A β-ketoacyl-acyl carrier protein (ACP) synthase III (KAS III; short-chain condensing enzyme) has been partly purified from pea leaves. The enzyme, which had acetyl-CoA:ACP acyltransferase (ACAT) activity, was resolved from a second, specific, ACAT protein. The KAS III enzyme had a derived molecular mass of 42 kDa (from its cDNA sequence) and operated as a dimer. Its enzymological characteristics were similar to those of two other plant KAS III enzymes except for its inhibition by thiolactomycin. A derivative of thiolactomycin containing a longer (C8 saturated) hydrophobic side-chain (compound 332) was a more effective inhibitor of pea KAS III and showed competitive inhibition towards malonyl-ACP whereas thiolactomycin showed uncompetitive characteristics at high concentrations. This difference may be due to the better fit of compound 332 into a hydrophobic pocket at the active site. A full-length cDNA for the pea KAS III was isolated. This was expressed in Escherichia coli as a fusion protein with glutathione S-transferase in order to facilitate subsequent purification. Demonstrated activity in preparations from E. coli confirmed that the cDNA encoded a KAS III enzyme. Furthermore, the expressed KAS III had ACAT activity, showing that the latter was inherent. The derived amino acid sequence of the pea cDNA showed 81–87% similarity to that for other plant dicotyledon KAS IIIs, somewhat less for Allium porrum (leek, 71%) and for Porphyra spp. (62%), Synechocystis spp. (65%) and various bacteria (42–65%). The pea KAS III exhibited four areas of homology, three of which were around the active-site Cys123, His323 and Asn353. In addition, a stretch of 23 amino acids (residues 207–229 in the pea KAS III) was almost completely conserved in the plant KAS IIIs. Modelling this stretch showed they belonged to a peptide fragment that fitted over the active site and contained segments suggested to be involved in substrate binding and in conformational changes during catalysis, as well as an arginine suggested to participate in the acid–base catalytic mechanism.

Carbocylic coformycin (4) is a potent herbicide whose primary mode of action involves inhibition of adenosine 5′-monophosphate deaminase (AMPDA) following phosphorylation of the 5′-hydroxyl group in vivo. The search for more stable and accessible structures led to the synthesis of carbocyclic nebularine (8) and deaminoformycin (10). The latter compound is a good herbicide and its corresponding 5′-monophosphate 14 is a strong inhibitor of plant AMPDA (IC50 100 nM).

Novel inhibitors of histidinol dehydrogenase are described. The most potent inhibitors, compounds 18 (K-i* = 4.4 nM) and 19 (K-i* = 2.9 nM) exploit a hitherto unreported lipophilic binding pocket adjoining the active site. Preliminary SAR data for this pocket are detailed. The electrophilic ketone 6 designed to bind to an active site nucleophile was a considerably weaker inhibitor (IC50 similar to 20 mu M). Copyright (C) 1996 Elsevier Science Ltd

Novel inhibitors of histidinol dehydrogenase are described. The most potent inhibitors, compounds 18 (Fi∗ = 4.4 nM) and 19 (Ki∗ = 2.9 nM) exploit a hitherto unreported lipophilic binding pocket adjoining the active site. Preliminary SAR data for this pocket are detailed. The electrophilic ketone 6 designed to bind to an active site nucleophile was a considerably weaker inhibitor (IC50 ∼20μM).

A protocol to separate the intermediates on the biosynthetic pathway to pantothenate on HPLC is described. Feeding experiments with L-[U-14C]valine to pea leaf disks result in incorporation of radioactivity into 2-ketoisovalerate, ketopantoyl lactone, and pantoyl lactone, providing the first experimental evidence that the biosynthetic pathway to pantothenate is the same in plants and bacteria.

