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PubMedDate not supplied

[The effect of various types of dry starch syrup on the rate of glucose utilization in lipid, carbohydrate, and protein components of rat liver].

Effect of a diet, containing dextran maltose and dry starch syrup, on some patterns of liver tissue metabolism were studied in young Wistar rats within 30 days. The animals of Control Group 1 were kept on a diet containing corn starch as a source of carbohydrates; in Group 2 the starch was replaced by the dry starch syrup enriched with disaccharides and especially with maltose; the dry starch syrup added into the Group 3 diet containing mainly oligosaccharides and polymers with high levels of glucose residues. The label mixtures of 6-3N- and 6-14C-glucose as well as of 6-3H- and I-14C-glucose were administered into the animals on the day of death. Analysis of the findings has shown that the products of starch hydrolysis may the specific parameters of glucose metabolism. Incorporation of the label into liver tissue lipids was similar to the control values in the group of animals kept on a diet enriched with maltose as compared with group 3. The glycolytic pathway of glucose utilization was more activated than the pentosephosphate pathway after substituting starch for dry starch syrup as shown by differences in the rates of carbon incorporation at positions 1 and 6 of a glucose molecule.

Animals↗
PubMed1998

The effects of starch and protein degradation rates, hay sources, and feeding frequency on rumen microbial fermentation in a continuous culture system.

Six continuous culture fermenters were used in three experiments to study the effects of dietary starch and protein degradability combination, hay sources, and feeding frequency on fermentation by rumen microorganisms. Experiments 1 and 2 used a 3 x 2 factorial design in which six diets were formulated to contain low (LS), medium (MS), or high starch degradability (HS) in combination with low (LP) or high protein degradability (HP). The dietary combinations were (1) LS + HP, (2) LS + LP, (3) MS + HP, (4) MS + LP, (5) HS + HP, and (6) HS + LP. In experiment 1, pangola was used as the hay source, and in experiment 2, alfalfa hay was used. In experiment 3, two starch degradabilities (LS, MS) and two hay sources (alfalfa, A; pangola, P) were combined with two feeding frequencies (2 X/d, 12 X/d). The dietary combinations were (1) LS + A + 12 X/d, (2) MS + A + 12 X/d, (3) LS + A + 2 X/d, (4) MS + A + 2 X/d, (5) LS + P + 12 X/d, and (6) MS + P + 12 X/d. A CRD design was used for experiment 3. Two rumen-cannulated Holstein cows fed alfalfa hay ad lib were used as donors of rumen fluid for all experiments. Each period was 6 d in length, with 5 d for adaptation and 1 d for sampling. In experiments 1 and 2, the effects of starch degradability on the composition of rumen microorganisms were significant. The MS or HS with HP had the highest total bacterial and protozoal density (P < 0.05). As for VFA, pH and nitrogen products, the effects of starch and protein degradability were not significant. There was no interaction between starch and protein degradability on most of microbial composition (ammonia-N, microbial nitrogen). In experiment 3, 12 X/d feeding frequency (MS + A + 12 X/d vs. MS + A + 12 X/d) resulted in higher pH, which tended to increase bacterial and protozoal density and starch and protein digestibilities. Different hay sources altered the starch and protein synchronization effect on the ammonia-N concentration (mg/dl) and non-ammonia N content (% DM) in the continuous culture system.

Animal Feed↗
PubMed1988

Maltose transport and starch binding in phage-resistant point mutants of maltoporin. Functional and topological implications.

