Characterization of regucalcin effect on proteolytic activity in rat liver cytosol: relation to cysteinyl-proteases

The increasing effect of regucalcin, isolated from rat liver cytosol, on neutral proteolytic activity in the hepatic cytosol was characterized. The proteolytic activity was markedly elevated by the addition of regucalcin (0.1–0.5 M) in the absence of Ca²⁺. This increase was not significantly altered by the presence of diisopropylfluorophsophate (DPF;2.5 mM)—although DFP caused a significant decrease in the proteolytic activity. Regucalcin (0.25 M) additively enhanced the dithiothreitol (DTT; 1.0 mM)—increased proteolytic activity, while the regucalcin or DTT effect was completely abolished by NEM (5 mM), indicating that regucalcin may act on the SH group in proteases. Also, regucalcin (0.25 M) enhanced the effect of Ca²⁺ (10 M) increasing liver proteolytic activity, suggesting that regucalcin does not influence on the active sites for Ca²⁺ in proteases. Moreover, the proteolytic activity of regucalcin (0.25 M) was significantly decreased by the presence of calpastatin (24 g/ml), an inhibitor of Ca²⁺-activated neutral protease (calpain). Now, regucalcin (0.25 M) increased about 7-fold the activity ofm-calpain isolated from rabbit skeletal muscle. These observations demonstrate that regucalcin directly activates cysteinyl-proteases. Regucalcin may have a role as a potent proteolytic activator in the cytoplasm of liver cells.     

Calcium-binding protein regucalcin inhibits deoxyribonucleic acid synthesis in the nuclei of regenerating rat liver

The effect of regucalcin, a Ca²⁺-binding protein isolated from rat liver cytosol, on deoxyribonucleic acid (DNA) synthesis in the nuclei of regenerating rat liver was investigated. At 1 day after partial hepatectomy, the liver weight was increased about 50% of that of sham-operated rats, and it reached to the same levels as sham operation at 3 days after hepatectomy. Nuclear DNA synthesis was markedly increased at 1 day after hepatectomy, and this increase was also seen at 3 days. Nuclear DNA synthesis was clearly enhanced in the presence of EGTA (0.4 mM) in the incubation mixture. The presence of Ca²⁺ ( 1.0–25 M) caused a significant decrease in the nuclear DNA synthesis of normal rat liver. Regucalcin (0.25 and 0.5 M) clearly inhibited the nuclear DNA synthesis of normal rat liver. This inhibition was also seen in the presence of Ca²⁺ (1.0 M). Moreover, in the liver nuclei obtained at 1 day after partial hepatectomy, the presence of regucalcin (0.05–0.5 M) caused a remarkable inhibition of nuclear DNA synthesis. This effect was also revealed in the presence of EGTA (0.4 mM). Thus, the inhibitory effect of regucalcin was remarkable in regenerating rat liver nuclei in comparison with that of normal rat liver. The present results demonstrate that regucalcin can suppress nuclear DNA synthesis in regenerating rat liver. We suppose that regucalcin may have a role in the regulation of nuclear DNA synthesis in liver cell proliferation.     

Activatory effect of regucalcin on GTPase activity in rat liver plasma membranes

The effect of regucalcin, a regulatory protein of Ca²⁺ signaling, on guanosine-5-triphosphatase (GTPase) activity in isolated rat liver plasma membranes was investigated. GTPase activity was significantly increased by the addition of Ca²⁺ (25–100 M) in the enzyme reaction mixture. Such an increase was not seen by other metals (Mg, Co, Zn, Cu, Ni, and Mn) with 50 M. The activatory effect of calcium (50 M) was significantly decreased by calmodulin (2.5 and 5 g/ml), indicating that it does not depend on calmodulin. The presence of regucalcin (0.1–0.5 M) in the enzyme reaction mixture caused a significant increase in GTPase activity. This increase was not significantly enhanced by calcium (50 M). GTPase activity was significantly increased by dithiothreitol (DTT; 5 mM), a protecting reagent of thiol (SH)-groups, while it was decreased by N-ethylmaleimide (NEM; 5 mM), a modifying reagent of SH-groups. The effect of calcium or regucalcin in increasing GTPase activity was not seen in the presence of NEM. Also, the activatory effect of calcium or regucalcin on GTPase was not seen in the presence of vanadate, an inhibitor of protein phosphorylation, which could inhibit GTPase activity. Moreover, the effect of regucalcin was not seen in the presence of digitonin (0.01%), a solubilizing reagent of membranous lipids, while the effect of calcium was not inhibited by digitonin. The present study demonstrates that regucalcin has an activatory effect on GTPase activity independently of Ca²⁺ in rat liver plasma membranes.     

