"Saponins," In: Ullmann's Encyclopedia of Industrial Chemistry
Total Page：16
File Type：pdf, Size：1020Kb
Article No : a23_485 Saponins MICHAEL W. SCHWARZ, E. Merck OHG, Darmstadt, Federal Republic of Germany 1. Introduction......................... 177 3.2. Glycoalkaloids ...................... 183 1.1. General Properties.................... 178 3.3. Triterpene Saponins................... 184 1.2. Distribution ......................... 178 4. Animal Saponins ..................... 188 1.3. Isolation ............................ 178 4.1. Asterosaponins....................... 188 1.4. Structure Elucidation.................. 179 4.2. Holothurins ......................... 189 1.5. Analysis ............................ 179 References .......................... 189 2. Pharmacology ....................... 179 3. Plant Saponins ....................... 180 3.1. Steroid Saponins ..................... 180 Abbreviations galA: b-D-galacturonic acid glc: b-D-glucose api: b-D-apiose glcA: b-D-glucuronic acid ara: a-L-arabinose 0 qui: b-D-quinovose drib: b-D-2 -deoxyribose rha: a- -rhamnose f: furanoside L xyl: b-D-xylose fuc: b-D-fucose fru: b-D-fructose gal: b-D-galactose 1. Introduction Saponins are categorized according to the structure of the aglycone moiety (sapogenin) and Saponins are glycosides (see ! Carbohydrates: the number of linked sugar chains. Aglycones Occurrence, Structures and Chemistry) occur- can be divided into triterpenoid and steroid ring primarily in plants but also in starfish sapogenins. Steroid saponins with basic proper- (Asteroidea) and sea cucumbers (Holothuridea). ties are termed glycoalkaloids. Properties generally considered to be shared by Hexoses common in all saponins are b-D- this group of natural products are surfactant glucose (glc), b-D-galactose (gal), and a-L- activity, hemolytic action, steroid- complexing rhamnose (rha); pentoses are a-L-arabinose ability, and biocidal capability. (ara) and b-D-xylose (xyl). Pentoses occur The characteristic soapy lather formed when primarily as pyranosides, less frequently as saponin- containing plant extracts are agitated in furanosides (f). water provides this group of secondary metabo- Saponins linked to one or two sugar chains are lites with its common name (Latin sapo ¼ soap), called monodesmosides and bisdesmosides, although this property is shared with structurally respectively (Greek desmos ¼ chain). Trisdes- related compounds (! Cardiac Glycosides and mosidic saponins are rare [1], [2]. Mild hydroly- Synthetic Cardiotonic Drugs). Other properties sis of saponins yields prosapogenins, neutral are characteristic of particular types of saponins glycosides with less than three sugar units, or rather than all members of the class. acidic or basic glycosides with one sugar unit. Ó 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/14356007.a23_485 178 Saponins Vol. 32 Prosapogenins are less soluble than saponins and [15] have been reported. Aquatic vertebrates do not have typical saponin properties. such as fish and tadpoles are affected through permeabilization of their gills [16], resulting in a rapid loss of physiological function. 1.1. General Properties Surfactant Activity. The well-known abili- 1.2. Distribution ty of saponins to cause frothing has long been used to detect these materials in plants or plant extracts. Saponins occur mainly in plants of the subdivision The combination of a hydrophobic aglycone and Angiospermae of the division Spermatophytae. hydrophilic substituents accounts for the amphi- Steroid saponins are restricted mostly to the class philic nature of saponins. Increased length and Monocotyledoneae, a known exception being the branching of the sugar chain correlate with saponins of foxglove (Digitalis sp.). Triterpene enhanced surfactant power. Hence, triterpenoidal and glycoalkaloid saponins are found primarily in bisdesmosides are more active in this sense than plants of the class Dicotyledoneae, a more recent- monodesmosides [3]. ly discovered exception being saponins isolated from montbretia (Crocosmia crocosmiiflora) Hemolytic Activity. Saponins vary consid- [17]. Certain families of the class Dicotyledoneae erably in their ability to lyse erythrocytes. (e.g., Solanaceae, Hippocastanaceae, Primula- Different glycosides of the same sapogenin differ ceae, Rosaceae,andCaryophyllaceae) are espe- in activity as a function of the type and structure of cially rich in saponin- containing genera. substituents [4], [5]. Bisdesmosides are generally Monodesmosides are concentrated in the less potent than monodesmosides. Introducing outer tissues of seeds, roots, and bark, consistent polar hydroxyl groups into the aglycone decreases with their function as a barrier against activity, whereas esterification restores it. The microorganisms, whereas bisdesmosides are linkage position of