UDP-Glucose: (1-->3)-beta-Glucan Synthases from Mung Bean and Cotton: Differential Effects of Ca and Mg on Enzyme Properties and on Macromolecular Structure of the Glucan Product

A re-examination of the kinetic properties of UDP-glucose: (1→3)-β-glucan (callose) synthases from mung bean seedlings (Vigna radiata) and cotton fibers (Gossypium hirsutum) shows that these enzymes have a complex interaction with UDP-glucose and various effectors. Stimulation of activity by micromolar concentrations of Ca²⁺ and millimolar concentrations of β-glucosides or other polyols is highest at low (<100 micromolar) UDP-glucose concentrations. These effectors act both by raising the V_max of the enzyme, and by lowering the apparent K_m for UDP-glucose from >1 millimolar to 0.2 to 0.3 millimolar. Mg²⁺ markedly enhances the affinity of the mung bean enzyme for Ca²⁺ but not for β-glucoside; with saturating Ca²⁺, Mg²⁺ only slightly stimulates further production of glucan. However, the presence of Mg²⁺ during synthesis, or NaBH₄ treatment after synthesis, changes the nature of the product from dispersed, alkali-soluble fibrils to highly aggregated, alkali-insoluble fibrils. Callose synthesized in vitro by the Ca²⁺, β-glucoside-activated cotton fiber enzyme, with or without Mg²⁺, is very similar in size to callose isolated from cotton fibers, but is a linear (1→3)-β-glucan lacking the small amount of branches at C-0-6 found in vivo. We conclude that the high degree of aggregation of the fibrils synthesized with Mg²⁺in vitro is caused either by an alteration of the glucan at the reducing end or, indirectly, by an effect of Mg²⁺ on the conformation of the enzyme. Rate-zonal centrifugation of the solubilized mung bean callose synthase confirms that divalent cations can affect the size or conformation of this enzyme.     

Inhibition of ethylene production in fruit slices by a rhizobitoxine analog and free radical scavengers

The rhizobitoxine analog, L-2-amino-4-(2-aminoethoxy)-trans-3-butenoic acid (Ro), which effectively inhibits ethylene production in apple (Malus domestica Borkh.) and other tissues at concentrations at about 68 micromolar, inhibited ethylene production by about 50 to 70% in green tomato (Lycopersicon esculentum Mill.) fruit slices but only by about 15% in pink and ripe tomato tissue slices. Ethylene production in climacteric-rise and postclimacteric avocado slices was likewise relatively insensitive to 68 micromolar Ro. At 340 micromolar Ro, inhibition of ethylene production increased up to 50% in pink tomato slices, whereas 680 micromolar Ro was required to inhibit ethylene production by 30% in avocado slices. Incorporation of ¹⁴C from [¹⁴C]methionine into ethylene in green and pink tomato tissues was inhibited by Ro to about the same extent as inhibition of total ethylene production. Results thus far are inconclusive as to the mechanism of Ro resistance in tomato and avocado tissues. At 1 millimolar, free radical scavengers such as benzoate, propyl gallate, nordihydroguaiaretic acid, and to a lesser extent, eugenol, inhibited ethylene production in both Ro-sensitive (green tomato and apple) tissues and Ro-resistant (pink tomato and avocado) tissues. Therefore, free radical steps are suggested in the ethylene-forming systems.     

Regulation of rat brainstem tryptophan 5-monooxygenase. Calcium-dependent reversible activation by ATP and magnesium.

Tryptophan 5-monooxygenase in rat brainstem cytosol was activated about twofold by incubation with 0.5 mm ATP and 5 mm MgCl₂. The activation required micromolar concentrations of Ca²⁺ but was not dependent on either cyclic AMP or cyclic GMP. Rat brain cytosol was shown to possess an endogenous protein kinase which was markedly stimulated by the addition of Ca²⁺ using endogenous protein substrates. Following activation by ATP and Mg²⁺ in the presence of Ca²⁺, tryptophan 5-monooxygenase was reversibly deactivated to the original level by incubation at 30 °C after removal of Ca²⁺ by adding ethylene glycol bis(β-aminoethyl ether)N,N′-tetraacetic acid and was then reactivated by incubation at 30 °C after subsequent addition of Ca²⁺ and ATP. The deactivation was markedly inhibited by the omission of Mg²⁺ or by the addition of NaF.     

