Untersuchungen zur Funktionsänderung der Microbodies in den Keimblättern von Helianthus annuus L.

Summary The enzyme patterns in sunflower cotyledons indicate that the glyoxysomal function of microbodies is replaced by the peroxisomal function of these organelles during the transition from fat degradation to photosynthesis. The separation of the microbody population into glyoxysomes and peroxisomes during this transition period is reported. The mean difference in density between the activity peaks of glyoxysomal and peroxisomal marker enzymes on a sucrose gradient was calculated to be 0.007±0.004 g/cm³ and turned out to be significant (t=7.8>4.04=t _5;0.01). The activity peak of catalase coincides with that of isocitrate lyase in early stages of development, but shifts to the activity peak of peroxisomal marker enzymes during the transition period. No isozymes of the catalase could be detected by gel electrophoresis in the microbodies with the two different functions.During the rise of the peroxisomal marker enzymes no synthesis of the common microbody marker, catalase, could be demonstrated using the inhibitor allylisopropylacetamide. Using D₂) for density labeling of newly-formed catalase, no difference is observed between the density of catalase from cotyledons grown on 99.8% D₂O during the transition period and the density of enzyme from cotyledons grown on H₂O. The activity of particulate glycolate oxidase is reduced 30–50% by allylisopropylacetamide, but is not affected by D₂O. The chlorophyll formation in the cotyledons is strongly inhibited by both substances.     

Die Hemmung der Isocitrat-Lyase bei Wassermelonenkeimlingen durch Weißlicht

Summary The isocitrate lyase activity (E.C. 4.1.3.1.) from watermelon cotyledons (Citrullus vulgaris Schrad.) is inhibited by white light (Fig. 5). To exclude artefacts during enzyme preparation the following experiments were performed (Table 1 and 2): 1. Mixing of raw extracts from cotyledons of light and dark grown seedlings. 2. Joint homogenization a) of cotyledons from light and dark grown seedlings, b) of purified isocitrate lyase together with cotyledons from light or dark grown seedlings. The total activity corresponded to an amount which was expected for the sum of the individual activities. The results justify the conclusion that the inhibition of the isocitrate lyase by light is real, and that the measured enzyme activities are close to the true enzyme concentrations in the plant tissue. — The relatively slow inhibition of the enzyme activity caused by light seems to be correlated with the formation of the photosynthetic apparatus.     

Glyoxysomal malate dehydrogenase of watermelon cotyledons: De novo synthesis on cytoplasmic ribosomes

The development of glyoxysomal malate dehydrogenase (gMDH, EC 1.1.1.37) during early germination of watermelon seedlings (Citrullus vulgaris Schrad.) was determined in the cotyledons by means of radial immunodiffusion. The active isoenzyme was found to be absent in dry seeds. By density labelling with deuterium oxide and incorporation of [¹⁴C] amino acids it was shown that the marked increase of gMDH activity in the cotyledons during the first 4 days of germination was due to de novo synthesis of the isoenzyme. The effects of protein synthesis inhibitors (cycloheximide and chloramphenicol) on the synthesis of gMDH indicated that the glyoxysomal isoenzyme was synthesized on cytoplasmic ribosomes. Possible mechanisms by which the glyoxysomal malate dehydrogenase isoenzyme reaches its final location in the cell are discussed.Abbreviations mMDH mitochondrial malate dehydrogenase - gMDH glyoxysomal malate dehydrogenase - D₂O deuterium oxide - EDTA ethylenediaminetetraacetic acid, disodium salt     

Deuterium labeling for investigating de novo synthesis of terpene volatiles in Achyranthes bidentata

