The Weekly Newsmagazine of Science
Volume 155, Number 23 (June 5, 1999)
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Canola could provide
a new fat on the farm By C. Wu
Healthier margarine is on its way. Researchers at Calgene in Davis, Calif., have genetically engineered canola plants to make large amounts of a saturated fat called stearic acid. A solid at room temperature, stearic acid is used to make margarine, chocolate, baked goods, and many other foods. Food manufacturers need saturated fat to keep margarine solid at room temperature, and stearic acid is the only one in use that doesn't raise blood cholesterol concentrations (SN: 12/24&31/94, p. 442). Canola oil naturally contains just 1 percent stearic acid. To create more of this fat, manufacturers chemically process unsaturated plant oils—hence the words "partially hydrogenated vegetable oil" on many ingredient lists. This process also produces large amounts of unsaturated, trans fatty acids, which studies have linked to high blood cholesterol concentrations and heart disease (SN: 8/10/96, p. 87). The Calgene researchers set out to genetically engineer canola to make abundant stearic acid, so manufacturers can avoid producing trans fatty acids. Ling Yuan, now at Maxygen in Redwood City, Calif., and his colleagues Marc T. Facciotti of the University of California, Berkeley and Paul B. Bertain of Calgene report their findings in the June Nature Biotechnology. The researchers built upon earlier work by another Calgene group led by Jean C. Kridl. From a tropical plant called mangosteen, whose seeds contain large amounts of stearic acid, she and her coworkers isolated an enzyme that helps make the fatty acid. Inserting the gene for this enzyme, one of a family known as thioesterases, into canola boosted stearic acid production. "But it wasn't that high," says Yuan. His group then created mutant genes and tested which of their enzymes were most active in making stearic acid. They found several with a "dramatic increase in activity," says Yuan. Canola plants into which the team had introduced the best of these genes produce oil with almost 40 percent stearic acid. The enzymes allow stearic acid to accumulate in the plant by interrupting the biosynthesis of oleic acid, which makes up most of the fat in commercial canola oil. Oleic acid, a monounsaturated fat, consists of an 18-carbon chain containing one double bond. Stearic acid also has 18 carbons but no double bonds. During the biosynthesis of these two fatty acids, a protein supports the growing carbon chain as various thioesterases lengthen it. When the chain reaches 18 atoms, an enzyme called desaturase puts in the double bond to form oleic acid (SN: 5/31/97, p. 335). However, the thioesterase derived from mangosteen releases stearic acid before desaturase can turn it into oleic acid, explains Yuan. "The technology is terrific," says John Shanklin, a lipid biochemist at Brookhaven National Laboratory in Upton, N.Y. "It not only works in the test tube, but it works in the plant, and that's an important step. "The new paradigm is to figure out the activity you want, create an enzyme, and then put that in the plant," he says. "You're not constrained by the availability of enzymes in nature." The engineered canola will need further development before becoming a salable product. "In order to achieve really useful varieties, it requires a breeder to do classical genetic breeding," says Yuan. "What we do is just the beginning." |
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From Science News, Vol. 155, No. 23, June 5, 1999, p. 357. Copyright © 1999, Science Service. |
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