Triterpenes form a large group of lipid substances found in all plants. There are approximately 750 known triterpenes.
Triterpenes arise via dimerization of two farnesyl pyrophosphate units (15 carbons) to produce an intermediate compound called SQUALENE (30 carbons). Squalene can cyclize to form CHOLESTEROL and other steroid hormones.
Cholesterol occurs in all plants. Other important steroid compounds found in plants include: PHTOECDYSONES; PROGESTAGENS (pregnenolone and progesterone); and BETA SITOSTEROL.
Progestagens (21 carbon steroids) occur as animal hormones in addition to being found in plants. Progestagens and beta sitosterol give rise to a number of other types of steroids including the CARDENOLIDES.
CARDENOLIDES - CARDIAC GLYCOSIDES
Cardenolides have 23 carbon atoms and a lactone ring. They are formed by condensation of pregnenolone or progesterone with acetate. The sugar moieties of cardenolides are normal sugars or unbranched aldohexoses. About 50 cardenolides exist and are primarily found in: Apocynaceae, Asclepiadiaceae, Brassicaceae, Celastraceae, Euphorbiaceae, Liliaceae, Moraceae, Ranunculaceae, Scrophulariaceae, Sterculiaceae, and Tiliaceae.
Cardiac glycosides affect the dynamics and rhythm of dysfunctional heart muscle. Examples of important cardiac glycosides used in man include: digitoxin, digoxin, gitoxin, strophanthidin, hellebrin, scillaren.
Important sources of the digitalis glycosides (digoxin, digitoxin) include: Digitalis lanata (white foxglove) and Digitalis purpurea (red foxglove). Digitalis drugs are agents used for people in congestive heart failure.
CUCURBITACINS
CUCURBITACINS are classified as tetracyclic triterpenoids. First found in the seeds of particular Curcubita species (i.e., cucumber, pumpkin, summer squash) are generally bitter, but some are sweet, and some have cytotoxic, antitumor, and antileukemic activities.
Cucurbitacins are also derived from SQUALENE. They are found in: Begoniaceae, Brassicaceae, Cucurbitaceae, Primulaceae, Rosaceae, and Scrophulariaceae.
Sapogenins are combined with sugars to form saponins. Generally, they include up to five sugar units and are always attached to the third carbon.
Cholesterol and beta-sitosterol are precursors to the formation of the sapogenins.
Sapogenins are plant glycosides which lather in water and are used in detergents, or as foaming agents or emulsifiers. They have antifungal, antimicrobial and some have antiinflammatory action. Glycyrrhizin, from licorice root, is an example of a saponin used for antiinflammatory purposes in place of cortisone. Aescin also has antiinflammatory action.
TETRATERPENES (40 carbons)
Carotenoids, or 40-carbon tetraterpenoids, are lipid-soluble terpenes found in all forms of plants. Their value to animals comes from the splitting of the C40 molecule into the twenty-carbon isoprenoid alcohol known as vitamin A.
Vitamin K, another higher isoprenoid vitamin, is required by mammals for prothrombin synthesis and blood clotting.