n e w s of t h e w e e k *&'

ο OH G D P

GDP-L-galactose

lactoi

GMP

L-Galactose-1-P

L-Galactose

plant growth, develop­ment, and protection, as well as in human nutri­tion, the work represents "one of the most impor­tant discoveries in plant science in recent times," says Christine H. Foyer of the Institute of Grass­land & Environmental Research, Aberystwyth, Wales, and an expert on the protective effects of ascorbic acid on plants.

The Exeter work "is a major breakthrough," concurs Robert L. Last, who points out that de­spite the abundance of vitamin C in plants, the pathway "has been very difficult to sort out." Last is a professor of genetics at the Boyce Thompson Institute for Plant Research, Cornell University, Ithaca, N.Y.

It is known that the ultimate precursor of L-ascorbic acid in both animals and plants is glucose. But be­cause glucose metabolism in plants leads to more diverse products than in animals, labeling experiments with plants often have been uninformative. "You are lucky if you get 2% of the labeled glucose into ascorbate in plants," says Last.

On the other hand, the pathway in an­imals is well understood. A lactone de­rived from L-gulonic acid is the immediate precursor of L-ascorbic acid. L-Gulonic acid comes from i>glucose through a series of reactions, including one that involves in­version of the sugar carbon skeleton.

"People got fixed on the idea" that a similar inversion must also occur in plants and that "L-galactono-l,4-lactone had to be made from D-galactose," says Smirnoff. But no evidence supporting that idea was ever found.

For the Exeter team, the break­through came when they observed that leaves fed with L-galactose synthesize vi­tamin C very rapidly. It is known that the penultimate step in the pathway in plants is the oxidation of L-galactono-1,4-lactone. But "nobody knew where the lactone came from," says Smirnoff. The answer appears to be L-galactose.

Smirnoffs group provides "exciting new evidence that L-galactose, a hexose hitherto regarded as rare in plant metabo­lism, is a committed intermediate and is indeed a precursor of L-galactono-1,4-

L-Galactose is a key intermediate in plant biosynthesis of vitamin C

CH2OH

D-Glucose-6-P

GDP GDP-D-mannose

CHoOH I

HCOH^O

NAD+ V NADH

HO/

OH 2H

L-Galactono-1,4-lactone

CHoOH I

HCOHO

OH OH L-Ascorbic acid

Key: Ρ = phosphate, GDP = guanosine diphosphate, GMP = guanosine monophosphate, P| = orthophosphate, NAD* = oxidized nicotinamide adenine dinucleotide, NADH = reduced nicotinamide adenine dinucleotide Enzymes: 1 = GDP-D-mannose-3,5-epimerase, 2 and 3 = unidentified, 4 = L-galactose dehydrogenase, 5 = L-galactono-1,4-lactone dehydrogenase Note: The conversion of D-glucose-6-phosphate to GDP-D-mannose proceeds through reactions that are well established in carbohydrate metabolism.

lactone," says Frank A. Loewus, emeritus professor of biological chemistry at Wash­ington State University, Pullman, and a pi­oneer in the study of ascorbate metabo­lism in plants. Moreover, the group shows that plants convert guanosine diphosphate (GDP>D-mannose to L-galactose, presum­ably through GDP-L-galactose. This step converts the sugar from a D to an L config­uration without inverting the carbon chain but simply by flipping the positions of the groups at carbons 3 and 5.

"Once we had established that plants could use L-galactose, then it was just a question of finding the enzyme that con­verts L-galactose to L-galactono-1,4-lactone," says Smirnoff. The group subse­quently isolated the enzyme, called L-ga­lactose dehydrogenase.

Smirnoff and coworkers also propose a two-step conversion of GDP-Lgalactose to L-galactose, which was still speculative when they submitted their Nature paper. However, Smirnoff tells C&EN that just last week they obtained evidence for L-ga-lactose-1-phosphate, the intermediate in this conversion. The group is now at­tempting to isolate the two still-unidenti­fied enzymes involved in this part of the pathway.

The findings of Smirnoff and coworkers are not only "a landmark advance in the knotty problem of ascorbic acid biosynthe­sis in plants," says Loewus, they also "open many new opportunities to explore L-ascor­

bic acid biosynthesis in other photosynthetic or­ganisms." They will also shed light on the role of L-ascorbic acid in the biosynthesis of plant or­ganic acids and in the regulation of complex physiological events, such as light-related re­sponses and cell divi­sion, he adds.

Elucidation of the pathway not only fills a big gap in the under­standing of a very com­mon and important nat­ural molecule, but also paves the way to genet­ic engineering of plants, says Patricia L. Conklin, a research associate working with Last. They are now working to elu­cidate ascorbate biosyn­thesis through mutant plants that are defective in making ascorbic acid.

"We can use the mutants to get at the genes and enzymes involved in the path­way," says Conklin. "Eventually, one could engineer plants to make more vitamin C to increase not only their nutritional value but also—[because vitamin C is an antiox­idant]—their resistance to environmental stresses such as drought, chilling, and air pollutants, which cause plants to generate active oxygen species," she explains.

Having worked out the pathway through classical biochemical techniques, Smirnoff says, "what we're hoping to do now is to work [with Last's group] to see if their mutants are affected in any of the enzymes that we predict ought to be in the pathway."

But even while the pathway remains to be confirmed, Foyer calls "the evi­dence provided in support of its exist­ence sound and convincing."

Maureen Rouhi

Trade surplus plummets as chemical imports soar The U.S. chemical trade surplus was al­most halved in March on a year-to-year ba­sis as imports hit their highest level ever on a less than 1% rise in exports.

