Metabolites with antioxidant and protective functions from leaves of vegetable amaranth (Amaranthus tricolor L.)

Antioxidant metabolites of plant origin are able to regulate many physiological functions of the body and reduce the risk of developing chronic diseases caused by free radical oxidation. Vegetable plants are the most affordable source of essential antioxidant metabolites lack of which leads to a sharp decrease in resistance to environmental stresses. Amaranth ( Amaranthus tricolor L.) is a promising food and medicinal plant. Variety Valentina (originated by V.K. Gins, P.F. Kononkov, M.S. Gins, All-Russian Research Institute of Breeding and Seed Production of Vegetable Crops) was successfully introduced and grown in several Russian regions. Our objective was to study the composition and content of low-molecular biologically active antioxidant metabolites that determine the nutritional and pharmacological value of amaranth leaves, and to assess the main antioxidant accumulation in plant organs under the conditions of the Moscow Region. For analysis, fresh and dried leaves (juvenile, those with a formed blade, and old ones), inflorescences, stems, veins, petioles and roots were used. Amaranthine, reduced ascorbic acid, and total antioxidant content was measured in water and ethanol extracts from fresh and dry leaves and plant organs. Also, simple phenols and oxybenzoic acids, flavonoids, condensed and polymeric polyphenols were assayed. Chlorogenic, gallic, ferulic acids and arbutin content was determined in aqueous extract by high performance liquid chromatography (HPLC). The metabolites were analyzed by gas chromatography-mass spectrometry (GC/MS). It was shown that actively photosynthesizing leaves with a fully formed blade predominantly accumulated ascorbic acid, while in the aging leaves its amount decreased. Veins, petioles and stems contained substantially less metabolites with antioxidant activity compared to leaves. In aqueous extracts, the main betacyanins were amaranthine and iso-amarantine. Chromatography of aqueous extracts from amaranth leaves showed the presence of highly active antioxidants, e.g. arbutin-glucoside hydroquinone and oxycinnamic acids including ferulic, chlorogenic, oxybenzoic (gallic) acids. In the tests, gallic acid concentration was 1.51 µg/100 ml, chlorogenic acid concentration was 2.05 µg/100 ml, ferulic acid concentration was 0.01 µg/100 ml, and arbutin concentration was 472.51 µg/100 ml. Water-extracted squalene (C30H50), a powerful antioxidant usually isolated from amaranth seeds only, was first discovered in amaranth leaves. Ethanol extraction revealed a greater number of the colored components in the spectral range of the 350-700 nm, in addition, gallic, chlorogenic and ferulic acids were found. A total of 37 low-molecular metabolites were identified by gas chromatography-mass spectrometry. Our findings indicate that vegetable amaranth, as a promising reproducible source of antioxidants, can be used in functional foods and phytobiologicals.