Inhibition of ethylene production by cobaltous ion

The effect of Co²⁺ on ethylene production by mung bean (Phaseolus aureus Roxb.) and by apple tissues was studied. Co²⁺, depending on concentrations applied, effectively inhibited ethylene production by both tissues. It also strongly inhibited the ethylene production induced by IAA, kinetin, IAA plus kinetin, Ca²⁺, kinetin plus Ca²⁺, or Cu²⁺ treatments in mung bean hypocotyl segments. While Co²⁺ greatly inhibited ethylene production, it had little effect on the respiration of apple tissue, indicating that Co²⁺ does not exert its inhibitory effect as a general metabolic inhibitor. Ni²⁺, which belongs to the same group as Co²⁺ in the periodic table, also markedly curtailed both the basal and the induced ethylene production by apple and mung bean hypocotyl tissues.     

Cytosolic Phosphofructokinase from Spinach Leaves : II. Affinity for Mg and Nucleoside Phosphates

Cytosolic ATP-phosphofructokinase (PFK) from spinach leaves (Spinacia oleracea L.) was inhibited by submillimolar concentrations of free Mg²⁺. The free Mg²⁺ concentration required for 50% inhibition of PFK activity was 0.22 millimolar. Inhibition by free Mg²⁺ was independent of the MgATP²⁻ concentration. Inorganic phosphate (Pi) reduces the inhibition of PFK activity by Mg²⁺. Free ATP (ATP⁴⁻) also inhibits PFK activity. For free ATP the inhibition of PFK activity was dependent on the MgATP²⁻ concentration. Fifty percent inhibition of PFK activity requires 1.2 and 3.7 millimolar free ATP at 0.1 and 0.5 millimolar MgATP²⁻, respectively. It was proposed that free ATP competes for the MgATP²⁻ binding site, whereas free Mg²⁺ does not. Pi diminished the inhibitory effect of free ATP on PFK activity. Free ATP and Pi had substantial effects on the MgATP²⁻ requirement of cytosolic PFK. For half-maximum saturation of PFK activity 3 and 76 micromolar MgATP²⁻ was required at 0.007 and 0.8 millimolar free ATP in the absence of Pi. At 5 and 25 millimolar Pi, half-maximum saturation was achieved at 9 and 14 micromolar MgATP²⁻. PFK activity was inhibited by Ca²⁺. The inhibition by Ca²⁺ depends upon the total Mg²⁺ concentration. Fifty percent inhibition of PFK activity required 22 and 32 micromolar Ca²⁺ at 0.1 and 0.2 millimolar Mg²⁺, respectively. At physiological concentrations of about 0.5 millimolar free Mg²⁺, Ca²⁺ would have little effect on cytosolic PFK activity from spinach leaves. PFK is not absolutely specific for the nucleoside 5′-triphosphate substrate. Besides MgATP²⁻, MgUTP²⁻, MgCTP²⁻, and MgGTP²⁻ could be used as a substrate. All four free nucleotides inhibit PFK activity. The physiological consequences of the regulatory properties of cytosolic PFK from spinach leaves will be discussed. A model will be introduced, in an attempt to describe the complex interaction of PFK with substrates and the effectors Mg²⁺ and Pi.     

