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A new abscisic acid (ABA) analogue has been isolated from tomato plants. High levels of the compound are found in flacca mutants compared with normal isogenic controls. The analogue also accumulates in response to water stress. Three alternative structures, consistent with the mass spectrum, have been proposed. The possibility that the compound may be a biosynthetic precursor of ABA is considered.
Aurintricarboxylic acid (ATA) at a concentration which produces 40% inhibition of protein synthesis, inhibits completely isoproterenol-stimulated DNA synthesis in mouse parotid glands. The drug was found to interfere with some essential changes occurring during the prereplicative phase of IPR-stimulated DNA synthesis. It inhibits the increase in ribosonal protein synthesis that takes place by 2 h after stimulation. The peak of ribosonal RNA that occurs 8 h after isoproterenol was also abolished by ATA. Since the drug completely inhibits isoproterenol-stimulated DNA synthesis, these results suggest that the control of ribosome production may be involved in cell growth activation. In view of the finding that ATA first inerferes with the binding of adenylate-rich RNA to polysomes, it was suggested that the drug may act by preferentially inhibiting that fraction of protein synthesis dependent on the newly transcribed messenger RNA.
Glutamic acid 1-semialdehyde hydrochloride was synthesized and purified. Its prior structural characterization was extended and confirmed by 1H NMR spectroscopy and chemical analyses. In aqueous solution at pH 1 to 2 glutamic acid 1-semialdehyde exists in a stable hydrated form, but at pH 8.0 it has a half-life of 3 to 4 min. Spontaneous degradation of the material at pH 8.0 generated some undefined condensation products, but coincidentally a significant amount isomerized to 5-aminolevulinate. At pH 6.8 to 7.0, glutamate 1-semialdehyde is sufficiently stable to permit routine and reproducible assay for glutamate 1-semialdehyde aminotransferase activity. Only about 20% of the enzyme extracted from chloroplasts was sensitive to inactivation by gabaculine with no pretreatment. However, when the enzyme was exposed to 5-aminolevulinate, levulinate or 4,5-dioxovalerate in the absence of glutamate 1-semialdehyde, it was completely inactivated by gabaculine; 4,6-dioxoheptanoate had no effect on the enzyme. These results lead to the hypothesis that the aminotransferase exists in the chloroplast in a complex with pyridoxamine phosphate, which must be converted to the pyridoxal form before it can form a stable adduct with gabaculine. We propose that the enzyme catalyzes the conversion of glutamate 1-semialdehyde to 5-aminolevulinate via 4,5-diaminovalerate.
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Enzyme extracts from Enterobacter aerogenes (62-1), Streptomyces aminophilus, and Streptomyces coelicolor were used to investigate the biosynthesis of p-aminobenzoic acid. The enzyme preparations from E. aerogenes and S. aminophilus contained both p-aminobenzoate synthase and iso-chorismate synthase activity, and were able to convert both chorismic and iso-chorismic acid to p-aminobenzoic acid. The apparent KM for chorismic acid was, however, significantly lower than that for iso-chorismic acid, while the Vmax was identical for both substrates in both enzyme systems. The enzyme preparations from S. coelicolor did not contain iso-chorismate synthase activity and p-aminobenzoic acid synthesis took place in this system from chorismic acid only. It is concluded that iso-chorismic acid is not an obligatory intermediate in p-aminobenzoic acid biosynthesis in these organisms.
Commercially available, as well as synthetically prepared, samples of aurintricarboxylic acid (a widely employed potent inhibitor of protein nucleic acid interactions) consist mostly of a heterogeneous collection of polymers, as shown by fractionation schemes utilizing both dialysis and ultrafiltration, and by molecular weight measurements. 13C-NMR studies suggest that the polymeric material is of the phenol-formaldehyde type; inhibitory assays that depend on the formation of a protein-nucleic acid complex revealed that potency varied directly with the molecular weight of the polymer. Fractions of molecular weight 400 were essentially inactive.
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Potassium channels were resolved in Vicia faba guard cell protoplasts by patch voltage-clamp. Whole-cell currents and single K+ channels had linear instantaneous current-voltage relations, reversing at the calculated Nernst potential for K+. Whole cell K+ currents activated exponentially during step depolarizations, with half-activation times of 400-450 msec at +80 mV and 90-110 msec at +150 mV. Single K+ channel conductance was 65 +/- 5 pS with a mean open time of 1.25 +/- 0.30 msec at 150 mV. Potassium channels were blocked by internal Cs+ and by external TEA+, but they were insensitive to external 4-aminopyridine. Application of 10 microM abscisic acid increased mean open time and caused long-lasting bursts of channel openings. Since internal and external composition can be controlled, patch-clamped protoplasts are ideal systems for studying the role of ion channels in plant physiology.
Activities of aminopropyltransferases (spermidine synthase and spermine synthase) were inhibited by aurintricarboxylic acid (ATA). Spermidine synthase was slightly more sensitive to the inhibitor than spermine synthase. These inhibitions were not prevented by 0.15 M NaCl. Inhibition by ATA of spermidine synthase was 'uncompetitive' with respect to putrescine and that of spermine synthase was 'non-competitive' with respect to spermidine. When the amount of spermidine synthase or spermine synthase was varied, inhibition ratio hardly changed on either case implying no appreciable interaction between ATA and these enzymes.