The relationships between the bacteriophage lambda binding site, the starch binding site and the pore formed by maltoporin (LamB protein, lambda receptor protein) were investigated. Bacteria with single amino acid substitutions in the maltoporin sequence, which were previously shown to be strongly reduced in phage lambda sensitivity, were assayed for maltose- (and maltodextrin) selective pore functions. Maltose transport assays was performed at low substrate concentrations, under conditions where LamB is limiting for transport. It revealed three classes of mutants. Class A is composed of mutants with no effect on transport (substitutions at amino acid residues 154, 155, 259, 382 and 401); class B corresponds to mutants with a significant but variable reduction in transport (sites 148, 151, 152, 163, 164, 245, 247 and 250); class C is represented by a single mutant for which transport is almost completely abolished (site 18). Starch binding was assayed by two different methods that gave compatible results. In class A mutants, binding was normal, while no binding was observed in the class C mutant. Binding was impaired to various extents in category B mutants. There was a correlation between the level of impairment of starch binding and impairment of maltose transport, consistent with the notion that the residues influencing starch binding are inside, or in close proximity to, the pore. These results, together with previous data on starch-binding mutants that were not affected in phage binding (substitutions at residues 8, 74, 82, 118 and 121), suggest that the binding sites for starch and phage lambda overlap but are distinct. Mutations affecting transport and starch binding are located in the first third of the protein and in the region of residues 245 to 250. Mutations affecting phage adsorption are located mainly in the last two-thirds of the protein. The topological constraints suggested by the results with the available mutants altered in the lamB gene were used to propose a revised model of maltoporin folding across the outer membrane as well as to define the outlines of footprints of macromolecular binding sites (phage, starch and monoclonal antibodies) on the surface of the protein.

Bacterial Outer Membrane Proteins↗
PubMed1998

Properties of Starches Conjugated with Lysine and Poly(lysine) by the Maillard Reaction.

Native potato starch (PS), carboxymethyl potato starch (CMS), and corn starch phosphate monoester (PCS) were conjugated with lysine (Lys) and poly(lysine) (PL) by the Maillard reaction. The increased yellowness of each reaction mixture showed the development of browning, suggesting the conjugation of starch and Lys or PL. The Lys and PL contents in the conjugates were 0.12-0.68% and 2.8-4.3%, respectively. Conjugation with PL reduced the swelling and solubility of each starch. The CMS and PCS conjugates gelatinized at higher temperatures than either CMS or PCS alone and were hard to retrograde. The digestibility of each conjugate with alpha-amylase was lower than that of the original starch. The functional changes to the starches were most marked in the cases of starches with partial distortion of the internal structure as indicated for CMS and PCS.

Journal Article↗
PubMed1998

Effect of Crystallinity on the Glass Transition Temperature of Starch.

The glass transition temperature (T(g)) of potato and wheat starches, stored for several periods after gelatinization, was measured by differential scanning calorimetry (DSC), and the relative crystallinity of the starches was measured by X-ray diffractometry. T(g) of stored starches was higher than that of starches without storage, and the T(g) increment of starches gelatinized at 120 degrees C was higher than that of starches gelatinized at 60 degrees C. The water content at which the glass transition of a starch occurs at 25 degrees C was estimated from DSC data, and it increased linearly with relative crystallinity in two groups that differed in the gelatinization method. These results also showed the quantitative relationship between T(g) and retrogradation. In addition, these results suggested that the glass transition of starch could be interpreted in the same way as the glass transition of cross-linked synthetic polymers.

Journal Article↗
PubMed1994

Effects of reducing the starch content in oat-based diets with cellulose on jejunal flow and absorption of glucose over an isolated loop of jejunum in pigs.

Four pigs fitted with two sets of re-entrant cannulas in the upper jejunum were used to study the effect of two oat-flour (F)- and oat-bran (B)-based diets without or with (FC and BC respectively) addition of 148 g wood cellulose/kg on jejunal flow and absorption of glucose over an isolated loop of jejunum. Mean transit time (MTT) of flow from the proximal re-entrant cannula increased from approximately 1.5 h with diet F to 2 h when feeding diet BC. Both the replacement of oat flour by oat bran and the supplementation of the diets with cellulose led to a significantly longer MTT for dry matter and the carbohydrates, except that cellulose did not have a significant effect on the mTT for the total non-starch polysaccharides. Only the addition of cellulose significantly delayed gastric emptying of the added liquid-phase (Polyethylene glycol 4000) and solid-phase (Cr2O3) markers, whereas no effect of the oat source used was seen. Feeding diets with a higher level of dietary fibre (DF) and lower content of starch, obtained by substitution with cellulose or by feeding oat bran instead of oat flour, reduced the recovery of starch from an isolated loop of jejunum. Consequently, the quantitative absorption of starch was not significantly different between diets when starch was related to the recovery of external markers. However, when related to the recovery of arabinoxylan (AX) there was a significantly lower absorption of starch from the bran-based diets in comparison with the flour-based diets. The capacity to digest and absorb the large quantities of starch entering the duodenum after a carbohydrate-rich meal appears to be the limiting factor for absorption in the upper jejunum. Therefore, any effect of DF on the rate of absorption of glucose is presumably an effect on gastric emptying rather than displacement of the site of starch absorption in the small intestine.