Alpha-amylase secretion by single barley aleurone layers

The secretion of α-amylase from single isolated (Hordeum vulgare L. cv Himalaya) aleurone layers was studied in an automated flow-through apparatus. The apparatus, consisting of a modified sample analyzer linked to a chart recorder, automatically samples the flow-through medium at 1 minute intervals and assays for the presence of α-amylase. The release of α-amylase from aleurone layers begins after 5 to 6 hours of exposure to gibberellic acid and reaches a maximum rate after 10 to 12 hours. The release of α-amylase shows a marked dependence on Ca²⁺, and in the absence of Ca²⁺ it is only 20% of that in the presence of 10 millimolar Ca²⁺. Withdrawal of Ca²⁺ from the flow-through medium results in the immediate cessation of enzyme release and addition of Ca²⁺ causes immediate resumption of the release process. The effect of Ca²⁺ is concentration-dependent, being half-maximal at 1 millimolar Ca²⁺ and saturated at 10 millimolar Ca²⁺. Ruthenium red, which blocks Ca²⁺ but not Mg²⁺ efflux from barley aleurone layers, renders α-amylase release insensitive to Ca²⁺ withdrawal. Inhibitors of respiratory metabolism cause a burst of α-amylase release which lasts for 0.5 to 5 hours. Following this phase of enhanced α-amylase release, the rate of release declines to zero. Pretreatment of aleurone layers with HCl prior to incubation in HCN also causes a burst of α-amylase release, indicating that the inhibitor is affecting the secretion of α-amylase and not its movement through the cell wall. The rapid inhibition of α-amylase release upon incubation of aleurone layers at low temperature (5°C) or in 0.5 molar mannitol also indicates that enzyme release is dependent on a metabolically linked process and is not diffusion-limited. This conclusion is supported by cytochemical observations which show that, although the cell wall matrix of aleurone layers undergoes extensive digestion after gibberellin treatment, the innermost part of the cell wall is not degraded and could influence enzyme release.     

Stimulation of ethylene production in the mung bean hypocotyls by cupric ion, calcium ion, and kinetin

The synergistic stimulation of ethylene production by kinetin and Ca²⁺ in hypocotyl segments of mung bean (Phaseolus aureus Roxb.) seedling was further studied. The requirement for Ca²⁺ in this system was specific. Except for Sr²⁺, which mimicked the effect of Ca²⁺, none of the following divalent cations, including Ba²⁺, Mg⁶⁺, Cu²⁺, Hg²⁺, Co²⁺, Ni²⁺, Sn²⁺, and Zn²⁺, showed synergism with kinetin on ethylene production. Fe²⁺, however, showed a slight synergism with kinetin. Some of them (Hg²⁺, Co²⁺, and Ni²⁺) had a strong inhibitory effect, while others (Zn²⁺, Mg²⁺, Sn²⁺, and Ba²⁺) had a slight or no inhibitory effect on ethylene production in the absence or presence of kinetin.     

Cis-unsaturated fatty acids modulate the function of the cell-surface cAMP receptor of Dictyostelium discoideum

The binding of cAMP to the chemotactic cAMP receptor in intact Dictyostelium discoideum cells and isolated membranes is strongly inhibited by unsaturated fatty acids. In isolated membranes, cis-unsaturated fatty acids decreased the number of accessible cAMP binding sites, without significantly altering their affinity. Most potent were C₁₈ and C₂₀ cis-poly unsaturated fatty acids, like arachidonic acid, linoleic acid and linolenic acid. Trans-unsaturated fatty acid was less potent than its cis isomer, while saturated fatty acids did not affect the binding of cAMP to receptors at all. Oxidation reactions were not important for the effect of unsaturated fatty acids. When membranes were preincubated with millimolar concentrations of Ca²⁺, the effect of unsaturated fatty acids was strongly diminished. Mg²⁺ was ineffective. Ca²⁺, if presented after the incubation of membranes with unsaturated fatty acids, did not reverse the inhibitory effect. The specificity of the fatty acid effect, and the interference with Ca²⁺, but not Mg²⁺, suggest that the properties of the cAMP receptor are changed as a result of alterations in the lipid bilayer structure of the membrane.     