Cytoskeletal-associated protein kinase and phosphatase activities from cerebral cortex of young rats

We describe the phosphorylation system associated with the Triton-insoluble cytoskeletal fraction that phosphorylates in vitro the 150 kDa neurofilament subunit (NF-M) and alpha and beta tubulin from cerebral cortex of rats. The protein kinase activities were determined in the presence of 20 M cyclic AMP (cAMP), 1 mM calcium and 1 M calmodulin (Ca²⁺/calmodulin) or 1 mM calcium, 0.2 mM phosphatidylserine and 0.5 M phorbol 12,13-dibutyrate (Ca²⁺/PS/PDBu). Phosphorylation of these cytoskeletal proteins increased approximately 35% and 65% in the presence of cAMP and Ca²⁺/calmodulin, respectively, but was unaffected in the presence of Ca²⁺/PS/PDBu. Basal phosphorylation of these proteins studied increased approximately 35% and 72% in the presence of 0.5 M okadaic acid and 0.01 M microcystin-LR, respectively, suggesting the presence of phosphatase type 1. Results suggest that at least two protein kinases and one protein phosphatase are associated with the Triton-insoluble cytoskeletal fraction from cerebral cortex of rats.     

Regulatory effect of regucalcin on (Ca2+−Mg2+)-ATPase in rat liver plasma membranes: comparison with the activation by Mn2+ and Co2+

The effect of various metals and regucalcin, a calcium-binding protein isolated from rat liver cytosol, on (Ca²⁺–Mg²⁺)-ATPase activity in the plasma membranes of rat liver was investigated. Of various metals (Zn²⁺, Cu²⁺, Ni²⁺, Mn²⁺, Co²⁺ and Al³⁺; 100 M as a final concentration), Mn²⁺ and Co²⁺ increased markedly (Ca²⁺–Mg²⁺)-ATPase activity, while other metals had no effect. When Ca²⁺ was not added into enzyme reaction mixture, Mn²⁺ and Co²⁺ (25–100 M) did not significantly increase the enzyme activity, indicating that heavy metals act on Ca²⁺-stimulated phosphorylation of the enzyme. Meanwhile, regucalcin (0.25–1.0 M) caused a remarkable elevation of (Ca²⁺–Mg²⁺)-ATPase activity. This increase was not inhibited by the presence of 100 M vanadate, although the effects of Mn²⁺ and Co²⁺ (100 M) were inhibited by vanadate. Also, the inhibition of the Mn²⁺ and Co²⁺ effects by vanadate was not seen in the presence of regucalcin. Moreover, regucalcin (0.5 M) increased significantly the enzyme activity in the absence of Ca²⁺. This effect of regulcalcin was not altered by increasing concentrations of Ca²⁺ added, indicating that the regucalcin effect does not depend on Ca²⁺. The present results suggest that regucalcin activates directly (Ca²⁺–Mg²⁺)-ATPase in liver plasma membranes, and that the activation is not involved in the Ca²⁺-dependent phosphorylation of the enzyme.     

Transformation of Atriplex gmelini plants from callus lines using Agrobacterium tumefaciens

Atriplex gmelini plants were regenerated via organogensis from hypocotyl explants. Callus lines were induced from the hypocotyl explants on Linsmaier and Skoog (LS) medium supplemented with 1 M benzyladenine and 5 M -naphthaleneacetic acid in the dark. Shoots were regenerated from the callus lines on LS medium supplemented with 20 M thidiazuron and 0.1 M -naphthaleneacetic acid under a high-intensity light condition (450 mol m^–2 s^–1). The regenerated shoots were rooted on LS medium without growth regulators to obtain fully developed plants. We succeeded in transforming Atriplex gmelini from callus lines using Agrobacterium tumefaciens.     

