A pharmacophylogenetic study of Aconitum L. (Ranunculaceae) from China

Aconitum L. (Ranunculaceae) is a large genus of about 300 species distributed in the temperate regions of the Northern Hemisphere. This genus consist of three well-circumscribed subgenera, subgen. Lycoctonum (DC.) Peterm., subgen. Aconitum and subgen. Gymnaconitum (Stapf) Rapaics. The southwestern China, particularly its Hengduan Mountains region, is the most important center of diversity and speciation of the genus. Many species in this genus have been used as poisonous and medicinal plants. This paper is to evaluate the taxonomic value of diterpenoid alkaloids mainly at subgeneric and serial levels of the genus Aconitum based on an analysis of the distribution of diterpenoid alkaloids in the Chinese species and of their biogenetic pathways. The correlation between phylogeny, chemical constituents and pharmaceutical uses in the genus Aconitum is also discussed from the data of the toxicity and therapeutic value of the species in the genus, a research field for which the term “pharmacophylogenetics” is here specifically coined. The major points of the paper are summarized as follows: 1. Diterpenoid alkaloids known in plants fall into four skeletal types: C18, C19, C20 and bisditerpenoid alkaloids. They can be further subdivided into 14 subgroups, namely, C18: appaconine-type (I) and ranaconine-type (II); C19: aconitine-type (III), lycoctonine-type (IV), 7,17-seco-type (V) and lactone-type (VI); C20: atisine-type (VII), denudatine-type (VIII), hetidine-type (IX), hetisine-type (X), veatihine-type (XI) (not found in Aconitum species), napelline-type (XII) and anopterine-type (XIII) (not found in Aconitum species); and bisditerpenoid alkaloids (XIV). 2. The species in Aconitum subgen. Lycoctonum contain mainly the C18-diterpenoid alkaloids (lappaconine-type and ranaconine-type) and C19-diterpenoid alkaloids (lycoc- tonine-type). Roots of the plants in this subgenus show a relatively lower toxicity (LD50 to mice=1660–3340 mg/kg (i.v.)) and have been used for the treatment of rheumatism, pains and irregular menstruation etc. Because of the lower toxicity of the roots, the species in thissubgenus are worthy a more detailed phytochemical investigation for the development of new medicines. 3. Aconitum subgen. Aconitum is the largest subgenus, with about 250 species which are usually divided into two sections, section Sinaconitum, a monotypic section including only A. polycarpum, and section Aconitum including the remaining species. The species in China in the latter section are usually divided into nine series. (1). Series Tangutica and series Rotundifolia are all dwarf alpine plants, generally less than 35 cm tall. Phytochemically, they contain mainly the lactone-type C19-diterpenoid alkaloids and sporadically C20-diterpenoid alkaloids. The lactone-type alkaloids have been found so far to occur exclusively within these two series, and thus can be considered as the characteristic chemical constituents of these two series. The roots in the plants of the two series show a relatively lower toxicity (LD50 to mice =ca. 2400 mg/kg (i.v.)). The whole plants of the species have been traditionally used in China’s Tibetan, Mongolian and Uygur regions for the treatment of high fever. (2). The species in series Bullatifolia contain mainly the denudatine-type and the napelline-type C20-diterpenoid alkaloids, and the aconitine-type C19-diterpenoid alkaloids. This series may occupy a somewhat intermediate position in Aconitum subgen. Aconitum from a chemotaxonomic view. In toxicity, the LD50 to mice range from 210–270 mg/kg (i.v.). The roots in this series have been used for the treatment of pains and rheumatism. (3). Series Brachypoda includes A. brachypodum, A. pendulum, A. polyschistum and several other morphologically very similar species. Their roots are recorded as “Xue-shang-yi-zhi-hao” in the Chinese Materia Medica and have been used as anti-rheumatic and analgesic remedies. Phytochemically, they contain mainly the aconitine-type C19-diterpenoid alkaloids, an evolutionarily advanced type. The LD50 to mice range from 130–280 mg/kg (i.v.). It is noteworthy that A. coreanum contains mainly the C20-diterpenoid alkaloids and has a much lower toxicity (LD50 = 2800 mg/kg (i.v.) to mice) than the other species in this series. From a chemotaxonomic view it seems reasonable to segregate A. coreanum