Regulation of the synthesis of barley aleurone alpha-amylase by gibberellic Acid and calcium ions

The effects of gibberellic acid (GA₃) and calcium ions on the production of α-amylase and acid phosphatase by isolated aleurone layers of barley (Hordeum vulgare L. cv Himalaya) were studied. Aleurone layers not previously exposed to GA₃ or Ca²⁺ show qualitative and quantitative changes in hydrolase production following incubation in either GA₃ or Ca²⁺ or both. Incubation in H₂O or Ca²⁺ results in the production of low levels of α-amylase or acid phosphatase. The addition of GA₃ to the incubation medium causes a 10- to 20-fold increase in the amounts of these enzymes released from the tissue, and addition of Ca²⁺ at 10 millimolar causes a further 8- to 9-fold increase in α-amylase release and a 75% increase in phosphatase release. Production of α-amylase isoenzymes is also modified by the levels of GA₃ and Ca²⁺ in the incubation medium. α-Amylase 2 is produced under all conditions of incubation, while α-amylase 1 appears only when layers are incubated in GA₃ or GA₃ plus Ca²⁺. The synthesis of α-amylases 3 and 4 requires the presence of both GA₃ and Ca²⁺ in the incubation medium. Laurell rocket immuno-electrophoresis shows that two distinct groups of α-amylase antigens are present in incubation media of aleurone layers incubated with both GA₃ and Ca²⁺, while only one group of antigens is found in media of layers incubated in GA₃ alone. Strontium ions can be substituted for Ca²⁺ in increasing hydrolase production, although higher concentrations of Sr²⁺ are required for maximal response. We conclude that GA₃ is required for the production of α-amylase 1 and that both GA₃ and either Ca²⁺ or Sr²⁺ are required for the production of isoenzymes 3 and 4 of barley aleurone α-amylase.     

Effects of Ca and Mg upon amino acid transport in rat renal cortex slices

1.

1. The roles of Ca²⁺ and Mg²⁺ in the transport of amino acids were examined in rat kidney cortex slices in vitro. The absence of either Ca²⁺ or Mg²⁺ from the incubation fluid was associated with increased inulin space and slightly decreased K⁺ content of the slices although no significant alterations of total tissue water nor Na⁺ content were noted. Decreased net accumulation of glycine, cycloleucine and α-aminoisobutyric acid were found upon removal of either divalent cation from the incubation fluid with no corresponding effects upon efflux from prelabeled tissues. No effects of divalent cations were noted upon lysine transport.     

Activation of the renal cortical and hepatic guanylate cyclase-guanosine 3′,5′-monophosphate systems by nitrosoureas divalent cation requirements and relationship to thiol reactivity

