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Plastidial degradation of transitory starch yields mainly maltose and glucose. Following the export into the cytosol, maltose acts as donor for a glucosyl transfer to cytosolic heteroglycans as mediated by a cytosolic transglucosidase (DPE2; EC 2.4.1.25) and the second glucosyl residue is liberated as glucose. The cytosolic phosphorylase (Pho2/PHS2; EC 2.4.1.1) also interacts with heteroglycans using the same intramolecular sites as DPE2. Thus, the two glucosyl transferases interconnect the cytosolic pools of glucose and glucose 1-phosphate. Due to the complex monosaccharide pattern, other heteroglycan-interacting proteins (HIPs) are expected to exist. Identification of those proteins was approached by using two types of affinity chromatography. Heteroglycans from leaves of Arabidopsis thaliana (Col-0) covalently bound to Sepharose served as ligands that were reacted with a complex mixture of buffer-soluble proteins from Arabidopsis leaves. Binding proteins were eluted by sodium chloride. For identification, SDS-PAGE, tryptic digestion and MALDI-TOF analyses were applied. A strongly interacting polypeptide (approximately 40 kDa; designated as HIP1.3) was observed as product of locus At1g09340. Arabidopsis mutants deficient in HIP1.3 were reduced in growth and contained heteroglycans displaying an altered monosaccharide pattern. Wild type plants express HIP1.3 most strongly in leaves. As revealed by immuno fluorescence, HIP1.3 is located in the cytosol of mesophyll cells but mostly associated with the cytosolic surface of the chloroplast envelope membranes. In an HIP1.3-deficient mutant the immunosignal was undetectable. Metabolic profiles from leaves of this mutant and wild type plants as well were determined by GC–MS. As compared to the wild type control, more than ten metabolites, such as ascorbic acid, fructose, fructose bisphosphate, glucose, glycine, were elevated in darkness but decreased in the light. Although the biochemical function of HIP1.3 has not yet been elucidated, it is likely to possess an important function in the central carbon metabolism of higher plants.

A technique has been developed to measure absolute intracellular oxygen concentrations in green plants. Oxygen-sensitive phosphorescent microbeads were injected into the cells and an optical multifrequency phase-modulation technique was used to discriminate the sensor signal from the strong autofluorescence of the plant tissue. The method was established using photosynthesis-competent cells of the giant algae Chara corallina L., and was validated by application to various cell types of other plant species.

In this communication both matrix-assisted laser desorption/ionization mass spectrometry (MALDI–MS) and high-performance anion-exchange chromatography (HPAEC) have been applied to analyze fructans from higher plants. Size distribution of a commercially available fructan preparation fromDahlia variabilisL. was determined by MALDI–MS. Molecular masses ranged from 2000 up to 6000 Da with a peak value of the distribution at 2635 Da. Essentially the same pattern was obtained using HPAEC. Low-molecular-weight fructans from onion bulbs (Allium cepaL.) were studied in more detail. Tissue extracts were analyzed by MALDI–MS without any analyte purification. Mass spectra of both proteins and oligosaccharides were obtained. For identification, metastable ion scanning was performed. Neither deproteinization nor deionization of the samples affected the oligosaccharide pattern. Using HPAEC, a more complex oligosaccharide pattern was obtained because isomeric glycans were differentiated. However, the overall size distribution was similar to that obtained by MALDI–MS. In further experiments epidermal or parenchyma cell layers of the onion bulb were placed into matrix solution and were then subjected to MALDI–MS and metastable ion scanning as well. By taking this approach, analyte desorption was achieved immediately from plant tissue. Oligosaccharide mass spectra were essentially the same as those of the extracts. To our knowledge, this is the first time that MALDI–MS has been applied as a microprobe to plant tissue. Finally MALDI–MS analysis was performed using single-cell extracts from onion tissues without any purification of the analyte.

In this study neutral and acidic oligosaccharide fractions prepared from human milk have been investigated using matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). The fraction of neutral oligosaccharides was separated by gel permeation chromatography (GPC) and the resulting subfractions were analyzed by MALDI-MS using the positive ion mode. Several low-molecular-weight glycans (degree of polymerization up to 13) were observed whose structures have already been elucidated. In addition, a variety of so far unknown large-sized carbohydrates was detected whose molecular weights range from M(r) 2242 to 8000. The large-sized glycans which possess a low abundance appear to be composed of both lactosamine and fucose residues attached to the lactose unit at the reducing end of the sugar chains with a highly variable stochiometry. Following subfractionation by GPC, acidic (i.e., containing sialic acid) glycans were analyzed by MALDI-MS using both positive and negative ion mode. Because of the inferior stability of acidic glycans, various matrices were applied and compared with respect to signal intensity, resolution, and analyte stability.