According to the latest data from the Department of Commerce, the value of U.S. chemical exports totaled $6.00 billion

10 JUNE 1, 1998 C&EN

in March, up just 0.3% from March 1997, whereas imports soared 20% to $5.01 bil­lion. Thus, the chemical trade surplus, long a boasting point for the industry, dropped 46% to $990 million.

On a month-to-month basis, the chemi­cal trade surplus was off about 20% from February, continuing a downward trend. Exports were up 10%; imports increased 19%.

The huge March decline pushed the surplus for the first quarter of the year down to about $3.48 billion, a 25% drop from the same three months of 1997. Ex­ports for the quarter totaled $17.1 bil­lion, up 3%, and imports rose 13% to $13.7 billion.

Observers blamed much of the lacklus­ter export performance in the quarter on lack of demand in the Asia-Pacific region, but they were less specific about the rea­sons for the rise in imports. They note that imports of chemicals from Asia have in­creased—chemical industries there are try­ing to raise needed currency—but not by as much as had been expected. Chemical imports from Asia rose only 5% in March from February levels to about $913 million compared with the overall 19% increase in chemical imports.

Three major chemical categories ei­ther slipped into trade deficits or saw deficits deepen in March, according to the Commerce Department data. Particu­larly worrisome is the medicinal chemi­cals category, which includes pharmaceu­ticals and over-the-counter medications. Imports of medicinals in March rose 21% from February and 36% from the same month last year to $904 million, resulting in a trade deficit of $179 million, deepen­ing February's deficit by $126 million.

During the first three months of this year, the medicinal chemicals deficit to­taled $271 million, compared with a sur­plus of $68 million in first-quarter 1997. Trade experts note three trends that could account for rising imports of me­dicinal chemicals: more products being shipped by foreign companies to their U.S. subsidiaries as the number of foreign-owned U.S. drug operations increases; in­creased levels of generic pharmaceutical imports; and the increasing number of off­shore pharmaceutical operations owned by U.S. companies shipping products into the U.S. They cite Ireland as being a prime site for U.S.-owned drug manufacturing plants.

Allen J. Lenz, managing director of economics for the Chemical Manufactur­ers Association, notes that, based on cur­rent trends, the trade deficit in medici-

March chemical imports hit all-time high $ Billions 81

Exports

0

Imports

- j _

J F M A M J J A S O N D J F M 1997 1998

Source: Department of Commerce

nals this year "could easily surpass $1 bil­lion." Medicinals posted a trade deficit of $507 million in 1997.

Inorganic chemicals slipped into a deficit of $98 million in March from a $34 million surplus in February and a $24 million surplus in March of last year. The biggest deficit—$373 million-—oc­curred in the organic chemicals catego­ry, which had a $216 million deficit the month before and a deficit of $53 million in March 1997.

What's ahead for the year is probably best summed up by Lenz: "The situation may improve, but at this point the out­look is for export growth that, at best, will be modest; strong import growth; and a significant decline in the chemical trade surplus."

William Storck

Environmental speed bumps don't slow highway bill

As C&EN went to press last week, Presi­dent Bill Clinton had said he would sign a $216 billion, six-year national highway spending bill—the Transportation Equity Act for the 21st Century. Although the bill's central purpose is to fund highway and mass-transit projects, it also became a vehicle for amendments that serve oth­er— frequently unrelated—purposes, in­cluding three controversial environmen­tal amendments that surfaced at the last minute during the final House-Senate conference on the bill.

In a terse letter to the conferees, Envi­ronmental Protection Agency Administra­tor Carol M. Browner wrote that this 11th-hour approach "would not lend itself to the careful consideration of important public health and environmental issues that the public deserves." She also warned that, in some cases, "public health may be in immediate jeopardy."

Whether or not owing to Browner's plea, the conferees did block the most no­table of the three amendments. That one would have delayed—pending the results of a National Research Council study-dredging of contaminated sediments from the Hudson River near Albany, N.Y., and the Housatonic River near Pittsfield, Mass. The rivers' sediments are contaminated with polychlorinated biphenyls (PCBs) from two General Electric plants.

PCBs are suspected of causing cancer in humans and, according to EPA, pose a number of serious noncancer health risks, particularly to children and preg­nant women. But dredging the rivers could cost GE $1 billion, and there are no assurances that the dredging won't cause more environmental problems than it solves.

Another amendment that bit the dust would have indefinitely delayed EPA's plans to regulate emissions from paints and coatings (see page 33). It would have required the agency to complete a lengthy review of the impact of emissions of vola­tile organic compounds from consumer and commercial products on ozone levels. Neither states nor the federal government could have adopted rules to limit volatile organic compounds in paints until the study was complete.

However, the third last-minute amend­ment did make it into the highway bill. And it's one that Browner may find ac­ceptable since it essentially codifies EPA's schedule for implementing the July 1997 particulate matter air quality standards. It directs EPA to set up and pay for a na­tionwide fine-particle monitoring net­work by December 1999 and to collect three years' worth of data. States will have until Dec. 31, 2008, to develop plans for meeting the standards. In addition, the amendment coordinates the implementa­tion schedules for fine-particulate air stan­dards with regulations aimed at control­ling regional haze.

The bill also extends until 2007 a par­tial gas tax exemption for fuel mixed with ethanol. The tax break, which would have expired in 2000, exempts these mixtures from 5.4 cents of the gas tax.

Linda Raber

JUNE 1, 1998 C&EN 11