ABSTRACTpropose that they should be more correctly termed sucrolysis and sucroneogenesis. Before recent work it was customary to assume that sucrose synthase action resulted in the formation of UDP-glucose and then other nucleotide sugars leading into sugar polymer synthesis, such as plant cell walls. However, a substrate level pool of PPi was measured in plants (2,7,27), and we successfully tested the pyrophosphorolysis of UDPglucose feeding glucose 1-P directly into plant metabolism (32). These steps are an integral part of the recently proposed sucrose synthase pathway (1,15,30). Then, of course, from glycolysis carbon can be directed into essentially every metabolic activity of a cell. Therefore, the ability of a tissue or organ to metabolize sucrose must be one determinant of sink strength. Here we have tested the feasibility of biochemically measuring sucrose sink strength by assaying sucrose cleavage activities, i.e. sucrolysis via either the invertases or by the sucrose synthase pathway. The reasons we developed this UDP and PPi-dependent assay for sucrose synthase activity were (a) to measure activity in the sucrose breakdown direction whereas many other workers measured the opposite, namely sucrose synthesis (3,4,22,26), and (b) others who measured sucrose breakdown often assayed for UDP-glucose accumulation as a precursor of the synthesis of cell walls or other nucleotide sugars (6, 22). But in our assay we couple through the PPi-dependent UDP-glucopyrophosphorylase, which is very active in plants, to the formation of glucose 1-P which feeds carbon directly into glycolysis and possibly on to starch formation (31,32

Pyruvate ferredoxin oxidoreductase (POR) has been previously purified from the hyperthermophilic archaeon, Pyrococcus furiosus, an organism that grows optimally at 100°C by fermenting carbohydrates and peptides. The enzyme contains thiamine pyrophosphate and catalyzes the oxidative decarboxylation of pyruvate to acetyl-CoA and CO 2 and reduces P. furiosus ferredoxin. Here we show that this enzyme also catalyzes the formation of acetaldehyde from pyruvate in a CoA-dependent reaction. Desulfocoenzyme A substituted for CoA showing that the cofactor plays a structural rather than a catalytic role. Ferredoxin was not necessary for the pyruvate decarboxylase activity of POR, nor did it inhibit acetaldehyde production. The apparent K m values for CoA and pyruvate were 0.11 mM and 1.1 mM, respectively, and the optimal temperature for acetaldehyde formation was above 90°C. These data are comparable to those previously determined for the pyruvate oxidation reaction of POR. At 80°C (pH 8.0), the apparent V m value for pyruvate decarboxylation was about 40% of the apparent V m value for pyruvate oxidation rate (using P. furiosus ferredoxin as the electron acceptor). Tentative catalytic mechanisms for these two reactions are presented. In addition to POR, three other 2-keto acid ferredoxin oxidoreductases are involved in peptide fermentation by hyperthermophilic archaea. It is proposed that the various aldehydes produced by these oxidoreductases in vivo are used by two aldehyde-utilizing enzymes, alcohol dehydrogenase and aldehyde ferredoxin oxidoreductase, the physiological roles of which were previously unknown.

Plant cells have two cytoplasmic pathways of glycolysis and gluconeogenesis for the reversible interconversion of fructose 6‐phosphate (F‐6‐P) and fructose 1,6‐bisphosphate (F‐1,6‐P2). One pathway is described as a maintenance pathway that is catalyzed by a nucleotide triphosphate‐dependent phosphofructokinase (EC 2.7.1.11; ATP‐PFK) glycolytically and a F‐1,6 bisphosphatase (EC 3.1.3.11) gluconeogenically. These are non‐equilibrium reactions that are energy consuming. The second pathway, described as an adaptive pathway, is catalyzed by a readily reversible pyrophosphate‐dependent phosphofructokinase (EC 2.7.1.90; PP‐PFK) in an equilibrium reaction that conserves energy through the utilization and the synthesis of pyrophosphate. A constitutive regulator cycle is also present for the synthesis and hydrolysis of fructose 2,6‐bisphosphate (F‐2,6‐P2) via a 2‐kinase and a 2‐phosphatase, respectively. The pathway catalyzed by the ATP‐PFK and F‐1,6‐bisphosphatase, the maintenance pathway, is fairly constant in maximum activity in various plant tissues and shows less regulation by F‐2,6‐P2. Plants use F‐2,6‐P2 initially to regulate the adaptive pathway at the reversible PPi‐PFK step. The adaptive pathway, catalyzed by PPi‐PFK, varies in maximum activity with a variety of phenomena such as plant development or changing biological and physical environments. Plants can change F‐2,6‐P2 levels rapidly, in less than 1 min when subjected to rapid environmental change, or change levels slowly over periods of hours and days as tissues develop. Both types of change enable plants to cope with the environmental and developmental changes that occur during their lifetimes. The two pathways of sugar metabolism can be efficiently linked by the cycling of uridylates and pyrophosphate required for sucrose breakdown via a proposed sucrose synthase pathway. The breakdown of sucrose via the sucrose synthase pathway requires half the net energy of breakdown via the invertase pathway. Pyrophosphate occurs in plant tissues as a substrate pool for biosynthetic reactions such as the PPi‐PFK or uridine diphosphate glucose pyrophosphorylase (EC 2.7.7.9; UDPG pyrophosphorylase) that function in the breakdown of imported sucrose. Also, pyrophosphate links the two glycolytic/gluco‐neogenic pathways; and in a reciprocal manner pyrophosphate is produced as an energy source during gluconeogenic carbon flow from F‐1,6‐P2 toward sucrose synthesis.