Streptozotocin, 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) and N-methyl nitrosourea, compounds with both oncogenic and cytotoxic properties, increased guanylate cyclase activity in the 100 000 × g soluble fractions of rat renal cortex and liver 35- to 65-fold over basal values. Particulate enzyme activities of these tissues were increased 2- to 4-fold by a maximally effective concentration of the nitrosoureas. In the presence of the cyclic nucleotide phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine, maximally effective concentrations of these nitrosoureas increased cyclic GMP accumulation of hepatic and renal cortical slices to peak levels 7- to 10-fold over control in 30 min. By contrast, with the structurally related carcinogen N-methyl-N′-nitro-N-nitrosoguanidine (MNNG) peak increases occurred in 5–10 min and were 40- to 70-fold over control levels in renal cortex and liver, respectively. Unlike the Ca²⁺-dependent actions of cholinergic stimuli on cyclic GMP, the nitrosoureas and MNNG increased cyclic GMP in either the presence or absence of extracellular Ca²⁺. Moreover, while basal soluble guanylate cyclase of renal cortex was highly Mn²⁺-dependent and decreased 85% when either Mg²⁺ or Ca²⁺ was employed as sole divalent cation in reaction mixtures, the actions of nitrosoureas on enzyme activity were well expressed with either Mn²⁺ or Mg²⁺, but not with Ca²⁺, as sole divalent cation. Improved utilization of Mg²⁺ by guanylate cyclase in the presence of nitrosoureas would favor enhanced enzyme activity under cellular conditions where Mg²⁺ is abundant. In the presence of maximally stimulatory concentrations of streptozotocin or BCNU, high concentrations of Mg²⁺ or Mn²⁺ further increased soluble guanylate cyclase, suggesting important differences in metal and nitrosourea stimulation of enzyme activity.Preincubation of supernatant fractions with nitrosoureas plus dithiothreitol inhibited the action of the N-nitroso compounds to increase renal cortical guanylate cyclase. Glutathione and cysteine were also inhibitory, but less effective than dithiothreitol. Initial incubation of nitrosoureas with dithiothreitol in buffer alone similarly suppressed the subsequent action of the N-nitroso compounds on guanylate cyclase, and implicated direct chemical interactions. Prior incubation of renal cortical supernatant fractions with the SH blockers N-ethylmaleimide or maleimide significantly suppressed guanylate cyclase activation mediated by streptozotocin or BCNU. Direct drug interactions seemed unlikely, since effects of the inhibitors were optimally expressed by initial exposure of the supernatant fraction of tissue to the SH blockers and were not potentiated by a 30 min preincubation of the SH blockers and nitrosoureas in buffer alone.Thus, nitrosoureas activate and alter the metal requirements of soluble guanylate cyclase and increase cellular cyclic GMP in the presence or absence of extracellular Ca²⁺. Activation of soluble guanylate cyclase by nitrosoureas may involve an interaction of these agents with tissue SH groups, and possibly SH to SS transformation. Stimulation of the guanylate cyclase system by nitrosoureas could be related to the oncogenic actions of these agents.     

INFLUENCE OF DIVALENT CATIONS ON REGULATION OF CYCLIC GMP AND CYCLIC AMP LEVELS IN BRAIN TISSUE

—Depolarizing concentrations of K⁺ elevate levels of both adenosine 3′,5′monophosphate (cyclic AMP) and guanosine 3′,5′monophosphate (cyclic GMP) in incubated slices of mouse cerebellum. Calcium is an essential requirement for the K⁺ -induced accumulation of cyclic GMP. Barium and Sr²⁺, but not Mn²⁺ or Co²⁺, can substitute for Ca²⁺ in this process. Relatively high concentrations of Mg²⁺ inhibit the effect of Ca²⁺ on K⁺-induced accumulation of cyclic GMP. In contrast, depolarizing concentrations of K⁺ are capable of elevating cyclic AMP levels in brain slices suspended in media containing Mg²⁺ and no other divalent cations. High concentrations of Ca²⁺ (1 mm or greater) augment this Mg²⁺ -dependent, K⁺-induced accumulation of cyclic AMP, however. Strontium and Mn²⁺, but not Ba²⁺ or Co²⁺, can substitute for Ca²⁺ in this process, and high concentrations of Mg²⁺ are not inhibitory. The divalent cation ionophore, A-23187 (10 μm ), in the presence of extracellular Ca²⁺ elevates the level of cyclic GMP, but not cyclic AMP, in incubated mouse cerebellum slices. The results of this study indicate that intracellular Ca²⁺ concentration is a major factor regulating cyclic GMP levels in brain. In addition the present results suggest that, in brain tissue, depolarization-induced accumulation of cyclic GMP, but not cyclic AMP, is closely linked to some Ca²⁺-dependent mechanism(s) mediating release of intracellular substances.     

