Karela (Momordica charantia):
Momordica charantia has been found effective in lowering blood glucose in man. The anti-diabetic activity of M. charantia is due to two active ingredients, Polypeptide -P and Charantin.
Polypeptide -P is a 17 amino acid polypeptide, 16 of which are similar to crystalline insulin of bovine origin. This polypeptide has been shown to be ' insulinomimetic.' A number of other polypeptides from M. charantia seeds have been studied in vivo for the insulin-like activities of stimulation of lipogenesis and inhibition of corticotropin-induced lipolysis. The mechanism was suggested to involve the interaction of peptides with a-adrenergic or corticotropin receptors.
The other active constituent, charantin is a mixture of two steroid glycosides: b-sitosterol-D-glucoside and 5, 25-stigmastadien-3-b-ol-D-glucoside. Charantin has shown anti-hyperglycemic activity in alloxan treated rabbits and depancreatized cats.
Studies performed in vitro with M. charantia fruit extracts indicated a significant enhancement of glucose uptake in muscle tissue and of glycogen accumulation in muscles and hepatic tissue but no effect on glucose uptake or triglyceride synthesis in adipose tissue. Inhibition of glucose uptake by intestinal fragments was also observed and attributed to a glycosidic constituent of the fruit extract.
M. charantia has been demonstrated to possess a dose-dependent increase in the scavenging activity against superoxide radicals and hydroxyl radicals. M. charantia is a potent scavenger of superoxide and Hydroxy radicals. The antidiabetic effect of the plant is mediated through an oxygen radical scavenging mechanism. Indeed the body itself possesses enzymes such as Superoxide dismutase which routinely protects the b-cells in the pancreas through their scavenging action. The free radical scavenging antidiabetic role of M. charantia supports an additional prophylactic role as many diabetogenic chemicals such as Alloxan, Streptozotocin, Pyrinuron, Food nitrosamines, cyanogenic glycosides such as Linamarin and other sources of Dietary cyanide induce diabetes through damage to pancreatic b-cells via free radical generation. 
Thus it appears that the therapeutic activity of M. charantia against diabetes is due to pancreatic, extra-pancreatic, and pancreas- protective properties.