and its closet ally, A. anthoroideum, from series Brachyloda to establish an independent series. (4). Series Stylosa, with plants generally of larger roots, are the major sources of “Da-wu-tou”. The species in the series contain mainly the aconitine-type diester C19-diterpenoid alkaloids, which are characterized by the presence of anisoyloxy residues at C-14. The roots display a high toxicity with the LD50 to mice ranging from 24–102 mg/kg (i.v.). From a chemootaxonomic view, A. contortum, mainly with the presence of the hetidine-type C20-diterpenoid alkaloids and the dranaconine-type C18-diterpenoid alkaloids and yet the absence of anisoyloxy residues, seems to be a very special species in series Stylosa. Its systematic position needs to be reconsidered. (5) Series Ambigua, eight species of which have been investigated phytochemically, contains mainly the aconitine-type C19-diterpenoid alkaloids with anisoyloxy residues, indicating its close affinity to series Stylosa. (6). Series Volubilia, which is characterized by having twining stems, contains the aconitine-type diester C19-diterpenoid alkaloids with the presence of an anisoyl or a benzoyl group at C-14. Several species in this series, such as A. sczukinii and A. volubile, contain the highly advanced 15-hydroxyl aconitine-type C19-diterpenoid alkaloids, indicating its possible affinity to series Inflata. The LD50 to mice range from 84–283 mg/kg (i.v.). The roots of many species in this series have been used as folklore drugs for the treatment of trauma and rheumatic pains. In this series, A. hemsleyanum, a very polymorphic species in gross-morphology, exhibits also a greatinterpopulational phytochemical variation. (7). Series Inflata includes the two most widely medicinally used Aconitum species, A. carmichaeli and A. kusnezoffii. Both are now officially listed in the Chinese pharmacopoeia. They contain the aconitine-type 15-hydroxyl monoester or diester C19-diterpenoid alkaloids, highly advanced chemical constituents in the genus Aconitum. The LD50 to mice range from 66–137 mg/kg (i.v.). The cardiovascular, muscle-smoothing and central effects of the two species have been thoroughly studied. (8). Series Grandituberosa, which is morphologically characterized by having several chain-like arranged tubers, contains the highly advanced aconitine-type C19-diterpenoid alkaloids. The roots, with the LD50 to mice being 29 mg/kg (i.v.), have a high toxicity. (9). Series Racemulosa, with only A. racemulosum var. pengzhouense phytochemically already studied in detail and from it a novel skeleton of C20-diterpenoid alkaloids, i.e. racemulosine, being found, shows extraordinary chemical features. It contains mainly, however, the lycoctonine-type and the aconitine-type amino alcohol C19-diterpenoid and C20-diterpenoid alkaloids. The roots of A. racemulosum are known as “Xue-Wu” and have the actions of activating blood circulation and removing stasis. From a chemotaxonomic view, this series show some primitive features. (10). Series Brunnea is phytochemically not well known. This series contains both C20-diterpenoid alkaloids, e.g. denudatine and songorine, and the highly advanced aconitine-type alkaloids, indicating its intermediate position between the series Bullatifolia and the series Brachypoda. 4. The monotypic subgenus Gymnaconitum, with only A. gymnandrum, is of both primitive and advanced features phytochemically and gross-morphologically. The whole plant of this species is used as insecticide. In summary, 76 Aconitum species in China have been medicinally used. They are mainly used for the treatment of plaque, sepsis, intoxication, cold- and immuno- suppression-induced ailments, rheumatoid arthritis, and various types of pain, including migraine, swelling induced by trauma and fracture, and facial paralysis. Pharmacologically, they can be developed as analgesic, antirheumatic and anti-arrhythmic agents. The key obstacle for their extensive medical utilization may lie in their usually extremely high toxicity. From a pharmacophylogenetic point of view, therefore, the less toxic species, i.e. those in subgenus Lycoctonum, and in series Tangutica, Rotundifolia and Racemulosa, and two species (A. coreanum and A. anthoroideum) in series Brachypoda in subgenus Aconitum, should be paid much more attention. Further comprehensive studies on these species are needed for their better medical utilization.