Mechanisms of Long-Lasting Depression of Synaptic Transmission in the CA1 Region of the Rat Hippocampus

Low-frequency tetanic stimulation (2 sec^-1, 5 min) of Schaffer collaterals (SchC) in superfused slices of the dorsal hippocampus of 12- to 15-day-old rats was demonstrated to evoke homosynaptic long-lasting depression (LLD) of synaptic transmission. The same procedure applied to hippocampal slices of mature (8-week-old or older) rats failed to elicit LLD. Low-frequency tetanic stimulation of the alveus in hippocampal slices, applied under conditions of intensified NMDA glutamate receptor functioning, led to the development of heterosynaptic LLD of synaptic transmission in the SchC–dendrites of the CA1 pyramidal neurons system. Both LLD cases were either absent or weakened when hippocampal slices were treated with a competitive blocker of the NMDA glutamate receptors, D-2-amino-5-phosphonovalerate (50 M). Morphine hydrochloride (10 M), as well as inhibitors of calmodulin and calcineurin (trifluoroperasine and cyclosporin A in concentrations of 1 and 50 M, respectively), interfered with induction of LLD or decreased its intensity. A blocker of the L-type voltage-dependent Ca²⁺ channels, nifedipine (10 M), did not influence homosynaptic LLD, but decreased heterosynaptic depression. Both types of depression of synaptic transmission were facilitated upon application of substances possessing a nootropic activity, 1 mM pyracetam or 5 M carbacetam. A blocker of NO synthase, N-nitro-L-arginine (10 M) did not alter either type of LLD. When hippocampal slices were influenced with a blocker of the A1 adenosine receptors, 1,3-dipropyl-8-phenylxanthine (1 M, 15 min), both LLD forms were intensified, and the development of homosynaptic LLD of synaptic transmission became possible in hippocampal slices of mature rats. When hippocampal slices were treated with an inhibitor of protein kinase C, polymyxin B (50 M, 15 min), intensification of LLD and, in particular, the development of homosynaptic LLD of synaptic transmission were observed. When an inhibitor of phospholipase A2, mepacrine (25 M, 15 min), was applied to hippocampal slices, both forms of LLD of synaptic transmission were significantly suppressed.     

Characterization of a calcium/calmodulin-dependent protein kinase homolog from maize roots showing light-regulated gravitropism

Roots of many species respond to gravity (gravitropism) and grow downward only if illuminated. This light-regulated root gravitropism is phytochrome-dependent, mediated by calcium, and inhibited by KN-93, a specific inhibitor of calcium/calmodulin-dependent protein kinase II (CaMK II). A cDNA encoding MCK1, a maize homolog of mammalian CaMK, has been isolated from roots of maize (Zea mays L.). The MCK1 gene is expressed in root tips, the site of perception for both light and gravity. Using the [³⁵S]CaM gel-overlay assay we showed that calmodulin-binding activity of the MCK1 is abolished by 50 M KN-93, but binding is not affected by 5 M KN-93, paralleling physiological findings that light-regulated root gravitropism is inhibited by 50 M KN-93, but not by 5 M KN-93. KN-93 inhibits light-regulated gravitropism by interrupting transduction of the light signal, not light perception, suggesting that MCK1 may play a role in transducing light. This is the first report suggesting a physiological function for a CaMK homolog in light signal transduction.Abbreviations CaM calmodulin - CaMK (II) Ca²⁺/calmodulin-dependent protein kinase (II) - CBP CaM-binding protein - CDPK Ca²⁺-dependent protein kinase - MCK1 maize homolog of mamalian CaMK This work is supported by the National Aeronautics and Space Administration grant No: NAGW 238.     

Somatic embryogenesis and plant regeneration from immature embryos of saw palmetto,an important landscape and medicinal plant

Somatic embryogenesis and plant regeneration from immature zygotic embryos was achieved for saw palmetto (Serenoa repens (Bartr.) Small). Embryos, isolated from immature fruit of native-grown plants, were cultured on Murashige and Skoog medium plus 0.15% (w/v) activated charcoal and supplemented with 452 M 2,4-dichlorophenoxyacetic acid (2,4-D) and 14.7 M N⁶-(2-isopentenyl)adenine (2iP). Clusters of somatic embryos developed from all immature zygotic embryos 5 weeks after culture initiation. After 12 weeks, explants were transferred to the same medium with the amount of 2,4-D reduced to 90.4 M which resulted in somatic embryo proliferation. Somatic embryos were then transferred to the basal medium containing 0.9, 9 M thidiazuron (TDZ), or no growth regulator for conversion into plantlets. The 9 M TDZ treatment was ineffective for plant regeneration. However, 12% of the embryos subcultured on 0.9 M TDZ were able to produce complete plantlets. Shoot production was obtained from 35% of the embryos subcultured in the absence of growth regulators. Rooting (100%) was achieved when these shoots were transferred onto medium containing 22.2 M -naphthaleneacetic acid (NAA).     

In vitro meristem cloning of Eucalyptus tereticornis Sm.