Freezing Injury and Phospholipid Degradation in Vivo in Woody Plant Cells: II. Regulatory Effects of Divalent Cations on Activity of Membrane-bound Phospholipase D

Activity of membrane-bound phospholipase D in microsomes from bark tissues of black locust tree (Robina pseudoacacia L.) was demonstrated to be regulated by a competitive binding of divalent cations. Binding of Ca²⁺ at high concentrations (1 to 50 millimolar) modified the pH activity profile, shifting the optimum pH by 0.5 unit toward neutral and increasing the activity in the neutral pH. Mg²⁺, on the other hand, inhibited the reaction of membrane-bound phospholipase D without added Ca²⁺, and competitively inhibited the Ca²⁺ stimulation. The regulatory effects of those ions were dependent on pH. Reduction in pH resulted in a decrease in the apparent dissociation constant for Ca²⁺ and an increase in that for Mg²⁺. From Lineweaver-Burk double reciprocal plots of Ca²⁺ and the initial velocity, it was suggested that the binding of Ca²⁺ in the higher concentration resulted in nearly the same conformational change of enzyme as reduction in pH. Mg²⁺, on the other hand, counteracted those effects of Ca²⁺ and lower pH on the enzyme conformation in such a manner as to inactivate. The membrane-bound phospholipase D because more sensitive to Ca²⁺ and less sensitive to Mg²⁺ as the hardiness of the tissues decreased. This fact may indicate that some qualitative changes in membranes are involved in the hardiness changes and also in the susceptibility of phospholipid to degradation by phospholipase D in plant cells.     

Effects of agents affecting Ca2+ homeostasis onTrichoderma viride growth and conidiation

Agents known to influence Ca²⁺ homeostasis affected significantly the vegetative growth and starvation-induced conidiation ofTrichoderma viride. Ca²⁺ in millimolar concentrations stimulated both growth and conidiation; a Ca²⁺ deprivation of the fungus by the chelation of extracellular Ca²⁺ (not Mg²⁺ or divalent trace metals) with EGTA (ethyleneglycolbis(2-aminoethylether)-N,N,N′,N′-tetraacetic acid) restricted both the vegetative growth rate and starvation-induced conidiation. Both processes were affected by either Ca²⁺ or EGTA with different efficiencies. Divalent cations (Sr²⁺, Ba²⁺, Co²⁺, Ni²⁺, Mg²⁺, Cd²⁺, Cu²⁺, Mn²⁺) and La³⁺ (inorganic Ca²⁺ blockers) in millimolar concentrations exerted complex (stimulatory, inhibitory, or biphasic) effects on growth and conidiation. In general, their effects on the two processes were mutually different either qualitatively, or quantitatively, or both. Organic Ca²⁺ antagonists (verapamil and dihydropyridines) inhibited the vegetative growth. The results show that Ca²⁺ is required for vegetative growth and conidiation, and that different Ca²⁺-dependent mechanisms may be involved in the two processes. Divalent cations could serve as a tool for investigating the relationship between growth and conidiation.     

Stimulation and inhibition by magnesium ions of intrinsic protein phosphorylating systems in synaptosomal membrane fragments from rat brain

Synaptosomal membrane fragments from rat brain were incubated with [-³²P]ATP in the presence of cyclic AMP or Ca²⁺ plus calmodulin and a range of Mg²⁺ concentrations. Incorporation of³²P into membrane polypeptides was examined by electrophoresis and radioautography. Cyclic AMP-stimulated reactions were stimulated by low concentrations and inhibited to varying degrees by high concentrations of Mg²⁺ in the range 1–50 mM. In general the Ca²⁺ plus calmodulin-stimulated reactions were maximally active in the range 30–50 mM Mg²⁺, but the Ca²⁺ plus calmodulin dependent phosphorylation of Protein I was progressively inhibited by concentrations of Mg²⁺ above 5 mM. These results emphasize the importance of establishing optimum Mg²⁺ concentrations in the study of specific membrane protein phosphorylating systems.     