We have investigated the regulation of sucrose storage in cell-suspension cultures of sugarcane. When grown in batch culture, sucrose accumulation commences after about 5 d, when the nitrogen supply is exhausted. Sucrose storage is also induced by decreasing the nitrogen supply to cells growing in a chemostat. The measured activity of sucrose-phosphate synthase is high enough to account for the rate of sucrose accumulation, provided precautions are taken to avoid the hydrolysis of UDP during the assay. The cells contained high sucrose-synthase activity but pulsing experiments with [(14)C]glucose and unlabelled fructose indicated that this enzyme did not contribute substantially to the synthesis of sucrose, because the glucosyl and fructosyl moieties of sucrose were equally labelled. Several lines of evidence demonstrate the presence of a cycle in which sucrose is synthesized and degraded simultaneously; sucrosephosphate-synthase activity doubles during the phase when the cells are actively storing sucrose but activity is also high after storage has ceased, or when the sucrose is being remobilised; pulse experiments with [(14)C]fructose also showed that sucrose synthesis occurs not only during the storage phase, but also after storage has stopped and during the rapid mobilisation of sucrose; the cells contain high activities of sucrose synthase and alkaline invertase and these are both at a maximum when sucrose storage is occurring; even during the storage phase. [(14)C]fructose pulses lead to labelling of free glucose which is evidence for rapid synthesis and degradation of sucrose. It is proposed that the rate and extent of sucrose storage is regulated by this cycle of synthesis and degradation. Measurements of enzyme activities and metabolite levels are presented, and it is discussed which factors could contribute to the regulation of these two opposing fluxes and, hence, the rate of net sucrose storage and mobilisation.

Experiments were carried out to determine whether pyrophosphate: fructose-6-phosphate phosphotransferase (PFP) catalyses the rapid recycling of triose phosphates that is found in the cytosol of heterotrophic cell cultures of Chenopodium rubrum L. (W.-D. Hatzfeld, M. Stitt, 1990, Planta, 180, 198–204). Oxygen uptake, carbohydrate turnover, fructose 2,6-bisphosphate (Fru2,6bisP), glycolytic intermediates, adenine and uridine nucleotides, pyrophosphate and the activity of PFP and glycolytic enzymes were monitored for 48 h after subculturing carbohydrate-depleted cells onto glucose. Immediately after transfer there was an increase in the amount of Fru2,6bisP, and of the hexose phosphate. The triose phosphates, fructose-1,6-bisphosphate and inorganic pyrophosphate increased gradually over the next 24 h. This was accompanied by a tripling in the extractable activity of PFP, but not of phosphofructokinase. The activity of fructose-1,6-bisphosphatase was 20–50fold lower than that of PFP. It is calculated that the activity of PFP is high enough to catalyse the observed rate of cycling between the triose phosphates and the hexose phosphates, based on the measured Vmax capacity of the enzyme, the known kinetic properties, and the measured levels of its reactants and Fru2,6bisP. The changes in the levels of Fru2,6bisP were not correlated with the rate of respiration. Instead, the rate of O2 uptake was inversely related to the phosphoenolpyruvate level, showing that pyruvate kinase or phosphoenolpyruvate carboxylase are regulating the use of glucose for respiration. There was also no relation between Fru2,6bisP, and partitioning to sucrose or starch. It is proposed that the main function of the cycle in these cells is to maintain high levels of inorganic pyrophosphate and triose phosphates, which are necessary for the remobilisation of sucrose and for biosynthesis in the plastid, and that ‘coarse’ and ‘fine’ control of PFP play an important role in regulating this cycle.

We have investigated whether sucrose accumulation in heterotrophic cell-suspension cultures of Chenopodium rubrum L. is regulated by a cycle in which sucrose is simultaneously synthesised and degraded. Net sucrose accumulation was measured by monitoring the sucrose content, unidirectional synthesis was monitored by supplying pulses of [14C] glucose, and unidirectional degradation was estimated from the difference between unidirectional synthesis and net accumulation. When 50 mM glucose was supplied to carbohydrate-depleted cells there was a rapid net accumulation of sucrose, which stopped after 24 h. The incorporation of 14C into sucrose was similar to the initial rate of net sucrose accumulation, but rapid 14C incorporation continued after the cells had stopped accumulating sucrose. A method was developed to rapidly separate sucrose-phosphate synthase (SPS) from uridine-diphosphate-hydrolysing activities which interfered with the assay. The cells contained enough SPS activity to catalyse the observed rate of sucrose synthesis. SPS activity increased in cells which had stopped accumulating sucrose, and the enzyme became less sensitive to inhibition by inorganic phosphate. Sucrose synthase and alkaline invertase activity were four- and twofold higher than SPS activity, and both degradative enzymes increased in cells which had stopped accumulating sucrose. Sucrose synthase is strongly modulated by the concentration of sucrose and by competitive feedback regulation by fructose in these cells. It is concluded that sucrose accumulation ceases in these cells because the rate of degradation of sucrose increases until it matches the rate of synthesis. It is discussed how this cycle is regulated, and how it may interact with the substrate cycle between triose-phosphates and hexose-phosphates (Hatzfeld and Stitt, 1990, Planta 180, 198–204). These cycles allow sucrose turnover to respond in a highly sensitive manner to small changes in the balance between the supply of sucrose and the demand for carbon for respiration and biosynthesis in the cell.