Summary Chloroplast and non-chloroplast a-glucan phosphorylases (EC 2.4.1.1) were separated by ion-exchange chromatography on DEAE-Sephacel and identified by analytical polyacrylamide gel electrophoresis and co-electrophoresis with extracts of isolated intact chloroplasts. The non-chloroplast phosphorylase was eluated with a linear gradient of 20-60 mmol citrate/l (adjusted to pH 6.5 with NaOH), then the chloroplast phosphorylase was recovered with a linear sodium chloride gradient (0-1.2 mol NaCl/1 in 60 mmol citrate/1). In a control experiment, chromatography of a preparation from isolated intact chloroplasts produced only one phosphorylase activity peak which appeared during elution with the sodium chloride gradient. In a leaf extract, an approximately 1 : 3 ratio of chloroplast to non-chloroplast activity was observed. The apparent molecular weights determined by gel filtration were 230,000 for the chloroplast phosphorylase and 150,000 for the non-chloroplast enzyme. Non-chloroplast phosphorylase showed a high affinity to both starch and glycogen when assayed under saturating orthophosphate concentrations, whereas the chloroplast phosphorylase had an affinity to starch about 10 times lower and an even lesser affinity to glycogen. Both phosphorylases exhibited similar affinities to orthophosphate (Km 1-2 mmol/1) when assayed at saturating starch concentrations. Key words

Chloroplast alpha-1,4-glucan phosphorylase (EC 2.4.1.1) has been purified to homogeneity from spinach leaves as revealed by sodium dodecyl sulfate polyacrylamide gel electrophoresis. The purification procedure is composed of (NH4)2SO4 precipitation, ion-exchange chromatography, and chromatography on Sepharose-bound starch. In order to achieve binding of the chloroplast phosphorylase, a previously described Sepharose-glucan gel (Steup, M. Schächtele, C. and Latzko, E. (1980) Planta 148, 168-173) was modified by introducing hydrophobic groups in addition to the covalently bound starch. The chloroplast phosphorylase exhibited complete binding to this type of gel and could be eluted by a mixture of soluble glucan and NaCl. For the purified chloroplast phosphorylase, sodium dodecyl-sulfate polyacrylamide gel electrophoresis and pyridoxal phosphate determination resulted in a molecular weight estimation of about 110,000 per monomer. The apparent molecular weight of the native enzyme, as determined by polyacrylamide density gradient electrophoresis and gel filtration on Sephadex G-200, was 200,000 and 220,000, respectively. The data indicate that the chloroplast phosphorylase is a dimer with a molecular weight higher than that of the non-chloroplast phosphorylase.

Leaf extracts of Pisum sativum L. contain three forms of α-1,4-glucan phosphorylase (EC 2.4.1.1) activity. One of these (form I) is located outside the chloroplast; the other two reside inside this organelle (Steup, M. and Latzko, E. (1979) Planta 145, 69–75). The extra-chloroplastic enzyme form, which represents the major proportion of the total extractable phosphorylase activity, was purified and characterized. Its in situ location was determined by indirect immunofluorescence performed with cryostat sections of formaldehyde-fixed leaf. By this technique the enzyme was localized in the cytoplasm of mesophyll and guard cells, whereas the other epidermal cells lacked the enzyme. In its kinetic properties, especially glucan specificity, the enzyme was very similar to the cytosolic phosphorylase from spinach leaves; it has a low affinity towards low-molecular-weight glucans but a very high affinity towards branched polysaccharides such as strach and glycogen. The immunological properties of the enzyme and its peptide pattern were determined and compared with those of other plant phosphorylase. The pea phosphorylase form I was immunologically different from the two chloroplastic phosphorylase forms, and it reacted more strongly with antibodies raised against the spinach cytosolic phosphorylase than with those directed against the spinach chloroplastic counterpart. Peptide patterns obtained after cleavage with N-chlorosuccinimide were very similar for the cytosolic spinach and pea leaf phosphorylase forms, suggesting a high degree of homology between both proteins.