the DPPH assay.

Polyphenolic antioxidants from the fruits of Chrysophyllum cainito L. (Star Apple).

Activity-guided fractionation of a methanol extract from the fruit of Manilkara zapota cv. Tikal resulted in the isolation of two new antioxidants, methyl 4-O-galloylchlorogenate (1) and 4-O-galloylchlorogenic acid (2), along with eight known polyphenolic antioxidants, namely, methyl chlorogenate (3), dihydromyricetin (4), quercitrin (5), myricitrin (6), (+)-catechin (7), (−)-epicatechin (8), (+)-gallocatechin (9), and gallic acid (10). Of the 10 polyphenols, 1 showed the highest antioxidant activity (IC50 = 12.9 μM) in the 1,1-diphenyl-2-picrylhydrazyl (DPPH) free-radical assay and displayed cytotoxicity in the HCT-116 and SW-480 human colon cancer cell lines with IC50 values of 190 and 160 μM, respectively. Compound 2 showed high antioxidant activity (IC50 = 23.5 μM) in the DPPH free-radical assay and displayed cytotoxicity in the HCT-116 and SW-480 human colon cancer cell lines with IC50 values of 154 and 134 μM, respectively.

Bioactive novel polyphenols from the fruit of Manilkara zapota (Sapodilla).

Four new polyphenolic glycosides (1-4) were isolated from the BuOH extract of the bark of Walsura yunnanenis C. Y. Wu. These compounds comprise two lignans, i.e., the 9-O-alpha-L-arabinopyranosides 1 and 2 of (-)-isolariciresinol and of (+)-5-methoxyisolariciresinol, respectively, and the two phenolic glycosides 3,4,5-trimethoxyphenyl 2-O-(alpha-L-fucopyranosyl)-beta-D-glucopyranoside (3) and 3,5-dihydroxyphenyl 6-O-(4-hydroxy-3,5-dimethoxybenzoyl)-beta-D-glucopyranoside (walsuraside; 4). In addition, three known compounds, 3,4,5-trimethoxyphenyl beta-D-glucopyranoside, asperglaucide, and butyl alpha-D-fructofuranoside were identified. Their structures were elucidated spectroscopically and by chemical transformation (hydrolysis). The new compounds 1, 2, and 4 displayed significant antioxidant activities, with IC50 values in the range of ca. 42 to 49 mu g/ml.

Xiao-Dong Luo

Papers

Anthocyanin antioxidants from edible fruits

Polyphenolic antioxidants from the fruits of Chrysophyllum cainito L. (Star Apple).

Bioactive novel polyphenols from the fruit of Manilkara zapota (Sapodilla).

New antioxidant phenolic glycosides from Walsura yunnanensis.