Amino-Acid Composition of Urtica dioica Leaves and Polygonum hydropiper and P. aviculare Herbs

Abstract

Medicinal plants of Uzbekistan such as Urtica dioica, Polygonum hydropiper, and P. aviculare are promising sources for obtaining pharmacologically active substances. The chemical composition of U. dioica leaves and P. hydropiper L. and P. aviculare L. herbs is insufficiently studied. The goal of the present work was to isolate and study the quantitative content of total protein and qualitative composition of free and bound amino acids. Total protein from ground aerial parts of the three plant species was isolated by extraction with NaOH (0.2 N) with stirring on a magnetic stirrer for 1 h at 500 rpm. The resulting extract was centrifuged in a refrigerated centrifuge at 3,000 rpm for 15 min. Proteins in the transparent supernatant were precipitated for 16 h at +6°C using 80% anhydrous ammonium sulfate. The precipitate was separated by centrifugation at 6,000 rpm for 30 min and dissolved in NaOH (20 mL, 0.2 N). The total protein content in the resulting solution of each plant was determined by spectrophotometry on an SF-46 spectrophotometer [1]. The solution protein concentration was assayed using the Kalkar S. A. method. The protein assay results established that the amount of total protein for U. dioica leaves was 15.4%; P. hydropiper herb, 20%; and P. aviculare herb, 18.2%. The obtained protein solutions were dialyzed against tap water for 24 h in order to determine quantitatively the bound amino-acid content. The protein solutions were lyophilized. An accurately weighed portion (50 mg) of lyophilized proteins was hydrolyzed at 110°C under vacuum. The hydrolysates were evaporated and analyzed for amino acids in a T339 analyzer (Czech Rep.). Free amino acids were prepared as previously described [2]. A weighed portion (100 mg) of each ground plant was wetted with distilled H 2 O (5 mL) and placed on a boiling-water bath for 10 min. The extract was separated from the plant material by centrifugation at 3,000 rpm for 15 min. The transparent supernatant was evaporated to dryness in a rotary evaporator. Free amino acids in the obtained samples were determined on an amino-acid analyzer. Bound and free amino acids were assayed on a T339 amino-acid analyzer by the method proposed by Ermakov [3]. The results were processed automatically by comparing them with those for standard solutions of amino acids. Table 1 presents the results. Table 1 shows that asparagine, isoleucine, leucine, and arginine dominated the free amino acids from U. dioica leaves; asparagine, glutamine, glycine, tyrosine, and arginine, free amino acids from P. hydropiper; and asparagine, glycine, lysine, and arginine, free amino acids of P. aviculare. Asparagine, glutamine, leucine, and arginine dominated the bound amino acids of all three plants. These same amino acids dominated the bound amino acids of the alcohol-soluble fractions. IR spectra of proteins from U. dioica leaves and P. hydropiper and P. aviculare herbs found that the most pronounced characteristic absorption bands in the first instance were at 2957, 2924, and 2851 cm –1 . These corresponded to C–H stretching vibrations in methyl (–CH 3 ), methylene (–CH 2 ), and ethylene (–CH 2 –CH 2 –) groups. Absorption bands in the range 1546 cm –1 for protein from U. dioica leaves corresponded to amide (=NH) bending vibrations and indicated that the conformational state of the proteins was different from that of proteins from P. hydropiper and P. aviculare herbs. In particular, the conformation in the first instance was -spirals (1645 cm –1 ) and -spirals (1540 cm –1 ) whereas the protein conformation from P. hydropiper and P. aviculare corresponded to an anti-parallel -structure. Characteristic absorption bands at 1018 and 959 cm –1 belonging to symmetric bending vibrations of intermolecular OH groups appeared for protein from P. aviculare herb. This indicated that this protein was more hydrated.