Rapid multiplication of axillary meristems and direct shoot development occurred from nodal explants of mature Eucalyptus tereticornis Sm. with 5.3 M NAA, 1.1 M IAA and 4.4 M BA in Murashige-Skoog medium. Repeated subcultures of the second generation shoot cultures into low cytokinin-auxin containing media (0.44–0.88 M BA+0.1 M NAA) yielded axillary microshoots in large numbers. Half-strength MS liquid medium with 4.9 M IBA, 5.5 M IAA and 5.3 M NAA for four days, half-strength semi-solid hormonefree MS medium with charcoal, and MS liquid medium without charcoal and hormones, in sequence, induced rooting of shoots in the dark. This system is suitable for the mass propagation of this difficult-to-root eucalypt.Abbreviations BA 6-benzyladenine - IAA indole-3-acetic acid - IBA -indole-3-butyric acid - 2-iP isopentyl adenine - Kn kinetin - MS Murashige-Skoog - NAA -naphthalene acetic acid - PVP polyvinylpyrrolidone     

Characterization of the type of calcium channel primarily regulating GABA exocytosis from brain nerve endings

In an attempt to further characterize the type of Ca²⁺ channels primarily regulating GABA exocytosis, the effects of increasing concentrations of CTx MVIIC,--Aga IVA and other Ca²⁺ channel blockers (nitrendipine, Cd²⁺ and Ni²⁺), commonly used for pharmacologically discerning among the various types of Ca²⁺ channels, were tested on the dissected Ca²⁺ dependent fraction of the depolarization evoked release of GABA from mouse brain synaptosomes. Our results show that -CTx MVIIC inhibits GABA exocytosis with a calculated IC₅₀ of 3 M and -Aga IVA with a calculated IC₅₀ of 50 nM. The divalent cation Cd²⁺ only diminishes GABA exocytosis at 70 M, but does not modify this response at lower concentrations (i.e. 1 and 10 M). Neither nitrendipine (10 M) nor Ni²⁺ (100 M and 500 M) modified GABA exocytosis. The failure of nitrendipine at a high concentration to inhibit GABA exocytosis discards L-type Ca²⁺ channels as the main regulators of this response; likewise that of Ni²⁺ discards Ca²⁺ channels of the N-type, and the failure of nM concentrations of -CTx MVIIC or 500 M Ni²⁺, also discards alpha_1A/Q-type Ca²⁺ channels as the main regulators of the GABA response. On the basis of these results and in particular of the higher potency of -Aga IVA than -CTx MVIIC, it is concluded that the type of Ca²⁺ channels that primarily determine the exocytosis of GABA belong to a P-like type of Ca²⁺ channels.     

NMDA-Independent Long-Term Depression of Synaptic Transmission in the Hippocampus: Mechanisms of Induction and Effects of Nootropic Drugs

In studies on transversal slices of the rat dorsal hippocampus, we found that low-frequency tetanic stimulation of the medial perforant pathway (2 sec^-1, 7.5 min) results in long-term depression (LTD) of field EPSP of granular cells in the dentate gyrus. This synaptic plasticity phenomenon was weakened by calmodulin, nitric oxide synthase, and protein kinase C inhibitors, trifluoperazine (1 M), N-nitro-L-arginine (5 M), and polymixin B (50 M), respectively, but was enhanced by a nonselective inhibitor of cAMP phosphodiesterases, 1-isobuthyl-3-methylxanthine (100 M), and a calcineurin inhibitor, cyclosporin A (50 M). The nootropic activity-possessing drugs piracetam, carbacetam, and etimizole suppressed, in a dose-dependent manner, the induction and expression of the studied form of LTD of synaptic transmission, but glycine did not. We assume that Ca²⁺- and protein kinase G-mediated increase in the activity of calmodulin is the main link in the induction of this LTD form. Calmodulin, via NO synthase and adenylate cyclase, increases the activities of protein kinase C, a substrate of the latter, and inhibitor 1. Under the influence of piracetam, carbacetam, and etimizole, the calmodulin concentration in the cytoplasm of dendritic spines attains a level sufficient for activation of Ca²⁺/calmodulin-dependent protein kinase, which provides for the phosphorylation of AMPA receptors and interferes with the development of LTD of synaptic transmission.     