Aurin tricarboxylic acid (A.T.A.), an inhibitor of protein biosynthesis (initiation and elongation steps), acts also as a competitive inhibitor of phenylalanine, in the ATP-PPi exchange and tRNAPhe aminoacylation reactions catalysed by cytoplasmic wheat germ phenylalanine:tRNA ligase.
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Glu-plasminogen I (Glu-plg I: with two carbohydrate chains) and Glu-plg II (with one carbohydrate chain) were separated by a gradient elution of 6 aminohexanoic acid (6AHA) through lysine-Sepharose. Each preparation was excited with ultraviolet light of wave length at 291 nm. The intensity of fluorescence was measured at 340 nm. The intensity of fluorescence increased to a small extent at 0.02 mM of tranexamic acid (t-x) for Glu-plg I and then quickly increased from 0.1 mM of t-x to reach the peak at 0.6 mM. The intensity of fluorescence for Glu-plg II started to increase at 0.2 mM to reach the peak at 0.7 mM. No small increase of fluorescence was observed at less than 0.2 mM of t-x for Glu-plg II. Kdobs of Glu-plg I for t-x and 6AHA were 0.34 mM and 1.35 mM, respectively, whereas Kdobs of Glu-plg II for t-x and 6AHA were 0.46 mM and 3.3 mM, respectively. When Glu-plg I and II were activated by urokinase (UK) and the hydrolysis of S-2251 was measured, the extent of hydrolysis increased in the presence of t-x and 6AHA. The rate of the increase of S-2251 hydrolysis (thus activation rate of Glu-plg I and II with UK) increased in parallel with increase in fluorescence intensity of Glu-plg I and II in the presence of omega-aminoacids. In conclusion, changes in the activation rate with UK and in fluorescence intensity were observed at lower concentrations of omega-aminoacids for Glu-plg I than for Glu-plg II.
Conformational changes of two isozymes of Glu-plasminogen (Glu-plg I and II) induced by omega-aminoacids were studied by using fluorescence polarization and spectropolarimetry. The rotational relaxation times (Pn) of FITC labeled Glu-Plg I and II decreased in the presence of 6 aminohexanoic acid (6AHA) or tranexamic acid (t-x), which may mean increase in Brownian motion of FITC labeled region (possibly N-terminal region) of Glu-plg I and II when 6AHA or t-x binds with lysine binding sites (LBS) of these plasminogens. Glu-plg II seems to have longer rotational relaxation time compared to that of Glu-plg I, which may mean smaller extent of Brownian motion of FITC labeled region of Glu-plg II in comparison to that of Glu-plg I. The far ultraviolet circular dichroism (CD) spectra indicate that there may be some difference in the polypeptide backbone between Glu-plg I and II, possibly more of beta-structure and less of random coil structure in Glu-plg II in comparison to Glu-plg I. The presence of 6AHA or t-x gave rise to larger change of the negative ellipticity at around 208 nm in Glu-plg I in comparison to its change in Glu-plg II, which may mean the larger extent of conformational change of Glu-plg I induced by 6AHA or t-x than that of Glu-plg II.
The plant hormone abscisic acid (ABA) is shown to enhance the aggregation and fusion of small unilamellar lipid vesicles composed of 80 mol% dimyristoylphosphatidylcholine (DMPC) and 20 mol% dimyristoylphosphatidylcholine (DMPE). Aggregation and fusion did not occur with single component (100 mol%) DMPC vesicles. Fusion was followed by two fundamentally different techniques, fluorescence resonance energy transfer which monitors intermixing of bilayers and ANTS-DPX which monitors intermixing of the sequestered aqueous interiors. It is suggested that a previously unreported role of ABA may be as a membrane fusagen.
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To obtain information about the degree of concert of both the nonenzymic and the enzyme-catalyzed rearrangement of chorismate to prephenate, we have determined the secondary tritium isotope effects at the bond-making position (C-9) and the bond-breaking position (C-5) of chorismate. The isotope effects were determined by the competitive method, using either [5-3H,7-14C )chorismate or [9-3H,7-14C]chorismate as the substrate. In the nonenzymic reaction (pH 7.5, 60 degrees C), KH/kT is 1.149 +/- 0.012 for bond breaking (C-9) and 0.992 +/- 0.012 for bond making (C-5). This indicates an asymmetric transition state in which the new bond is hardly, if at all, formed, while the bond between C-5 and oxygen is substantially broken. In the enzymic reaction (pH 7.5, 30 degrees C), the values of kH/kT in both positions are unity within experimental error. It is most likely that the isotope effects are suppressed in the enzymic process and that the rate-limiting transition state occurs before the rearrangement itself. The kinetically significant transition state presumably involves either the binding step of the small equilibrium proportion of the axial conformer of the substrate or an isomerization of enzyme-bound chorismate from the more stable conformer in which the carboxyvinyloxy group is equatorial to that in which this group is axial. Rearrangement would then proceed relatively rapidly from the higher energy axial conformer.