Animals↗
PubMed1983

Effects of a commercial starch blocker preparation on carbohydrate digestion and absorption: in vivo and in vitro studies.

We have tested the effectiveness of a commercial starch blocker on the digestion and absorption of dietary carbohydrates in six normal, healthy volunteers. The effectiveness of the starch blocker to attenuate or block the digestion of carbohydrate was assessed against a placebo by the measurement of end tidal breath hydrogen, plasma glucose, and insulin responses to a constant test meal. There were no significant differences in breath hydrogen, or plasma glucose and insulin responses. In vitro enzyme inhibition studies assessed the ability of the brush border enzyme maltase/glucoamylase to degrade starch in the presence of the starch blockers. A highly purified solution of rat and human maltase/glucoamylase was capable of degrading a starch solution, while 40 mM Tris-HCl (a known maltase/glucoamylase inhibitor) completely abolished the enzyme activity. These data challenge the claims that starch blocker preparations are effective in reducing or attenuating the absorption of carbohydrates or calories from a mixed meal. The ineffectiveness in vivo could be explained, in part, by the ability of the brush border enzyme maltase/glucoamylase to hydrolyze starch in the presence of starch blockers.

Animals↗
PubMed1998

Patterns of photosynthesis and starch allocation in seedlings of four bottomland hardwood tree species subjected to flooding.

Effects of short-term (32 days) flooding on photosynthesis, stomatal conductance, relative growth rate and tissue starch concentrations of flood-intolerant Quercus alba L. (white oak), bottomland Quercus nigra L. (water oak), bottomland Fraxinus pennsylvanica Marshall. (green ash) and flood-tolerant Nyssa aquatica L. (water tupelo) seedlings were studied under controlled conditions. Net photosynthetic rates of flooded N. aquatica seedlings were reduced by 25% throughout the 32-day flooding period. Net photosynthetic rates of flooded Q. alba seedlings fell rapidly to 25% of those of the control seedlings by Day 4 of the flooding treatment and to 5% by Day 16. In F. pennsylvanica and Q. nigra, net photosynthetic rates were reduced to 50% of control values by Day 8 but remained at approximately 30 and 23%, respectively, of control values by Day 32. Leaves of flooded Q. alba seedlings accumulated approximately twice as much starch as leaves of non-flooded control plants, whereas root starch concentrations decreased to 67% of those of control plants by the end of the 32-day flooding treatment. In contrast, flooding caused only a small increase in leaf starch concentrations of N. aquatica plants, but it increased root starch concentrations to 119% of those of the control plants by the end of the experiment. The co-occurring bottomland species, Fraxinus pennsylvanica and Q. nigra, differed from each other in their patterns of stomatal conductance and root starch concentrations. We conclude that the maintenance of low leaf starch concentrations, and high pre-flood root tissue starch concentrations are important characteristics allowing flood-tolerant species to survive in flooded soils.

Journal Article↗
PubMed1973

Starch Synthetase, Phosphorylase, ADPglucose Pyrophosphorylase, and UDPglucose Pyrophosphorylase in Developing Maize Kernels.