Possible modulation of phosphorylation of acetylcholine receptor-enriched membrane preparations

During phosphorylation of acetylcholine receptor (AChR)-enriched membrane preparations fromTorpedo fuscomaculata, phosphate is incorporated into a single protein, with a molecular weight corresponding to that of one of the receptor subunits (37,000 daltons). This protein also seems to contain the receptor binding site. ATP binds to four protein species, one of which corresponds to a different subunit of the receptor (molecular weight 45,000). Phosphorylation of these membrane preparations is affected by several factors, known to be involved in postsynaptic events. Ca²⁺ (10 M) inhibits the reaction, whereas cGMP (20 M), causes stimulation. Furthermore it has been shown that the agonists, acetylcholine, and carbamylcholine (10 M and 1 M) stimulate the phosphorylation reaction, while the antagonists, tubocurarine, hexamethonium, and decamethonium (1 M), cause inhibition.     

Establishment and multiplication of colchi-autotetraploids of Rauvolfia serpentina L. Benth. ex Kurz. through tissue culture

A tissue culture procedure was developed for the establishment and propagation of a colchi-autotetraploid of Rauvolfia serpentina for possible commercial exploitation. Multiplication of autotetraploid shoots was obtained either through axillary bud elongation on Murashige and Skoog [1] medium (MS) containing 2.65 M (0.5 mgl^–1) -naphthaleneacetic acid and 0.33 M (0.05 mgl^–1) kinetin, or via multiple shoot formation on MS medium supplemented with 4.44 M (1.0 mgl^–1) 6-benzylaminopurine and 0.53 M (0.1 mgl^–1) -naphthaleneacetic acid. Rooting could be induced by transferring the shoots to MS medium containing 7.95 M (1.5 mgl^–1) -naphthaleneacetic acid alone. The plantlets, thus formed, were tetraploid in nature by cytological observations of the root tips. They exhibited 80–90% success in establishment under glass house and field conditions.     

Phenylacetic acid-induced somatic embryogenesis in cultured hypocotyl explants of geranium (Pelargonium x hortorum Bailey)

Summary The effect of a non-indole compound, phenylacetic acid (PAA), on the induction of somatic embryogenesis in tissue cultures of geranium (Pelargonium x hortorum Bailey cv. Scarlet Orbit Improved) was investigated. Hypocotyl explants derived from young, dark-grown seedlings were cultured on Murashige and Skoog (1962) medium (MS) supplemented with PAA or IAA (0.01–120 M) alone or in combination with BAP (8 M). Somatic embryogenesis was induced by both PAA and IAA at 0.01–20 M with 8 M BAP, however, the optima differed considerably for the two compounds. Maximal activity of IAA for somatic embryogenesis was found at 0.1–2.5 M, whereas PAA gave best results at 10 and 20 M under identical culture conditions. Higher concentrations (30–120 M) of IAA or PAA in the medium induced callusing in the explants, but the callus was neither embryogenic nor morphogenic.Abbreviations BAP N⁶-benzylaminopurine - IAA indole-3-acetic acid - MS Murashige and Skoog (1962) - PAA phenylacetic acid     

Calcium control of differentiation in Phytophthora palmivora

A method has been developed for the preparation of zoospores from Phytophthora palmivora which allows the ionic composition of the suspension medium to be closely controlled. Sub-micromolar concentrations of calcium ions have been shown to play a key role in maintaining the zoospore state and in the transition to the cyst stage. Restriction of free Ca²⁺ to between 0.2 and 1 M resulted in zoospores which could be maintained for several hours before they finally encysted and germinated. When exposed to citrus-pectin, or 3 mM SrCl₂, or to vigorous shaking, these zoospores underwent rapid synchronous encystment. At free Ca²⁺ concentrations below 0.1 M, zoospores lysed slowly. If exposed to inducers of encystment before lysis had occurred, the zoospores failed to respond to pectin or to vigorous shaking. However, they did differentiate in response to SrCl₂ addition. Provided the free Ca²⁺ was maintained between 0.02 and 0.2 M, zoospores survived gentle centrifugation, a procedure which previously had resulted in encystment.Abbreviations IM (ion-mix) release medium containing 100 M KCl, 10 M CaCl₂, and 10 M MgCl₂     

Modulation of brain mitochondrial membrane permeability and synaptosomal Ca2+ transport by dopamine oxidation