This work provides further evidence that plants contain appreciable amounts of inorganic pyrophosphate (PPi), and that breakdown of phosphoribosyl pyrophosphate (PPRibP) does not contribute significantly to the PPi detected in plant extracts. Inorganic pyrophosphate in extracts of the roots of Pisum sativum L., clubs of the spadices of Arum maculatum L., and the developing endosperm of Zea mays L. was assayed with pyrophosphate fructose 6-phosphate 1-phosphotransferase (EC 2.7.1.90), and with sulphate adenyltransferase (EC 2.7.7.4). The two different assays gave the same value for PPi content, and for recovery of added PPi. It was shown that PPRibP is converted to PPi during the extraction of PPi. However, the amounts of PPRibP in clubs of A. maculatum and the developing endosperm of Z. mays were negligible in comparison with the contents of PPi.

The aim of this work was to discover the role of pyrophosphate : fructose-6-phosphate 1-phosphotransferase [PFK(PP;)]. Substrates and enzymes of sucrose breakdown were measured during starch accumulation by the endosperm of wild type and the sh 1 shrunken mutant of Zea mays L. The results suggested that the mutant depended primarily on alkaline invertase to break down sucrose, and showed that the activities of PFK(PPi) and UDPglucose pyrophosphorylase and contents of pyrophosphate and fructose2, 6-bisphosphate (Fru-2,6-P2) were no lower in the mutant than in the wild type. Appreciable activities of PFK(PPi), which were not responsive to Fru-2,6-P2, were found in the following red algae, which lack sucrose: Audouninella purpurea, Polysiphonia sp., Mastocarpus stellatus, Rhodymenia pseudopalmata. No PFK(PP;) was found in the bacterium Paracoccus denitrificans. Addition of sucrose to starved suspension cultures of Glycine max L. led to increases in oxygen uptake and Fru-2,6-P2 content. Glucose produced the same effects. It is argued that, although one of the functions of PFK(PP;) is production of pyrophosphate for sucrose breakdown via sucrose synthase, the key role of the enzyme is the maintenance of the cytosolic concentration of pyrophosphate according to the supply and demand of the latter.

This work was done to determine whether the inorganic-pyrophosphate (PPi) content of plant tissues changes when the rate of glycolysis is altered. Treatment of excised clubs of the spadix of Arum maculatum L. and root apices of Pisum sativum L. with 2,4-dinitrophenol increased the rates of respiration but had no detectable effects on PPi contents. When the two tissues were subjected to up to 60 min anoxia, no changes in PPi were detected. Anoxia was shown to lead to a fall in ATP and concomitant rises in ADP and AMP in pea roots. It is argued (i) that variation in the rate of glycolysis was not accompanied by detectable changes in PPi content, (ii) that this observation does not favour the view that pyrophosphate fructose 6-phosphate 1-phosphotransferase mediates appreciable entry into glycolysis, and (iii) that PPi content can be maintained when respiratory-chain phosphorylation is inhibited.

Experiments were carried out to provide direct evidence that pyrophosphate: fructose‐6‐phosphate phosphotransferase (PFP) can operate as a glycolytic enzyme in some circumstances. A large increase in the rate of glycolysis was produced by adding uncoupler and alkalinising the medium of heterotrophic Chenopodium rubrum cells. Initially, a marked decrease in phosphoenolpyruvate, 3‐phosphoglycerate and hexose phosphates occurred, but no change in fructose 2,6‐bisphosphate or inorganic pyrophosphate was observed. However, a gradual increase in the rate of O2 uptake during the subsequent 5 min was accompanied by an increase in fructose 2,6‐bisphosphate and a decrease in pyrophosphate, providing evidence that activation of PFP contributes to the increase in rate of glycolysis. This was accompanied by partial recovery of the adenine nucleotide energy status.