Soluble ADPglucose-alpha-glucan 4-alpha-glucosyltransferase (starch synthetase), ADPglucose pyrophosphorylase, UDPglucose pyrophosphorylase and phosphorylase were assayed in extracts from developing kernels of maize (Zea mays). Normal, waxy and amylose-extender maize at stages of development ranging from 8 days to 28 days after pollination were studied. Shrunken-4 maize at the 22-day stage was also studied. There is adequate activity of both ADPglucose pyrophosphorylase and starch synthetase at all stages of development to account for the synthesis of starch. Thus all starch could be synthesized via the ADPglucose pathway. High levels of UDPglucose pyrophosphorylase and of phosphorylase activities were also found at all stages of development. The possible role of phosphorylase in starch synthesis could not be discounted. The levels of phosphorylase, ADPglucose pyrophosphorylase, starch synthetase, and UDPglucose pyrophosphorylase activities in shrunken-4 kernels were about 20 to 40% of that found in normal maize kernels. It appears that the mutation in shrunken-4 affects the activities of more than one enzyme. The defective starch synthesis seen in this mutant could be due to the low activities of ADPglucose pyrophosphorylase and starch synthetase rather than the low activity of phosphorylase.

Journal Article↗
PubMed1986

Heat inactivation of starch synthase in wheat endosperm tissue.

The effect of temperature on accumulation of starch was studied in grain slices of wheat (Triticum aestivum cv SUN9E), taken 15 days after anthesis. As compared with pretreatment of such slices at 25 degrees C, pretreatment at 30 or 35 degrees C reduced the subsequent conversion of sucrose to starch. In contrast to rice (Oryza sativa cv Calrose), pretreatment of wheat soluble starch synthase in vitro at 30 degrees C or higher temperatures reduced its activity. In zymograms using nondenaturing polyacrylamide gel electrophoresis followed by activity staining, the slowest migrating band represented the most temperature sensitive isozyme. Although preincubation of a soluble enzyme sample in vitro at 25 degrees C did not result in loss of starch synthase activity, it did result in a gradual shift of zymogram banding pattern toward faster migrating species. Pretreatment of isolated starch granules at 40 degrees C increased their bound starch synthase activity. Both soluble and bound enzymes in the grains of whole wheat plants lost activity when the plants were held above 30 degrees C for 30 minutes or longer. Both activities lost from the grains after a 1 hour treatment at 37 degrees C were restored in 1 to 2 days by a return to 21 degrees C. In slices, inactivation of the soluble starch synthase was increased by incubation with 2,4-dinitrophenol. It is tentatively suggested that in vivo heat inactivation of soluble starch synthase may be a direct effect of heat on the enzyme protein and that of bound enzyme an indirect effect involving metabolic factors.

Journal Article↗
PubMed1988

Starch Biosynthesis in Developing Wheat Grain : Evidence against the Direct Involvement of Triose Phosphates in the Metabolic Pathway.

We have used (13)C-labeled sugars and nuclear magnetic resonance (NMR) spectrometry to study the metabolic pathway of starch biosynthesis in developing wheat grain (Triticum aestivum cv Mardler). Our aim was to examine the extent of redistribution of (13)C between carbons atoms 1 and 6 of [1-(13)C] or [6-(13)C]glucose (or fructose) incorporated into starch, and hence provide evidence for or against the involvement of triose phosphates in the metabolic pathway. Starch synthesis in the endosperm tissue was studied in two experimental systems. First, the (13)C sugars were supplied to isolated endosperm tissue incubated in vitro, and second the (13)C sugars were supplied in vivo to the intact plant. The (13)C starch produced by the endosperm tissue of the grain was isolated and enzymically degraded to glucose using amyloglucosidase, and the distribution of (13)C in all glucosyl carbons was quantified by (13)C-NMR spectrometry. In all of the experiments, irrespective of the incubation time or incubation conditions, there was a similar pattern of partial (between 15 and 20%) redistribution of label between carbons 1 and 6 of glucose recovered from starch. There was no detectable increase over background (13)C incidence in carbons 2 to 5. Within each experiment, the same pattern of partial redistribution of label was found in the glucosyl and fructosyl moieties of sucrose extracted from the tissue. Since it is unlikely that sucrose is present in the amyloplast, we suggest that the observed redistribution of label occurred in the cytosolic compartment of the endosperm cells and that both sucrose and starch are synthesized from a common pool of intermediates, such as hexose phosphate. We suggest that redistribution of label occurs via a cytosolic pathway cycle involving conversion of hexose phosphate to triose phosphate, interconversion of triose phosphate by triose phosphate isomerase, and resynthesis of hexose phosphate in the cytosol. A further round of triose phosphate interconversion in the amyloplast could not be detected. These data seriously weaken the argument for the selective uptake of triose phosphates by the amyloplast as part of the pathway of starch biosynthesis from sucrose in plant storage tissues. Instead, we suggest that a hexose phosphate such as glucose 1-phosphate, glucose 6-phosphate, or fructose 6-phosphate is the most likely candidate for entry into the amyloplast. A pathway of starch biosynthesis is presented, which is consistent with our data and with the current information on the intracellular distribution of enzymes in plant storage tissues.

