Triterpenoid, numbering

Terpenes (and terpenoids) are further classified according to the number of 5-carbon units they contain. Thus, monoterpenes are 10-carbon substances biosynthesized from two isoprene units, sesquiterpenes are 15-carbon molecules from three isoprene units, diterpenes are 20-carbon substances from four isoprene units, and so on. Monoterpenes and sesquiterpenes are found primarily in plants, but the higher terpenoids occur in both plants and animals, and many have important biological roles. The triterpenoid lanosterol, for example, is the precursor from which all steroid hormones are made. 

Triterpenoid saponins are synthesized via the isoprenoid pathway.4 The first committed step in triterpenoid saponin biosynthesis involves the cyclization of 2,3-oxidosqualene to one of a number of different potential products (Fig. 5.1).4,8 Most plant triterpenoid saponins are derived from oleanane or dammarane skeletons although lupanes are also common 4 This cyclization event forms a branchpoint with the sterol biosynthetic pathway in which 2,3-oxidosqualene is cyclized to cycloartenol in plants, or to lanosterol in animals and fungi. 

A number of important resins are composed of triterpenoids, including the dammar resins which derive from a sub-family of the family Dipterocarpaceae. Dammar resins are fluid, balsamic oleoresins highly suited for caulking and waterproofing. Frankincense (olibanum) is known as a gum-resin collected from various Boswellia spp. and contains amyrin epimers and triterpenoid acids. The gum component is polysaccharide in origin and is water soluble. The Anacardiaceae family contains the genus Pistacia (Mills and White, 1977 21 Mills and White, 1989). 

The interaction between Drosophila, yeasts and columnar cacti of the Sonoran Desert has been the subject of much recent interest (37) As a coevolved system, perhaps more is known about this interaction than any other. The chemistry of the cacti (70 spp.) has been postulated to play a significant role in the establishment of this system (38 ), but this was based upon reports of a relatively small number of relatively simple alkaloids, and a small number of terpenoid compounds. Only recently, the diversification of plant compounds has been discovered to be much greater (5). In their section on alkaloids in the above work, BajaJ and McLaughlin report the presence of some thirty-five structures. In addition, I have been able to isolate some sixty triterpenoid glycosides (structures were not determined) and more were detected but not Isolated. 

Currently there is no experimentally determined three-dimensional structural information available for OSCs, although studies with a related enzyme, squa-lene-hopene cyclase (SC EC 5.4.99.7) have proved informative. SCs are involved in the direct cyclisation of squalene to pentacyclic triterpenoids known as hopanoids, which play an integral role in membrane structure in prokaryotes [ 51 ]. A number of SC genes have been cloned from bacteria [52 - 54]. The SC and OSC enzymes have related predicted amino acid sequences, and so should have similar spatial structures [55]. The crystal structure of recombinant SC from the Gram-positive bacterium Alicyclobacillus acidocaldarius has established that the enzyme is dimeric [55]. Each subunit consists of two a-a barrel domains that assemble to form a central hydrophobic cavity [55,56]. 

Comparison of the amino acid sequences of OSCs that generate sterols and triterpenoids reveals a number of residues in addition to the DCTAE and QW motifs that are highly conserved in all classes. These residues may be required to open up the epoxide ring of 2,3-oxidosqualene, a catalytic step that is common to all of these enzymes [46]. Amino acid residues that are conserved exclusively in sterol OSCs or in triterpenoid OSCs maybe required for formation of the pro-tosteryl or dammarenyl cations, respectively. 

Now that a number of cloned CS, LuS and /lAS genes are available it will be possible to generate transgenic plants in which the levels of these enzymes have been manipulated. The effects of altering the levels of these different OSCs on sterol and triterpenoid synthesis can then be assessed. 

A number of triterpenoids are bioactive compounds and are used in medicine. For example, fusidic acid is an antimicrobial fungal metabolite, isolated from Fusidium coccineum, and cytotoxic dimeric triterpenoids, crellastatins, are isolated from marine sponges Crella species. 

The in vitro EBV-EA activation inhibition assay uses EBV genome-carrying lymphoblastoid cells (Raji cells derived from Burkitt s lymphoma). Many compounds which inhibit EBV-EA induction by tumor promoters have been demonstrated to act as inhibitors of tumor promotion in vivo [16,22,42,66-68]. This assay has an advantage since it obtaines the information on die cytotoxicity from the viability of Raji cells. High viability of these cells is an important factor in developing a compound for the chemoprevention of cancer [16]. So far, a number of triterpenoids and steroids from plants and their derivatives have been shown to possess... 

As discussed in the previous sections, a number of triterpenoids with great structural diversity have antitumor-promoting activities. In contrast, 28-deacetylbelamcandal (288), a spiroiridal-type triterpenoid, has recently been reported to possess tumor promoting activity [92]. Compound 288, which stimulated differentiation of human promyelocytic leukemia (HL-60) cells (a fast method for screening TPA-type tumor promoters),... 

Saponins are found in many major food crops and include a large number of examples of constitutive plant compounds with antimicrobial activity. Their name refers to the property of forming a stable foam when shaken with water, and they consist of triterpenoid, steroid or steroidal glycoalkaloid molecules bearing one or more sugar chains. Excellent general reviews on saponins have been published recently [2-6]. 

The role of lanosterol in non-photosynthetic organisms (animals, fungi) is taken in photosynthetic organisms (plants, algae) by the cyclopropane triterpenoid cycloartenol (Figure 5.55). This cyclopropane feature is found in a number of... 

From the point of interest of antifeedants, triterpenoids can be divided into three classes, depending on the number of carbon atoms in the skeleton there are full C-30 triterpenes, C-26 limonoids, and C-20 quassins. In addition, there are saponins, which can have either a triterpenoid or a steroid skeleton. Interesting and powerful antifeedants are found in each class. 

---