Effects of dopamine on the membrane permeability transition, thioredoxin reductase activity, production of free radicals and oxidation of sulfhydryl groups in brain mitochondria and the Ca²⁺ uptake by Na⁺-Ca²⁺ exchange and sulfhydryl oxidation in brain synaptosomes were examined. The brain mitochondrial swelling and the fall of transmembrane potential were altered by pretreatment of dopamine in a dose dependent manner. Depressive effect of dopamine on mitochondrial swelling was reversed by 10 g/ml catalase, and 10 mM DMSO. The activities of thioredoxin reductase in intact or disrupted mitochondria were decreased by dopamine (1-100 M), 25 M Zn²⁺ and 50 M Mn²⁺. Dopamine-inhibited enzyme activity was reversed by 10 g/ml SOD and 10 g/ml catalase. Pretreatment of dopamine decreased Ca²⁺ transport in synaptosomes, which was restored by 10 g/ml SOD and 10 mM DMSO. Dopamine (1-100 M) in the medium containing mitochondria produced superoxide anion and hydrogen peroxide, while its effect on nitrite production was very weak. The oxidation of sulfhydryl groups in mitochondria and synaptosomes were enhanced by dopamine with increasing incubation times. Results suggest that dopamine could modulate membrane permeability in mitochondria and calcium transport at nerve terminals, which may be ascribed to the action of free radicals and the loss of reduced sulfhydryl groups.     

In vitro clonal propagation of tea (Camellia sinensis (L.) O. Kuntze)

A system for in vitro clonal propagation has been developed in tea plants. Shoots obtained from primary explants were induced from terminal buds and axillary buds of mature field-grown plants. Cultures were initiated from both types of explants on Murashige and Skoog (MS) medium supplemented with 10% coconut milk (CM), 200 mg l^-1 of yeast extract (YE), 1.4 M indoleacetic acid (IAA) and 17.8 M benzyladenine (BA). The shoot tips were multiplied on 1/2 strength MS medium containing 10% CM, 2.9 M IAA and 17.8 M BA. The larger shoots were separated after multiplication and rooted on 1/2 MS medium supplemented with 11.4 M ascorbic acid and 34.5 M indolebutyric acid (IBA). A pretreatment of the plants with an aqueous solution of 493 M IBA greatly increased the frequency of rooting. More than 60% of the rooted plants have been transferred to soil successfully.Abbreviations BA benzyladenine - IAA indoleacetic acid - IBA indolebutyric acid - YE yeast extract - CM coconut milk - MS Murashige and Skoog medium (1962)     

Copper but not iron limitation increases astaxanthin production by Phaffia rhodozyma in a chemically defined medium

The influence of copper (0–32 M) and iron (0–108 M) on growth and astaxanthin production by Phaffia rhodozyma was studied. Copper below 3.2 M increased the astaxanthin content of the cells (from 220 to 287 g g^–1) but at the expense of a slightly decreased growth (from 11.3 to 10.2 mg ml^–1). In contrast, iron below 1 M decreased both the growth and astaxanthin content of the cells. Using copper limitation instead of toxic respiratory inhibitors to improve astaxanthin production has obvious advantages from the product quality, environmental and process operation points of view.     

The influence of temperature on glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase and the regulation of these enzymes in a mesophilic and a thermophilic cyanobacterium

The activities and kinetics of the enzymes G6PDH (glucose-6-phosphate dehydrogenase) and 6PGDH (6-phosphogluconate dehydrogenase) from the mesophilic cyanobacterium Synechococcus 6307 and the thermophilic cyanobacterium Synechococcus 6716 are studied in relation to temperature. In Synechococcus 6307 the apparent K _m's are for G6PDH: 80M (substrate) and 20M (NADP⁺); for 6PGDH: 90M (substrate) and 25M (NADP⁺). In Synechococcus 6716 the apparent K _m's are for G6PDH: 550M (substrate) and 30M (NADP⁺); for 6PGDH: 40M (substrate) and 10M (NADP⁺). None of the K _m's is influenced by the growth temperature and only the K _m's of G6PDH for G6P are influenced by the assay temperature in both organisms. The idea that, in general, thermophilic enzymes possess a lower affinity for their substrates and co-enzymes than mesophilic enzymes is challenged.Although ATP, ribulose-1,5-bisphosphate, NADPH and pH can all influence the activities of G6PDH and 6PGDH to a certain extent (without any difference between the mesophilic and the thermophilic strain), they cannot be responsible for the total deactivation of the enzyme activities observed in the light, thus blocking the pentose phosphate pathway.Abbreviations G6PDH glucose-6-phosphate, dehydrogenase - 6PGDH 6-phosphogluconate dehydrogenase - G6P glucose-6-phosphate - 6PG 6-phosphogluconate - RUDP ribulose-1,5-bisphosphate - Tricine N-Tris (hydroxymethyl)-methylglycine