For centuries, the Chinese have used an extract from the ancient ginkgo tree to alleviate health problems ranging from asthma to Alzheimer's disease.

For years, Western scientists intrigued by the possible usefulness of the extract have been trying to synthesize and study the active ingredient in the tree's leaves.Finally, researchers at Harvard University have succeeded where others failed, making possible development of the extract into a medicine.

Chemist Elias Corey and his colleagues have synthesized - determined the chemical structure of - what is believed to be the most important active incredient in the fan-shaped leaves, a complex substance known as ginkgolide B.

Drug companies could use the structure described by Corey to produce the substance synthetically much more cheaply, he said. In fact, one French company has already started producing a synthetic version, he said.

Although scientists are unsure how the substance may have beneficial health effects, ginkgolide B is believed to block the activity of a substance known as platelet activating factor, which apparently plays an important role in inflammation.

Studies have indicated the substance may be effective for regulating blood pressure, treating kidney diorders, circulatory problems, and various forms of shock, inflammation, eye diseases and possibly as an antidote to some toxins.