Starch-sugar interconversion in Solanum tuberosum

The changes in starch, sugars, and respiration of both immature and mature potato tubers (variety King Edward) caused by transfer from +10° to +2° and back to +10°, were followed throughout. At each storage temperature the tubers were allowed to reach a steady state before transfer to another temperature. In potatoes transferred from +10° to +2°, the sugar at first rose rapidly and then reached a constant value after 30 days. The respiration showed a characteristic pattern, for the first 5–8 days being below the initial value, then rising to a maximum at 14 days and finally returning to the initial value at 28 days. In potatoes transferred from +2° to +10° the sugar declined steadily, the respiration reaching a maximum after 10 days and then slowly falling to a value slightly above the initial value. Quantitative analysis of the results showed that the sum of starch + sugar + CO₂ expressed in equivalent anhydrohexose units did not change throughout the various changes in temperature. This work provided a quantitative experimental basis for what had hitherto been an assumption. Starch was the only polysaccharide involved in these carbohydrate changes. No change in the amylose/amylopectin ratio was detected either during the breakdown of starch (temperature change +10° to +2°) or during its synthesis (+2° to + 10°). The increased respiration which accompanied any change in temperature was related quantitatively to the formation of sucrose from starch (+10° to +2°) and starch from sugar (+2° to + 10°). The ATP equivalent of the extra CO₂ output was of the same order as that predicted on the basis of known biochemical pathways linking starch and sugar.     

Activity of phosphorylase in Solanum tuberosum during low temperature storage

Changes in the activity of phosphorylase were measured during storage of potatoes at + 2° when the sugar content rises rapidly and subsequently at + 10° when the accumulated sugar is converted mainly to starch. The observed changes were relatively small and could not be related to any of the components of the phosphorylase system, which was shown to be complex.     

Membrane-degrading enzymes in the leaves of Solanum tuberosum

When homogenates of potato leaves were prepared under conditions which are typical for organelle isolation (pH 7.5 and 4°), membrane lipids underwent rapid hydrolysis (17% of phosphatidylcholine was hydrolysed in 2 hr). Leaves of 41 potato cultivars were surveyed for phospholipase activity to determine whether certain cultivars might be more suitable for the preparation of organelles. Phospholipase activities ranged from 1.04 to 11.60, μmol/min · g fr. wt and p-nitrophenyl palmitate hydrolase activity ranged from 0.0119 to 0.0502,μmol/min · g fr. wt. These phospholipase values were several hundred-fold higher than previously reported for potato leaves and nearly as high as in potato tubers. Most of the phospholipase activity in leaves was soluble and not membrane-associated as previously reported.     

Phosphofructokinase of Solanum tuberosum tuber

Potato tuber phosphofructokinase was purified 19·.6-fold by a combination of ethanol fractionation and DEAE-cellulose column chromatography. The enzyme was very unstable; its pH optimum was 8·0. K_m for fructose-6-phosphate, ATP and Mg²⁺ was 2·1 × 10^?4 M, 4·5 × 10^?5 M and 4·0 × 10^?4 M respectively. ITP, GTP, UTP and CTP can act as phosphate donors, but are less active than ATP. Inhibition of enzyme activity by high levels of ATP was reversed by increasing the concentration of fructose-6-phosphate; the affinity of enzyme for fructose-6-phosphate decreased with increasing concentration of ATP. 5′-AMP, 3′,5′-AMP, 3′-AMP, deoxy AMP, UMP, IMP, CMP, GMP, ADP, CDP, GDP and UDP did not reverse the inhibition of enzyme by ATP. ADP, phosphoenolpyruvate and citrate inhibited phosphofructokinase activity but Pi did not affect it. Phosphofructokinase was not reactivated reversibly by mild change of pH and addition of effectors.     

Alterations in Solanum tuberosum polyphenol oxidase activity induced by gamma irradiation

On gamma irradiation of potato tubers at sprout-inhibiting dose (10 krad) the cresolase activity showed a 45% increase while catecholase was reduced by 25%. This reduced the ratio of catecholase to cresolase from 11–12 in unirradiated to 5–6 in irradiated potatoes. Chlorogenic acid oxidation was enhanced by about 25% on irradiation. The increase in the oxidation of p-cresol corresponded with the production of diphenolic compounds. The process of activation of cresolase was slow, reversible and oxygen dependent. A comparative study of the isoenzyme pattern suggested that this activation was due to conformational change, rather than synthesis of new protein.     

Lipoxygenase isoenzymes from Solanum tuberosum

Two lipoxygenase isoenzymes were separated from potato tubers (Solanum tuberosum). Experiments with chemical modifications showed that tryptophan is essential for enzyme activity and that one or more tryosine residues was involved. On the other hand, no lysine or sulfhydryl groups were necessary. Both enzymes had an optimum pH of 5·5. They were not affected by calcium ions but were inhibited by cysteine.     