Journal Article↗
PubMed1988

Correlation between the Carbon Isotope Discrimination in Leaf Starch and Sugars of C(3) Plants and the Ratio of Intercellular and Atmospheric Partial Pressures of Carbon Dioxide.

Carbon isotope discrimination (Delta) was analyzed in leaf starch and soluble sugars, which represent most of the recently fixed carbon. Plants of three C(3) species (Populus nigra L. x P. deltoides Marsh., Gossypium hirsutum L. and Phaseolus vulgaris L.) were kept in the dark for 24 hours to decrease contents of starch and sugar in leaves. Then gas exchange measurements were made with constant conditions for 8 hours, and subsequently starch and soluble sugars were extracted for analysis of carbon isotope composition. The ratio of intercellular, p(i), and atmospheric, p(a), partial pressures of CO(2), was calculated from gas exchange measurements, integrated over time and weighted by assimilation rate, for comparison with the carbon isotope ratios in soluble sugars and starch. Carbon isotope discrimination in soluble sugars correlated strongly (r = 0.93) with p(i)/p(a) in all species, as did Delta in leaf starch (r = 0.84). Starch was found to contain significantly more (13)C than soluble sugar, and possible explanations are discussed. The strong correlation found between Delta and p(i)/p(a) suggests that carbon isotope analysis in leaf starch and soluble sugars may be used for monitoring, indirectly, the average of p(i)/p(a) weighted by CO(2) assimilation rate, over a day. Because p(i)/p(a) has a negative correlation with transpiration efficiency (mol CO(2)/mol H(2)O) of isolated plants, Delta in starch and sugars may be used to predict differences in this efficiency. This new method may be useful in ecophysiological studies and in selection for improved transpiration efficiency in breeding programs for C(3) species.

Journal Article↗
PubMed1992

Diurnal starch accumulation and utilization in phosphorus-deficient soybean plants.

The effects of phosphorus deficiency on carbohydrate accumulation and utilization in 34-day-old soybean (Glycine max L. Merr.) plants were characterized over a diurnal cycle to evaluate the mechanisms by which phosphorus deficiency restricts plant growth. Phosphorus deficiency decreased the net CO(2) exchange rate throughout the light period. The decrease in the CO(2) exhange rate was associated with a decrease in stomatal conductance and an increase in the internal CO(2) concentration. These observations indicate that phosphorus deficiency increased mesophyll resistance. Assimilate export rate from the youngest fully expanded leaves was decreased by phosphorus deficiency, whereas starch concentrations in these leaves were increased. Higher starch concentrations in phosphorus-deficient youngest fully expanded leaves resulted from a longer period of net starch accumulation and a shorter period of net starch degradation relative to those for phosphorus-sufficient controls. Phosphorus deficiency decreased sucrose-P synthase activity by 27% (averaged over the diurnal cycle), and essentially eliminated diurnal variation in sucrose-P-synthase activity. Diurnal variations in nonstructural carbohydrate concentrations in leaves and stems were also less pronounced in phosphorus-deficient plants than in controls. In phosphorus-deficient plants, only 30% of the whole plant starch present at the end of a light phase was utilized during the subsequent 12-hour dark phase as compared with 68% for phosphorus-sufficient controls. Although phosphorus deficiency decreased the CO(2) exchange rate and whole plant leaf area, accumulation of high starch concentrations in leaves and stems and restricted starch utilization in the dark indicate that growth processes (i.e. sink activities) were restricted to a greater extent than photosynthetic capacity. Further experimentation is required to determine whether decreased starch utilization in phosphorus-deficient plants is the cause or the result of restricted growth.

