Researchers with the U.S. Department of Agriculture have identified another compound in grapes that they believe shows promise in fighting cancer. The compound, pterostilbene, is similar to resveratrol, an antioxidant recently found in grapes and red wine that has also been linked to cancer-prevention, they say.

Previous studies by others have demonstrated that pterostilbene also has antidiabetic properties. The current study is the first to identify it as a cancer-preventive agent, the researchers report. Their study is tentatively scheduled to appear in the June 19 issue of the Journal of Agricultural and Food Chemistry, a peer-reviewed publication of the American Chemical Society, the world's largest scientific society.

"The study adds to the growing health benefits of grapes," said Agnes M. Rimando, Ph.D., lead investigator for the study and a research chemist with the USDA's Natural Products Utilization Research Unit in Oxford, Miss.

The research, limited to laboratory studies of cells, could lead to more healthful varieties of grapes and the design of better drugs to fight cancer and diabetes, two of the major medical problems in this country, says Rimando. However, she cautions that studies will be needed in both animals and humans to evaluate the potential health benefits of pterostilbene, whose biological activities are not well understood.

Prompted by the structural similarity of pterostilbene to resveratrol, Rimando decided to test the compound to determine if it had similar anticancer and antioxidant activities.

In lab tests using mouse mammary cells, pterostilbene prevented a type of cell damage that is normally induced by cancer-causing agents, she says. Earlier studies by the researcher showed that the compound is toxic to a human breast cancer cell line.

In conclusion, the study says that pterostilbene is equal in potency to resveratrol as a cancer-preventive agent as both have shown similar anticancer activity in lab tests, says Rimando. Their similar effect is likely due to the high antioxidant activity of these compounds, she says. Antioxidants destroy free-radicals, highly reactive molecules whose excess has been linked to cancer.

Pterostilbene has another benefit not found in resveratrol. Animal studies by others have shown that the compound can lower blood glucose and may be a potent antidiabetic agent. At least one study showed that it could lower plasma glucose levels in rats with high blood sugar by 42 per cent, comparable to at least one known antidiabetic agent (metformin), according to the researcher.

"My study is saying that there's another compound in grapes with equal cancer-fighting power as resveratrol, but which has antidiabetic properties as well," said Rimando. "The compound has always been in grapes, but no one has paid much attention to it." Both pterostilbene and resveratrol belong to a group of chemicals called phytoalexins, which are produced by plants in response to fungal infection, ultraviolet light, and various chemical and physical stressors. While both exhibit strong antifungal activity, pterostilbene appears to be 60 to 100 times more potent as a fungicide, a property that may one day be exploited by farmers in search of a more disease-resistant grape, says Rimando.

In plants that contain these chemicals, their content usually differs dramatically. Quantitative studies have shown that for every 10 parts resveratrol, there's only about one to two parts pterostilbene. The relationship between the two similar compounds and their unequal content in plants is unclear, but remains the subject of ongoing studies, the researcher says.

Resveratrol has been found in many fruits, including blueberries and cranberries, but it is perhaps best known for its presence in grapes and red wine. Pterostilbene has so far been identified in grapes and in a relatively unknown medicinal plant, according to Rimando.

Dark-skinned grapes (such as red and blue-black) are likely to contain the most pterostilbene, while green grapes (also called white grapes) probably contain less, she says.

For reasons that are unclear, pterostilbene is not normally found in wine, Rimando says. This may be because it is unstable in light and air, which makes it less likely to survive the wine-making process, she says.

The researcher is currently investigating other fruits and fruit juices to determine whether they contain pterostilbene, but results are not yet available.