Purpose of work

Plants synthesize and accumulate secondary metabolites as defensive volatiles against diverse stresses. We aim to unravel the jasmonate-inducible volatile de novo synthetic metabolites in plants using a deuterium-labeling technique. Jasmonic acid and its methyl ester (MeJA) are well-documented for inducing defensive volatiles. Here, we have developed an efficient deuterium oxide (D₂O)-based labeling approach to determine the extent of de novo synthetic metabolites in a model plant A. bidentata bidentata. The labeling approach was demonstrated on quantitative profiling of terpene volatile organic compounds (VOCs) elicited by airborne MeJA in Achyranthes plants. We show, for the first time that airborne MeJA-elicited terpene VOCs are predominantly and differentially de novo synthesized except for a homoterpene, (3E)-4,8-dimethyl-1,3,7-nonatriene, which is weakly and least labelled with deuterium. D₂O is therefore an efficient labeling source for investigating de novo synthetic metabolites of terpene VOCs in planta.     

Synthesis of Isocitrate Lyase in Sunflower Cotyledons during the Transition in Cotyledonary Microbody Function

Density-labeling with 10 millimolar K¹⁵NO₃/70% ²H₂O has been used to investigate isocitrate lyase synthesis during greening of sunflower (Helianthus annuus L.) cotyledons when the glyoxysomal enzyme activities sharply decline and the transition in cotyledonary microbody function occurs. A density shift of 0.0054 (kilograms per liter) was obtained for the profile of isocitrate lyase activity in the CsCl gradient with respect to the ¹H₂O control. Quantitative evaluation of the density-labeling data indicates that about 50% of the isocitrate lyase activity present towards the end of the transition stage in microbody function is due to enzyme molecules newly synthesized during this stage.     

The action of exogenous gibberellic acid on isocitrate lyase —mRNA in germinating castor bean seeds

Gibberellic acid (GA₃) stimulates isocitrate lyase activity of the endosperm during germination of castor bean seeds. Isocitrate lyase from castor bean was purified and an antibody to it was prepared from rabbit serum. This antibody was used to measure the amounts of isocitrate lyase-mRNA using an in vitro translation system. No specific stimulation of isocitrate lyase-mRNA by application of GA₃ was detected. The stimulation of isocitrate lyase activity by exogenous GA₃ may be accounted for by the action of the growth substance in advancing the overall production of rRNA and mRNA which accelerates the rate of total protein synthesis during germination. The application of Amo 1618 retards the production of isocitrate lyase activity but also retards protein synthesis in general. This suggests that endogenous gibberellins also act non-specifically in the regulation of protein synthesis during castor bean germination.Abbreviations SDS sodium dodecyl sulphate - GA₃ gibberellic acid - PAGE polyacrylamide gel electrophoresis     

Evidence for de novo synthesis of isocitratase and malate synthesis in germinating peanut cotyledons

Evidence for de novo synthesis of isocitratase and malate synthetase in cotyledons of germinating peanut (Arachis hypogaea L.) was obtained by the density labeling method. When dry peanut cotyledons were cultured in H₂¹⁸O, a 2.4% increase in the buoyant density of malate synthetase in a cesium chloride gradient was observed. In 100% D₂O the buoyant density shift was 5.5% for isocitratase and 3.5% for malate synthetase in comparison to the water controls. These data suggest that isocitratase and malate synthetase do not pre-exist in some inactive form in the cotyledons, but are completely synthesized after onset of germination from a pool of amino acids which do not derive directly from hydrolysis of storage proteins.     

Mitochondrial malate dehydrogenase of watermelon cotyledons: Time course and mode of enzyme activity changes during germination

Summary Specific antibodies were prepared against the purified mitochondrial malate dehydrogenase (EC 1.1.1.37) from cotyledons of watermelon seedlings (Citrullus vulgaris Schrad.). The isoenzyme was assayed by means of quantitative radial immunodiffusion. Cotyledons of ungerminated seeds were found to contain mitochondrial MDH. During the first 4 days of germination the enzyme activity increased threefold finally contributing 16% to the total MDH activity extracted from cotyledon tissue. Isopycnic CsCl density centrifugation was used to investigate the mode of activity increase. After a four-day period of labelling with deuterium oxide and purification of the mitochondrial isoenzyme, a density shift of 0.021kgx1^-1, accompanied by considerable band broadening of the enzyme profile was observed. These findings are evidence for the de novo synthesis of mitochondrial MDH and its relatively slow turnover in germinating seeds.Abbreviations mMDH mitochondrial malate dehydrogenase - D₂O deuterium oxide     

