Medicinal Plants Classification Biosynthesis and ... - Index of

Gastroprotective Triterpenoids: Pharmacological Mechanism
[120]. The enzymes that catalyze these reactions are known as triterpene synthases and can
be subdivided as squalene cyclases (SC) or oxysqualene cyclases (OSC), which convert
squalene and oxyqualene to cyclic triterpenes and triterpene alcohols. Squalene was isolated
by first time from the liver oil of shark (Squalus sp.). Then, it was found in rat liver and yeast.
Triterpenoids are classified according to number of cyclic ring in their chemical
structure. Most triterpenoids are 6-6-6-5 tetracycles, 6-6-6-6-5 pentacycles or 6-6-6-6-6
pentacycles but acyclic, monocyclic, bicyclic, tricyclic and hexacyclic triterpenoids have also
been isolated from natural sources.
Examples of monocyclic triterpenoid are achilleol A [121] and camelliol C [122].
Bicyclic triterpene have limited taxonomic distribution, species from Cratoxylum and
Pistacia have bicyclic triterpene [123]. However, the experimental biosynthesis of these
compounds has not been verified.
There is not evidence of natural tricyclic triterpenes, they just have been generated in the
laboratory by A. acidocaldarius squalene-hopene cyclase mutant [124]. Lanosterol and
cycloartenol are tetracyclic triterpenoid found in several plant sources. While lupine,
germanicane, taraxastane, α-amyrin, β-amyrin and ursane are some examples of pentacyclic
triterpenes [123].
5.2. Kind of Triterpenoids with Antiulcer Activity
Gastroprotective triterpenes have been isolated from several plants, for example lupeol
acetate, ursolic acid, taraxerol were isolated from Fabiana imbricata [125, 126] and Protium
heptaphyllum [127], and 18-β-glycyrrhetic acid from Glycyrrhiza glaba [128]. Which for
clinical use 18-β-glycyrrhetic acid (enoxolon) was replaced by its soluble succinate sodium
salt, carbenoxolone [129].
In regards to its chemical structure of triterpenoids, it has been proposed that a hydroxyl
group at position C-3 (free or derivatised) is necessary for sterols and triterpenoids to exhibit
antiulcer activity. This was proposed based in the experiments where the gastroprotective αamyrin,
β-amyrin, β-sitosterol and its glycoside, isolated from methanol extract of H. excelsa
have a hydroxyl group at position C-3 on their chemical structure. In contrast, triterpenoids
such as friedelin, cariophyllal and cariophillol do not possess the hydroxyl group in the
position C-3 either they do not present gatroprotective activity [130].
Furthermore, oleanolic acid, ursolic acid, sericic acid and taraxerol are compounds that
exhibit antiulcer activity and they contain a free hydroxyl group at C-3. Moreover,
glycyrrhizic acid, carbenoxolone, lupeol acetate, sericoside and several triterpenoid saponins
have a hydroxyl derivative at C-3 and protect gastric mucosa against injury [128].
More triterpenoids have exhibited gastroprotective activity (Figure 4); for example, 3-Oacetyl
aleuritolic acid has been reported as a constituent of several Euphorbiaceae species.
This compound is the main component of Croton cajuara which reduces gastrointestinal
transit in mice [131]. This substance was isolated from the rhizomes of the Paraguayan crude
drug Jatropha isabelli, it exhibited gastroprotective activity in the HCl/EtOH induce gastric
lesions in mice [132]. Besides, boswellic acid is an enriched mixture of tetra- and penta-
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