Isolation and characterization of amyloplast envelope membranes from Solanum tuberosum

Amyloplasts were separated from other subcellular organelles by sedimentation through a discontinuous sucrose density gradient. Purified amyloplast envelope membranes were similar in most characteristics to those from other types of plastids. A substantial proportion of the carotenoid content of these membranes was present in the esterified form. In contrast to published work on chloroplasts of photosynthetic tissue, our results showed that the acylase (acyl-CoA:sn-glycerol 3-phosphate acyltransferase) is firmly bound to the envelope membranes. Evidence was obtained to indicate that digalactosyldiacylglycerol, phosphatidylglycerol and sulpholipid, but not monogalactosyldiacylglycerol, are exclusively found in the cell as amyloplast lipids.     

Properties of two apyrases from Solanum tuberosum

Two homogeneous isoenzymes of apyrase from Pimpernel and Desirée varieties of Solanum tuberosum were obtained by affinity chromatography on agarose-Cibacron Blue or agarose-ATP-phosphonate columns. Both enzymes split POP bonds of organic and inorganic di- and triphosphates. The ratio of ATPase/ADPase is different for the two apyrases: 10 for Pimpernel and 1 for Desirée. All these activities require bivalent metals. Both isoapyrases have the same MW (49 000) but differ in their pI (8.74 for Pimpernel and 6.69 for Desirée). The optimum pH of hydrolysis of organic di- and triphosphates is 6 (except for Pimpernel ADPase) and 5 for inorganic substrates. Chemical modification of tryptophan, tyrosine, arginine and carboxylic residues decreased all enzymic activities of both enzymes. Protection by substrates and inactivation rates of the individual activities are different for each isoenzyme.     

Structural studies of two apyrases from Solanum tuberosum

The proportion of acid and basic amino acid residues obtained for two homogeneous isoenzymes of apyrase isolated from different clonal varieties of Solanum tuberosum (Pimpernel and Desirée) was essentially the same. This does not agree with the difference in pI values observed. Treatment with asparaginase and glutaminase caused partial inactivation of both enzyme activities in both isoenzymes, and pI values were changed, but not equalized. The differences in pI values of the native isoenzymes may still be attributed to different proportions of glutamine and asparagine in the primary structure. Leucine is the amino-terminal residue in both isoenzymes. Both have two disulphide bridges and one buried sulphydryl group which is not essential for enzyme activity. Differences in pI values should thus be attributed to factors other than amino acid composition.     

The effect of temperature on adenosine diphosphate glucose pyrophosphorylase from Solanum tuberosum

A partially purified extract of adenosine diphosphate glucose pyrophosphorylase has been prepared from Solanum tuberosum. The effect of temperature on the initial rate of reaction has been determined in the presence and absence of activator. The results are discussed in relation to the sweetening of potatoes at 2°.     

Zunahme von cytokinin und auxin in verwundetem speichergewebe von Solanum tuberosum

“Butanol-soluble” cytokinins increase in grated and in lighted cut potato tissue. In slices, an increase of “water soluble” cytokinins (darkness and of tRNA-cytokinins can be found. Together with auxin, the cytokinin factor stimulates cell division in wound tissue.     

Activities of enzymes of sugar metabolism in cold-stored tubers of Solanum tuberosum

Storage of tubers of Solanum tuberosum at 10° or 2° for 15 days did not alter significantly the maximum catalytic activities of sucrose phosphate synthetase, sucrose synthetase, glucose-6-phosphate dehydrogenase, aldolase, and glyceraldehydephosphate dehydrogenase. The temperature coefficients of phosphofructokinase, glyceraldehydephosphate dehydrogenase, and pyruvate kinase from the tubers were shown to be higher between 2° and 10° than between 10° and 25°. The rate of sugar accumulation at 2° exceeded the activity of sucrose synthetase but was less than that of sucrose phosphate synthetase. It is suggested that sucrose accumulation at 2° is catalysed by sucrose phosphate synthetase, is not due to changes in the maximum catalytic activities of any of the above enzymes, but may be due, in part, to the susceptibility of key glycolytic enzymes to cold.     

An arabinogalactoxyloglucan from the cell wall of Solanum tuberosum

Cell-wall material from potatoes was fractionated by successive extractions with water at 80°, 0.2 M (NH₄)₂C₂O₄ at 80°, 1 M and 4 M KOH, to leave a residue of α-cellulose. The compositions of the isolated carbohydrate polymers were determined by sugar and methylation analysis. From the 4M KOH-soluble fraction an arabinogalactoxyloglucan was isolated and (partially) characterized by methylation analysis of the undegraded polymer and partially degraded methylated polymer. Methylation analysis of the oligosaccharides produced on treatment of the xyloglucan with cellulase threw additional light on the structural features of the polysaccharide. The results show that the xyloglucan has a cellulosic backbone which is highly substituted at position 6 with xylopyranose residues, some of which, in turn, carry either arabinofuranose or galactopyranose residues, as a substituent on position 2. The significance of these results is discussed.     