Here it is reported that sucrose synthase can be readily measured

1988. A reassessment of glycolysis and gluconeogenesis in higher plants. -Physiol. Plant. 72: 650-654.Sucrose is the starting point of glycolysis and end point of gluconeogenesis in higher plants. During both glycolysis and gluconeogenesis alternative enzymes are present at various steps to carry out parallel pathways; alternatives are available for utilizing nucleotide triphosphates and pyrophosphate; fructose 2.6-bisphosphate serves as a strong internal regulator; and plants use these cytoplasmic alternatives as they develop and as their environments change.

In peach (Prunus persica [L.] Batsch.), sorbitol and sucrose plants in proportion to water supply. Osmotic adjustment was are the two main forms of photosynthetic and translocated detected by the second week of treatment in mature leaves and by the third week in shoot tips. Both SDH and S6PDH carbon and may have different functions depending on the activities were reduced by drought stress within 4 days of organ of utilization and its developmental stage. The role and treatment and positively correlated with overall c w levels. interaction of sorbitol and sucrose metabolism was studied in However, only SDH activity was correlated with c s . Among mature leaves (source) and shoot tips (sinks) of 'Nemaguard' the sucrose enzymes, only SS was affected by drought, being peach under drought stress. Plants were irrigated daily at reduced after 3 weeks. Sorbitol accumulation in both mature rates of 100, 67, and 33% of evapotranspiration (ET). The relative elongation rate (RER) of growing shoots was mea-leaves and shoot tips of stressed plants was observed starting sured daily. In mature leaves, water potential (c w ), osmotic from the second week of treatment and reached up to 80% of total solutes involved in osmotic adjustment. Sucrose content potential (c s ), sorbitol-6-phosphate dehydrogenase (S6PDH, was up to 8-fold lower than sorbitol content and accumulated EC 1. 1.1.200), and sucrose-phosphate synthase (SPS, EC only occasionally. We conclude that a loss of SDH activity in 2.4.

The breakdown of sucrose to feed both hexoses into glycolytic carbon flow can occur by the sucrose synthase pathway. This uridine diphosphate (UDP) and pyrophosphate (PPi)-dqpendent pathway was biochemically characterized using soluble extracts from several plants.

Activities of the sucrose-cleaving enzymes, acid and neutral invertase and sucrose synthase, were measured in pods and seeds of developing snap bean (Phaseolus vulgaris L.) fruits, and compared with *'*C-import, elongation and dry weight accumulation. During the first lOd postanthesis, pods elongated rapidly with pod dry weight increase lagging behind by several days. The temporal patterns of acid invertase activity and import coincided closely during the first part of pod development, consonant with a central role for this enzyme in converting imported sucrose during pod elongation and early dry weight accumulation. Later, sucrose synthase became the predominant enzyme of dry weight accumulation and was possibly associated with the development of phloem in pod walls. Sucrose synthase activity in seeds showed two peaks, corresponding to two phases of rapid import and dry weight accumulation; hence, sucrose synthase was associated with seed sink growth. Acid invertase activities in seeds were low and did not show a noticeable relationship with import or growth. All neutral invertase activities, during pod and seed development, were too low for it to have a dominant role in sucrose cleavage. Changes in activities of certain sucrose-cleaving enzymes appear to be correlated with certain sink functions, including import, storage of reserves, and biosynthetic activities. The data supports the association of specific sucrose-cleaving enzymes with the specific processes that occur in the developing pods and seeds of snap bean fruits; for example, acid invertase with pod elongation and sucrose synthase with fruit dry matter accumulation.