De Novo Synthesis of Isocitritase in Peanut (Arachis hypogaea L.) Cotyledons

Germination of peanut seed is accompanied by a rapid increase in isocitritase (isocitrate lyase, EC 4.1.3.1) during the first 4 days. The presence of cycloheximide (50 μg/ml) during water imbibition inhibited the increase in isocitritase activity. Actinomycin D conversely did not inhibit isocitritase activity until the second day of imbibition while RNA synthesis was inhibited. Germination of peanut seed in ¹⁴C-reconstituted amino acids followed by fractionation of a 20 to 35% ammonium sulfate preparation on a Sephadex G-200 column (57-fold purification) showed that the active enzymic fraction coincided with a large peak of radioactivity. Germination of peanut seed in 45% D₂O followed by enzyme purification and CsCl equilibrium centrifugation revealed that all the enzyme from D₂O seed had a higher density than normal isocitritase. These data indicate that isocitritase in peanut seed is synthesized de novo.     

Isolation and properties of watermelon isocitrate lyase

The glyoxylate cycle enzyme, isocitrate lyase (EC 4.1.3.1) was purified from cotyledons of Citrullus vulgaris (watermelon). The final preparation, which had been 97-fold purified with a specific activity of 16.1 units/mg protein in a yield of 36%, was homogeneous by gel- and immunoelectrophoretic criteria. The tetrameric enzyme had: a molecular weight of 277 000, a sedimentation coefficient of 12.4 s, and a K_m for D_s-isocitrate equal to 0.25 mM. Isocitrate lyase from this source is not a glycoprotein as shown by total carbohydrate content after precipitation by trichloroacetic acid of the purified enzyme. Reduction of the enzyme with thiols increased activity and maximal activity was obtained with at least 5 mM dithiothreitol. EDTA partially substituted for thiol in freshly isolated enzyme. Watermelon isocitrate lyase was also protected against thermal denaturation at 60° for at least 1 hr by 5 mM Mg²⁺ plus 5 mM oxalate. Oxalate was a competitive inhibitor with respect to isocitrate (K_i: 1.5 μM, pH 7.5, 30°).     

Microbody transition in greening watermelon cotyledons Double immunocytochemical labeling of isocitrate lyase and hydroxypyruvate reductase

Microbody transition during the greening of watermelon cotyledons (Citrullus vulgaris Schrad.) was studied by double immunocytochemical labeling of the glyoxysomal marker enzyme isocitrate lyase and the peroxisomal marker enzyme hydroxypyruvate reductase. In order to analyze the immunocytochemistry, developmental stages representing the glyoxysomal, microbodytransition and peroxisomal stages were chosen, taking into account the time course of enzyme activity and the amounts of the respective antigens. It was shown that during microbody transition, between 83 and 91% of all the tested microbodies contained isocitrate lyase as well as hydroxypyruvate reductase, which was significantly higher than in the glyoxysomal and peroxisomal stages of development. Comprehensive controls precluded labeling artifacts. Our results support the one-population hypothesis first proposed by Trelease et al. (1971, Plant Physiol. 48, 461–465).Abbreviations ICJ isocitrate lyase - HPR hydroxypyruvate reductase - pAg small protein A-gold complex - pAG large protein A-gold complex     

Enzyme Profiles in Seedling Development and the Effect of Itaconate, an Isocitrate Lyase-directed Reagent