the sugar chain in triterpenoids more abundant in leaves and stems. Secretion of also influences reactivity; thus, C-3 glycosides proteolytic enzymes by fungal hyphae triggers are more reactive than saponins with a C-28 conversion of the more soluble (and, therefore, glycosidic substituent. transportable) bisdesmosides by specific glycosi- dases [18] into the more active monodesmosidic Steroid-Complexing Ability. Saponins saponins. form insoluble complexes with cholesterol [6] Animal saponins are restricted to representa- and other sterols. This affinity is more pro- tives of the phylum Echinodermata. Species of nounced with steroid saponins and glycoalka- the class Holothuridea (sea cucumbers) contain loids than with triterpenoids. triterpene saponins, whereas saponins from species of the class Asteroidea (starfish) contain Biocidal Activity. Monodesmosidic sapo- steroids. These saponins induce avoidance nins exhibit fungitoxic or fungistatic and weak responses in susceptible predator species [19]. antimicrobial activity [7]. Examples include the specific action of a-tomatine [8] and the oat (Avena sativa) root saponin avenacin [9]. 1.3. Isolation Steroid saponins and glycoalkaloids generally demonstrate more pronounced action, whereas Methods for saponin isolation and purification triterpenes exhibit a broader spectrum of are covered in several reviews [20–22]. Ground effectiveness. dried or fresh specimens are extracted after The permeabilization of crucial cell mem- optional defatting with solvents such as metha- branes probably explains the toxicity of saponins nol, ethanol, their aqueous mixtures, or, in the to mollusks [10] and other invertebrates [11]. case of glycoalkaloids, slightly acidified water. Ecdysterone-like activity [12] and the repellent Formation of artifacts due to enzymatic or or antifeedant action of saponins against termites solvolytic cleavage of glycosidic bonds or acid- [13], leaf- cutting ants [14], and the potato beetle catalyzed elimination and rearrangement of the Vol. 32 Saponins 179 aglycone must be prevented by suitable choice of mixtures rather than single substances. Quantita- conditions. tive estimation of their makeup is accomplished Distribution of a crude extract between water by measuring the surface tension of solutions, and butanol separates the saponins from such their foaming power, and their hemolytic activity water-soluble compounds as oligosaccharides. [32]. Precipitation with organic solvents (e.g., ether or Saponins or saponin mixtures for pharmaceu- acetone) or complexing agents (e.g., cholesterol) tical applications require analytical methods permits further concentration. more specific to particular active compounds, Saponins with free carboxylic acid groups can such as spectrophotometric quantitation of a be purified by ion-exchange chromatography. For saponin color reaction [33], [34]. Combinations cases in which further enrichment by crystalliza- of methods including thin-layer chromatogra- tion is not possible, chromatographic techniques phy – densitometry (TLC – DM) [35], gas are used for the separation of crude saponin chromatography – mass spectrometry (GC – mixtures. Methods such as high-performance MS) [36], LC – MS [37], HPLC [38], and radio- thin-layer chromatography (HPTLC), high- immunoassay (RIA) [39], have been reported for performance liquid chromatography (HPLC), and ginseng saponins and other compounds. droplet countercurrent chromatography (DCCC) [23] have gained wide acceptance. Methods have been reported for isolating saponins from roots of 2. Pharmacology ginseng (Panax schinseng) [24] and Bupleurum falcatum [25] by preparative HPLC of crude Toxicology. In contrast to poikilothermic extracts. animals, saponin toxicity to homeothermic spe- cies by oral intake is quite low (50 – 100 mg/kg). The significance of saponins as components of the 1.4. Structure Elucidation human diet or of animal feeds has been reviewed extensively [20]. Saponins exhibit different Historically, saponin structures were deduced spectra of toxicity on parenteral or intravenous after methylation and acid hydrolysis to application (LD50 0.7 – 50 mg/kg). The hemo- aglycones and partially methylated sugars. How- lytic action of many saponins prevents their ever, this method leads to artifacts, so milder intravenous administration, although detoxifica- hydrolysis methods have been developed. Rela- tion by complex formation with serum choles- tively large amounts of purified material are still terol, albumin, or other plasma constituents needed for classical structural analysis. reduces this effect. The advent of modern spectroscopic methods Membranolytic saponins combine irreversibly has simplified the task. Fast-atom bombardment with cell membrane systems and produce mass spectrometry (FAB – MS) facilitates the lesions [40]