Interaction of kinetin and calcium in relation to their effect on stimulation of ethylene production

Application of kinetin and Ca²⁺ caused a striking synergistic increase in ethylene production by mung bean (Phaseolus aureus Roxb) hypocotyl segments. The effect of kinetin on Ca²⁺ uptake and of Ca²⁺ on the uptake and metabolism of kinetin in relation to their effect on ethylene production was studied. Tracer experiments showed that kinetin greatly increased the uptake of ⁴⁵Ca²⁺ after 6 hours of incubation. Reciprocally, Ca²⁺ stimulated the uptake of kinetin-8-¹⁴C and remarkably enhanced the metabolism of kinetin-8-¹⁴C into several polar metabolites. Consequently, the quantity of free kinetin-8-¹⁴C remaining in Ca²⁺-treated segments was much less than in control segments. A possible mechanism accounting for the synergism between kinetin and calcium on ethylene production is discussed.     

Protection of Pyruvate,Pi Dikinase from Maize against Cold Lability by Compatible Solutes

Most C₄ species are chilling sensitive and certain enzymes like pyruvate,Pi dikinase of the C₄ pathway are also cold labile. The ability of cations and compatible solutes to protect maize (Zea mays) dikinase against cold lability was examined. The enzyme in desalted extracts at pH 8 from preilluminated leaves could be protected against cold lability (at 0°C) by the divalent cations Mn²⁺, Mg²⁺, and Ca²⁺. There was substantial protection by sulfate based salts but little protection by chloride based salts of potassium or ammonium (concentration 250 millimolar). The degree of protection against cold lability under limiting MgCl₂ (5 millimolar) was pH sensitive (maximum protection at pH 8), but independent of ionic strength (up to 250 millimolar by addition of KCl). In catalysis Mg²⁺ is required and Mn²⁺ could not substitute as a cofactor. Several compatible solutes reduced or prevented the cold inactivation of dikinase (in desalted extracts and the partially purified enzyme), including glycerol, proline, glycinebetaine and trimethylamine-N-oxide (TMAO). TMAO and Mg²⁺ had an additive effect in protecting dikinase against cold inactivation. TMAO could largely substitute for the divalent cation and addition of TMAO during cold treatment prevented further inactivation. Cold inactivation was partially reversed by incubation at room temperature; with addition of TMAO reversal was complete. The temperature dependence of inactivation at pH 8 and 3 millimolar MgCl₂ was evaluated by incubation at 2 to 17°C for 45 minutes, followed by assay at room temperature. At preincubation temperatures below 11°C there was a progressive inactivation which could be prevented by TMAO (450 millimolar). The results are discussed relative to possible effects of the solutes on the quaternary structure of this enzyme, which is known to dissociate at low temperatures.     

Rapid osmotic adjustment by a succulent halophyte to saline shock

The objective of this research was to measure the short term osmotic adjustment of Salicornia europaea L. ssp. rubra (A. Nels) Breitung when suddenly exposed to 100 millimolar NaCl. Plants were grown hydroponically, shocked with 100 millimolar NaCl added to the culture solution, and stem tips analyzed for free inorganic ions and small organic molecules at intervals up to 72 hours. In the first 2 hours, the calculated leaf osmoticum showed a net increase of 158.8 millimolar most of which was free Mg²⁺ (+135.3 millimolar). Total sugars increased almost 5-fold by the 6th hour, enough to provide sufficient osmoticum for the cytoplasm if only partially confined there. By 24 hours, all measured osmotica had decreased except Na⁺, Mg²⁺, Cl⁻, and proline, with the net increase being 208 millimolar. By 72 hours, there was a net gain of 356 millimolar in osmotica of the stem tips, due to Na⁺ (+233.3 millimolar), Cl⁻ (+306.7 millimolar), and a small increase in sugar and proline (+3.5 millimolar), with all other osmotica decreasing in concentration. Compatible osmotica did not change sufficiently to account for osmotic balance between vacuole and cytoplasm; consequently, there must have been a reapportionment of osmotica within the cell in the short time duration of this experiment.     

