Chemical Constituents
The constituents of primary interest in Ginkgo leaf are ginkgolides and flavonoids (Huh and Staba, 1992).
Flavonoids present in Ginkgo leaf include flavones, biflavones, flavonols, tannins, and associated glycosides (Huh and Staba, 1992). In general, flavonoids are principally found in the leaves, although they are also present in many other parts of the plant. There are about 20 flavonoid glycosides along with glucosides, quercetin and kaempferol 3-rhamnosides and 3-rutinosides, p-coumaric esters of glucorhamnosides of quercetin and kaempferol (most with a 1´´´->2´´ bond between sugars), and biflavones (all 3´->8´´ biflavones). The biflavones are amentoflavone, bilobetol, 5-methoxybilobetol, ginkgetin, isoginkgetin, and sciadopitysin. The plant part with the highest acylated flavonoid content is the bud, with a content 3 times greater in the fall than in the spring. The monomer content is also highest in the fall (Huh and Staba, 1992).
Ginkgo leaf extracts differ in flavonoid content. One of the main flavone marker chemicals used for standardization is the unique flavonoid, quercetin 3-0-(6´´´-p-coumaroyl-glucosyl-1,4 rhamnoside) (Huh and Staba, 1992).
Ginkgolides (ginkgolides A, B, C, J, and M) are diterpenes: C (20) cage molecules that contain 6 rings of 5 carbons each, which involve a spirononane system, a tetrahydrofuran, 3 lactone groups, and a tert-butyl group (Huh and Staba, 1992). Terpene lactone ginkgolides only occur in the Ginkgo tree (Chang and Chang, 1997).
One interesting way to standardize and differentiate Ginkgo leaf extract preparations was proposed by Steinke et al. (1993), where the inhibition and binding of platelet-activating factor (PAF) by ginkgolides A, B, C and J is used as an in vitro biological assay.
Ginkgo leaves contain flavan-3-ols, proanthocyanins (Huh and Staba, 1992) and poly-isoprenoid-derived betulaprenols (similar to dolichol) in unusually high amounts. Other compounds of interest in the leaf include ginnon, ginnol, 2-hexenal, and bilobalide (a terpenoid). In the heartwood, the essential oil is comprised of monoterpenes and sesquiterpenes; the seed contains bilobol, ginkgolic acid (phenolics), and 4´-methoxypyridoxine (Huh and Staba, 1992).