Changes in levels of isocitrate lyase, malate synthase, and catalase have been investigated during germination of flax (Linum usitatissimum L.) in the presence and absence of itaconate. Germination was accompanied by a rapid increase in these enzymes during the first 3 days. The presence of 38 millimolar itaconate inhibited the incidence of seed germination and the growth of embryo axes as well as the appearance of isocitrate lyase but did not alter the levels of malate synthase, catalase, or NADP⁺-isocitrate dehydrogenase. The specific activity for the latter enzyme was constant throughout germination. Oxalate or succinate, each at 38 millimolar, had no effect upon germination of flax seeds. Itaconate did not inhibit the activities of malate synthase, catalase, or NADP⁺-isocitrate dehydrogenase in vitro but was a potent noncompetitive inhibitor of isocitrate lyase (K_i:17 micromolar at 30 C, pH 7.6). Itaconate (at 38 millimolar) did not alter the appearance of malate synthase but reduced the incidence of germination, onset of germination, and growth of the embryo axis as well as the specific activity of isocitrate lyase in seedlings of Zea mays, Vigna glabra, Glycine hispida, Vigna sinensis, Trigonella foenumgraecum, Lens culinaris, and Medicago sativa. The incidence and onset of germination of wheat seeds were unaltered by the same concentration of itaconate but seedlings did not contain isocitrate lyase or malate synthase. The data suggest that itaconate may be isocitrate lyase-directed in inhibiting the germination of fatty seeds.     

The effect of deuterium oxide on light-sensitive Paulownia tomentosa seed germination

The effect of deuterium oxide on the germination of light-sensitive Empress tree (Paulownia tomentosa) seeds was studied. Kinetics of germination in both light- and gibberellic acid (GA₃)-induced seeds was decelerated by increasing concentrations of deuterium oxide (D₂O) in the medium. Deuterium oxide in increasing concentrations also changed the duration of the light necessary for germination, from 5 min in H₂O to more than 3 days in 60% D₂O. A far-red (FR) pulse reversed the effect of inductive, prolonged red (R) irradiation; maximum germination could subsequently be restored by only 5 min of red light. The escape from FR reversibility was delayed from 1 day in water to 2 or 3.5 days in D₂O. The light reactions of D₂O-treated seeds resemble those of skotodormant seeds of the same species.     

Isocitrate dehydrogenase and glyoxylate cycle enzyme activities in Bradyrhizobium japonicum under various growth conditions

Bradyrhizobium japonicum, the nitrogen-fixing symbiotic partner of soybean, was grown on various carbon substrates and assayed for the presence of the glyoxylate cycle enzymes, isocitrate lyase and malate synthase. The highest levels of isocitrate lyase [165–170 nmol min^–1 (mg protein)^–1] were found in cells grown on acetate or β-hydroxybutyrate, intermediate activity was found after growth on pyruvate or galactose, and very little activity was found in cells grown on arabinose, malate, or glycerol. Malate synthase activity was present in arabinose- and malate-grown cultures and increased by only 50–80% when cells were grown on acetate. B. japonicum bacteroids, harvested at four different nodule ages, showed very little isocitrate lyase activity, implying that a complete glyoxylate cycle is not functional during symbiosis. The apparent K _m of isocitrate lyase for d,l-isocitrate was fourfold higher than that of isocitrate dehydrogenase (61.5 and 15.5 μM, respectively) in desalted crude extracts from acetate-grown B. japonicum. When isocitrate lyase was induced, neither the V _max nor the d,l-isocitrate K _m of isocitrate dehydrogenase changed, implying that isocitrate dehydrogenase is not inhibited by covalent modification to facilitate operation of the glyoxylate cycle in B. japonicum. Received: 10 October 1997 / Accepted: 16 January 1998     

Analysis of Indole-3-acetic Acid Metabolism in Zea mays Using Deuterium Oxide as a Tracer

A method using deuterium oxide (D₂O) as a tracer was used to study indole-3-acetic acid (IAA) metabolism in Zea mays seedlings. Seeds were imbibed and grown for 4 days in 30% D₂O in the dark. IAA was then isolated from roots and shoots and analyzed for deuterium content by mass spectrometry. We found that a significant portion of the IAA isolated from plants had incorporated deuterium at nonexchangeable sites of the indole ring. This indicates that some of the IAA in the germinating seedling is made via de novo indole synthesis. Moreover, we found that the deuterium content of IAA was 2.6 times greater in shoots than in roots. These results indicate that at least some of the IAA in roots and shoots came from different biosynthetic pathways. It appears that the fraction of IAA produced via de novo indole synthesis is greater in shoots than in roots.     