Journal Article↗
PubMed2005

Mutan produced in potato amyloplasts adheres to starch granules.

Production of water-insoluble mutan polymers in Kardal potato tubers was investigated after expression of a full-length (GtfI) and a truncated mutansucrase gene referred to as GtfICAT (GtfI without glucan-binding domain) from Streptococcus downei. Subsequent effects on starch biosynthesis at the molecular and biochemical levels were studied. Expression of the GtfICAT gene resulted in the adhesion of mutan material on starch granules, which stained red with erythrosine, and which was hydrolysed by exo-mutanase. In addition, GtfICAT-expressing plants exhibited a severely altered tuber phenotype and starch granule morphology in comparison to those expressing the full-length GtfI gene. In spite of that, no structural changes at the starch level were observed. Expression levels of the sucrose-regulated, AGPase and GBSSI genes were down-regulated in only the GTFICAT transformants, showing that GtfICAT expression interfered with the starch biosynthetic pathway. In accordance with the down-regulated AGPase gene, a lower starch content was observed in the GTFICAT transformants. Finally, the rheological properties of the GTFICAT starches were modified; they showed a higher retrogradation during cooling of the starch paste.

Journal Article↗
PubMed1986

Production and Characteristics of Raw-Starch-Digesting alpha-Amylase from a Protease-Negative Aspergillus ficum Mutant.

Mutational experiments were carried out to decrease the protease productivity of Aspergillus ficum IFO 4320 by using N-methyl-N'-nitro-N-nitrosoguanidine. A protease-negative mutant, M-33, exhibited higher alpha-amylaseactivity than the parent strain under submerged culture at 30 degrees C for 24 h. About 70% of the total alpha-amylase activity in the M-33 culture filtrate was adsorbed onto starch granules. The electrophoretically homogeneous preparation of raw-starch-adsorbable alpha-amylase (molecular weight, 88,000), acid stable at pH 2, showed intensive raw-starch-digesting activity, dissolving corn starch granules completely. The preparation also exhibited a high synergistic effect with glucoamylase I. A mutant, M-72, with higher protease activity produced a raw cornstarch-unadsorbable alpha-amylase. The purified enzyme (molecular weight, 54,000), acid unstable, showed no digesting activity on raw corn starch and a lower synergistic effect with glucoamylase I in the hydrolysis of raw corn starch. The fungal alpha-amylase was therefore divided into two types, a novel type of raw-starch-digesting enzyme and a conventional type of raw-starch-nondigesting enzyme.

Journal Article↗
PubMed1999

Effects of ruminal administration of supplemental degradable intake protein and starch on utilization of low-quality warm-season grass hay by beef steers.