The effect of wounding on glucose-6-phosphate dehydrogenase of Solanum tuberosum tuber tissue

Two forms of glucose-6-phosphate dehydrogenase were separated by disc electrophoresis of potato tuber extracts. The slower moving enzyme has a MW of 260 000 the faster one of 130 000. Wounding of potato tubers enhances the relative activity of the slower moving enzyme. Addition of NADP⁺ to the cathode buffer during electrophoresis has the same effect as wounding, whereas addition of glucose-6-phosphate has an opposite effect. The role of the wound induced increase of the pyridine nucleotide level in the interconversion of the two forms of glucose-6-phosphate dehydrogenase is discussed.     

Purification and methylation analysis of cell wall material from Solanum tuberosum

Cell wall material (CWM) of potatoes was prepared by sequentially extracting the wet ball-milled tissue with 1 % aq. Na deoxycholate, PhOHHOAcH₂O and 90 % (v/v) aq. DMSO. The purity of the CWM (e.g. absence of residual starch) was established by carbohydrate analysis using different acid hydrolysis conditions and by methylation studies. The partially methylated alditol acetates from the CHCl₃MeOH soluble fraction (S) of the methylated CWM were separated into 15 main peaks by GLC. Fourteen of these peaks were carbohydrate derivatives and the identity of most of these was established by MS. Reduction of the hydrolysate of S with NaBD₄ was used to identify the carbohydrate derivatives present in peaks 7 and 11 above. The occurrence of 4-linked galacturonosyl residues in the methylated polymers was established after reduction of S with LiAlH₄ and LiAlD₄. The main glycosidic linkages present in the non-cellulosic polysaccharides of the wall in descending order of concentration are: 4-linked galactose, 4-linked galacturonic acid, 5-linked arabinose and 4,6-linked glucose. The major branch points are those through 0–6 of glucose and 0–4 of rhamnose. Arabinose, galactose and xylose residues constituted the non-reducing ends. Graded acid hydrolysis of the CWM made it possible to assess the relative strengths of some of the glycosidic linkages. The general structural features of the CWM are discussed in the light of these results.     

Effect of ageing on enzymes of phenylpropanoid metabolism in Solanum tuberosum discs

-Separation of cell fractions or cell organelles of potato tuber by differential centrifugation and by sucrose density gradient centrifugation showed that, in dormant tissue, nearly all the activity of shikimate and prephenate dehydrogenases, phenylalanine ammonia lyase, cinnamate-4-hydroxylase and an O-methyltransferase for caffeate was in the soluble fraction. All these enzymes increased in activity in slices aged in light for 18 hr. In contrast to the other enzymes, cinnamate hydroxylase becomes associated with the microsomal fraction in aged discs.     

Effect of cold on glucose metabolism by callus and tubers of Solanum tuberosum

This work was done in order to discover the immediate effects of low temperature on glucose metabolism by tissue of Solanum tuberosum. [U-¹⁴C]-Glucose was supplied to tubers, and to callus derived from tubers, for 3 hr at 2 and 25°. The detailed distribution of label showed that lowering the temperature of both callus and tuber tissue to 2° caused a striking increase in the percentage of the metabolized label that was recovered in glucose-6-phosphate, fructose-6-phosphate. and glucose-1-phosphate. It is suggested that these results, together with the cold-lability of glycolytic enzymes, indicate that lowering the temperature of potato tissue reduces glycolysis in relation to the activities of other reactions involving hexose phosphates.     

Peroxidases of Solanum melongena leaves

Three major peroxidases, designated as A, B₂ and B₂ from Solanum melongena leaves have been reported. Peroxidases-A, -B₂ and -B₂ were considered to be true peroxidases on the basis of k₁:k₄ ratio. The pH optima for the three enzymes were found to be 7·0, 6·0 and 6.0 respectively. These peroxidases differ in their k₁:k₄ ratio, in the effect of pH on this ratio and in the uric acid/guaiacol and o-dianisidine/guaiacol activity ratio.     

Phenolic oxidase activities in glandular trichomes of Solanum berthaultii

Polyphenol oxidase and peroxidase activities were detected in foliar glandular trichomes of the wild, insect-resistant potato species, Solanum berthaultii. These enzyme activities may provide the basis for conversion of clear, viscous trichome exudate into a hard, brown substance which is formed rapidly after insect attack.     

A Simple in Vitro Method to Study Tuber Growth of Solanum tuberosum L.

A simple in vitro method is described which allows the study of effects of mineral nutrient supply on growth rate of the tuber. Single node cuttings carrying axillary tubers are cultured under semi-sterile conditions in cooled (+ 1°C) nutrient solutions. Tuber growth is monitored volumetrically and mineral nutrient uptake calculated from the nutrient solution taken up by the cuttings.