Development of Microbodies in Sunflower Cotyledons and Castor Bean Endosperm during Germination

In cotyledons of sunflower seedlings glyoxysomal and peroxisomal enzymes exhibit different rates of development during germination. The total activity of isocitrate lyase, a glyoxysomal marker enzyme, rapidly increased during the first 3 days, and then decreased 89% by day 9. Exposure to light accelerated this decrease only slightly. The specific activity of glyoxysomal enzymes (malate synthetase, isocitrate lyase, citrate synthetase, and aconitase) in the microbody fraction from sucrose density gradients increased between days 2 and 4 about 2- to 3-fold, and thereafter it remained about constant in light or darkness.     

Isolation and properties of isocitrate lyase from Lupinus seed

Isocitrate lyase (threo-d_s-isocitrate glyoxylate-lyase, EC 4.1.3.1) was purified from cotyledons of Lupinus seedlings. The final preparation showed two bands after polyacrylamide-gel electrophoresis. The optimum pH using phosphate, Tris or imidazole buffer was at pH 7.5; with triethanolamine (TRA) it was at pH 7. The enzyme required Mg²⁺ for maximal activity, and N-ethylmaleimide (NEM) inactivated the enzyme. Activity was increased by incubation with the reducing agents, glutathione (GSH), acetylcysteine (acetylcys), dithionite (Na₂S₂O₄), thioglycolate (TG) or 1,4-dithioerythritol (DTE). Na₂S₂O₄ and DTE were the most active among the tested substances and DTE prevented much of the inactivation by NEM. The apparent K_m value for isocitrate was ca 1 mM in phosphate buffer at pH 6.8 or 7.5 but was substantially lower (0.1–0.2 mM) using Tris, TRA or imidazole buffers. Glyoxylate, oxalate and malonate were competitive inhibitors of the enzyme. Synthase activity of the enzyme (i.e. formation of isocitrate from succinate and glyoxylate) was demonstrated. The K_m values for glyoxylate and succinate were 0.05 and 0.2 mM, respectively. The addition of glyoxylate to the culture medium in which Lupinus seeds germinate resulted in a reduced development of isocitrate lyase activity during germination.     

Isocitrate lyase in germinating spores of the fern Anemia phyllitidis

Isocitrate lyase was partially purified from germinating spores of the fern Anemia phyllitidis. The enzyme requires Mg²⁺ and thiol compounds for maximal activity and has a pH optimum between 6.5 and 7.5. The K_m of the enzyme for threo-Δ_s-isocitrate is 0.5 mM. Succinate inhibits the enzyme non-competitively (K_i. 1.8 mM). The increase of isocitrate lyase activity is closely correlated with the induction of the germination process. The fall of enzyme activity during germination is associated with the decline in triglyceride reserves.     

The Regulatory Role of the Embryo on the Development of Isocitrate Lyase Activity during Germination of Ponderosa Pine Seeds

The specific activity of isocitrate lyase rapidly increased in the megagametophytic tissue of cold-stratified seeds of ponderosa pine (Pinus ponderosa Laws) prior to and after germination. When the embryo was removed at germination, isocitrate lyase activity continued to develop. However, in the total absence of the embryo, only a small increase in the specific activity of the enzyme was observed. The development of the enzyme was inhibited by cycloheximide, actinomycin D and abscisic acid. The embryo produced an unidentified factor which enhanced the development of isocitrate lyase activity in the megagametophytic tissue. This embryo factor could not be replaced by the hormones indoleacetic acid (IAA), gibberellic acid (GA₃) or benzylaminopurine (BA). Indoleacetic acid had little effect upon enzyme development. Gibberellic acid and benzylaminopurine inhibited isocitrate lyase development in the megagametophytic tissue of the seed.