Communicated by Daniel I. Arnon, April 19, 1982 ABSTRACT Fructose 2,6-bisphosphate (Fru-2,6-P2), a regulatory metabolite discovered in animal cells and recently reported to occur in etiolated seedlings, was found to be present in the cytoplasmic fraction of leaves of spinach and peas (typical C3 plants, in which a three-carbon carboxylic acid is a major early photosynthetic product). At concentrations approximating those calculated to occur physiologically, Fru-2,6-P2 modulated two enzymes of the leaf cytoplasm: (i) Fructose-1,6-bisphosphatase (EC 3.1.3.11), a key enzyme of sugar synthesis, was competitively inhibited by Fru-2,6-P2, and (ii) pyrophosphate-linked phosphofructokinase (inorganic pyrophosphate-D-fructose-6-phosphate 1-phosphotransferase, EC 2.7.1.90), a cytoplasmic enzyme that now seems important in glycolysis of C3 plants, was activated by Fru-2,6-P2. There was no indication of a role for Fru-2,6-P2 in photosynthesis ofeither chloroplasts or oxygenic prokaryotes. The results suggest that Fru-2,6-P2 functions in the regulation of glycolysis and glu-

Acetyl-coenzyme A (CoA) carboxylase from maize (Zea mays L.) is inhibited by nanomolar concentradons of both haloxyfop, an aryloxyphenoxypropionate, ad tralkoxydim, a cyclohexanedione herbicide.These results suggest that acetyl-CoA carboxylase, which catalyzes the first committed step in fatty acid biosynthesis, may be the target of these herbicides, contrary to an earlier report su ng that aryloxyphenoxypropionate herbicides do not inhibit acetyl-CoA carboxylase.Aryloxyphenoxypropionates and cyclohexanediones are two classes of herbicides that control many monocotyledoneous species. Although these herbicides are structurally very different, there has been some conjecture based upon similarities in selectivity and symptomology that these herbicides have a similar mode of action. There have been independent reports that these herbicides disrupt lipid metabolism (2, 6-1 1).Research by Hoppe (7-9) and Hoppe and Zacher (10, 11) has indicated that the aryloxyphenoxypropionate diclofop-methyl (2- [

No abstract

No abstract

Fructose-6-phosphate 2-kinase and fructose-2,6-bisphosphatase have been partially purified from spinach leaves and their regulatory properties studied. Fructose-6-phosphate 2-kinase was activated by phosphate and fructose 6-phosphate, and inhibited by 3-phosphoglycerate and dihydroxyacetone phosphate. Fructose-2,6-bisphosphatase was inhibited by fructose 6-phosphate and phosphate. The interaction between these effectors was studied when they were varied, alone or in combination, over a range of concentrations representative of those in the cytosol of spinach leaf cells. In conditions when dihydroxyacetone phosphate or 3-phosphoglycerate rise, as is typical during photosynthesis, the fructose 2,6-bisphosphate level will decrease, which will favour sucrose synthesis. In conditions when fructose 6-phosphate accumulates, fructose 2,6-bisphosphate should rise, which will favour a restriction of sucrose synthesis and promotion of starch synthesis.During photosynthesis, triose phosphates are exported from the chloroplasts and converted to sucrose in the cytosol. The first irreversible reaction in the cytosol is catalysed by the cytosolic fructose-l,6-bisphosphatase and Fru(2,6)P is a potent inhibitor of this enzyme [l, 21. In spinach leaves the level of Fru(2,6)P2 decreases when photosynthesis and sucrose synthesis are stimulated by illumination [3] and increases again when sucrose has accumulated [3]. These observations suggest that Fru(2,6)P2 is involved in the regulation of photosynthetic sucrose synthesis but are incomplete because they do not reveal the mechanisms whereby the concentration of Fru(2,6)P2 is being altered. In liver, Fru(2,6)P2 is synthesized and degraded by specific enzymes, the Fru6P 2-kinase and the Fru(2,6)P2 phosphatase [4, 51. Recently it has been shown that similar enzyme activities are present in spinach leaves [6, 71 and that they are modulated by metabolites. In the present article the regulation of Fru6P 2-kinase and Fru(2,6)P2 phosphatase has been studied in more detail, with particular attention being paid to assessing the effect that measured cytosolic concentrations [8 -101 of metabolites would have on the concentration of Fru(2,6)P2.