Hereford x Angus steers were used in a 13-treatment, four-period, incomplete Latin square design to examine the effects of starch and degradable intake protein (DIP) supplements on forage utilization and ruminal function. Steers were given ad libitum access to low-quality hay (4.9% CP) and were not supplemented (NS) or received different amounts of starch (cornstarch grits; 0, .15, and .3% of initial BW) and DIP (Na-caseinate; .03, .06, .09, and .12% of initial BW) administered via ruminal fistulae in a 3 x 4 factorial arrangement of treatments. Supplemented steers consumed more (P < .01) forage OM, total OM, NDF, and digestible OM (DOM) than NS steers. Forage OM, total OM, NDF, and DOM intakes increased linearly (P < .01) as the amount of supplemental DIP increased. The addition of starch to supplements linearly decreased ( P < .01) the intake of forage OM, NDF, and DOM. The digestion of DM, OM, and NDF increased linearly (P < .01) with supplemental DIP and decreased linearly (P < or = .06) with supplemental starch. Particulate and liquid passages generally increased with DIP; however, starch level influenced the nature of the response (P = .03 and .06, respectively). Similarly, ruminal acid detergent-insoluble ash content generally decreased as starch increased, but the effect was dependent on DIP level (P < .01). Supplementation increased (P < .01) ruminal NH3 and total VFA and decreased (P < .01) ruminal pH relative to NS. All treatments supported average pH values in a range (6.3 to 6.7) unlikely to inhibit fibrolytic bacteria. Ruminal NH3 concentration increased quadratically (P = .03) with DIP and decreased linearly (P = .02) with starch. As DIP increased, total VFA concentration increased linearly (P = .02). Providing supplemental DIP to steers fed low-quality forage increased OM intake and digestion, whereas addition of starch to supplements decreased forage intake and digestion.

Animal Feed↗
PubMed2002

Integrating diel starch metabolism with the circadian and environmental regulation of Crassulacean acid metabolism in Mesembryanthemum crystallinum.

The diel (24-h) Crassulacean acid metabolism (CAM) cycle in Mesembryanthemum crystallinum (L.) requires rhythmic patterns of transitory starch degradation to produce carbon skeletons for phospho enolpyruvate (PEP) synthesis during the nocturnal Phase I, when PEP carboxylase (PEPc) mediates CO(2) fixation. Under a normal light-dark cycle, nocturnal malate accumulation and nocturnal CO(2) uptake were observed for CAM-induced, but not C(3), M. crystallinum. In both C(3) and CAM plants, transcripts encoding beta-amylase and starch phosphorylase accumulated during the afternoon and declined nocturnally. Under a continuous light regime, ribulose-1,5-bisphosphate carboxylase/oxygenase activity remained co-ordinated with the CAM phases, and circadian abundance patterns were observed for transcripts encoding starch degradative enzymes. Despite circadian PEPc kinase expression, the accumulation of vacuolar malate ceased under continuous light. Exposure to CO(2)-free air for 24 h inhibited starch accumulation over the photoperiod, but re-fixation of respiratory CO(2) resulted in the overnight accumulation of malate to levels comparable to those of control plants. Upon return to normal air, the depleted starch concentration led to stoichiometric decreases in Phase-I CO(2) uptake and malate accumulation. The up-regulation of PEPc kinase transcripts under these conditions was ineffective at sustaining Phase-I CO(2) uptake under starch-limited conditions. We conclude that starch turnover regulates and limits carbon flux through the diel CAM cycle.

Carbon Dioxide↗
PubMed1995

Biochemical and molecular characterization of a novel starch synthase from potato tubers.

An isoform of starch synthase from potato tubers which is present both in the stroma of the plastid and tightly bound to starch granules has been identified biochemically and a cDNA has been isolated. The protein encoded by the cDNA is 79.9 kDa and has a putative transit peptide and a distinct N-terminal domain which is predicted to be highly flexible. It is similar in both amino acid sequence and predicted structure to the granule-bound starch synthase II (GBSSII) of pea embryos. When expressed in Escherichia coli, the mature protein has starch synthase activity. The importance of the isoform has been assessed by biochemical measurements and antisense transformation experiments in which the amount of the isoform in the tuber is severely and specifically reduced. Both approaches indicate that the isoform contributes a maximum of 15% of the total starch synthase activity of the tuber. It is suggested that this isoform and the GBSSII of pea embryos represent a widely distributed class of isoforms of starch synthase. The contribution to total starch synthase activity of members of this class probably varies considerably from one type of storage organ to another.

Amino Acid Sequence↗