Ca2+ effects on ethylene, carbon dioxide and 1-aminocyclopropane-1-carboxylic acid synthase activity

The response of pericarp disks from ripening tomato (Lycopersicon esculentum Mill. cv. Traveler‘76) to CaCl₂, additions was studied to determine the effect of Ca²⁺ on ethylene and CO₂ production. Application of 5 mM CaCl₂ resulted in a 2, 20, 33, 39, and 50% increase in ethylene production in disks obtained from preclimacteric minimum, climacteric rise, climacteric peak, one, and two days postclimacteric fruit, respectively. CaCl₂ concentrations of 10 and 50 mM gave no additional stimulation of ethylene production; CO₂ production at 5 mM CaCl₂ was not different from controls, but is increased at 10 and 50mM CaCl₂. CaCl₂ also increased ethylene production in disks treated with 1-aminocyclopropane-1-carboxylic acid (ACC) or aminoethoxy-vinylglycine. Chloride salts of K⁺, Na⁺, Mg²⁺, Sr²⁺ and La³⁺ did not stimulate ethylene production. SrCl₂ stimulated ethylene production to a lesser degree than CaCl₂. Disks from potato (Solanum tuberosum L. cv. Katahdin) tubers produced greater quantities of ethylene and ACC when 5 mM CaCl₂ was included in the incubation medium (K. B. Evensen, 1983. Physiol. Plant. 60:125–128). Ca²⁺-treated disks had more than three times as much ACC synthase activity as control disks after 18 to 24 h incubation, when ethylene and ACC were maximal. The apparent K_m for S-adenosylmethionine was 13 μM at 29°C, pH 8.0 in extracts from both Ca²⁺-treated and control disks. Inclusion of 1 to 50 mM CaCl₂ in the assay medium did not significantly affect enzyme activity. ACC synthase extracted from control and Ca²⁺-treated disks had a pH optimum of 8.5 and an apparent molecular weight of 72 kdalton, estimated by gel filtration. It is likely that the presence of Ca²⁺ in the buffer allows greater synthesis of ACC synthase as part of the wound-healing response in potato, while in tomato the predominant effect is on membrane stabilization.     

Characterization of a partially purified adenosine triphosphatase from a corn root plasma membrane fraction

The (K⁺,Mg²⁺)-ATPase was partially purified from a plasma membrane fraction from corn roots (WF9 × Mol7) and stored in liquid N₂ without loss of activity. Specific activity was increased 4-fold over that of the plasma membrane fraction. ATPase activity resembled that of the plasma membrane fraction with certain alterations in cation sensitivity. The enzyme required a divalent cation for activity (Co²⁺ > Mg²⁺ > Mn²⁺ > Zn²⁺ > Ca²⁺) when assayed at 3 millimolar ATP and 3 millimolar divalent cation at pH 6.3. When assayed in the presence of 3 millimolar Mg²⁺, the enzyme was further activated by monovalent cations (K⁺, NH₄⁺, Rb⁺ Na⁺, Cs⁺, Li⁺). The pH optima were 6.5 and 6.3 in the absence and presence of 50 millimolar KCl, respectively. The enzyme showed simple Michaelis-Menten kinetics for the substrate ATP-Mg, with a K_m of 1.3 millimolar in the absence and 0.7 millimolar in the presence of 50 millimolar KCl. Stimulation by K⁺ approached simple Michaelis-Menten kinetics, with a K_m of approximately 4 millimolar KCl. ATPase activity was inhibited by sodium orthovanadate. Half-maximal inhibition was at 150 and 35 micromolar in the absence and presence of 50 millimolar KCl. The enzyme required the substrate ATP. The rate of hydrolysis of other substrates, except UDP, IDP, and GDP, was less than 20% of ATP hydrolysis. Nucleoside diphosphatase activity was less than 30% of ATPase activity, was not inhibited by vanadate, was not stimulated by K⁺, and preferred Mn²⁺ to Mg²⁺. The results demonstrate that the (K⁺,Mg²⁺)-ATPase can be clearly distinguished from nonspecific phosphohydrolase and nucleoside diphosphatase activities of plasma membrane fractions prepared from corn roots.