The Effect of Phenylalanine on DOPA Synthesis in PC12 Cells

DOPA synthesis from phenylalanine was studied in PC12 cells incubated with m-hydroxybenzylhydrazine, to inhibit aromatic L-amino acid decarboxylase. DOPA synthesis rose with increasing concentrations of either phenylalanine or tyrosine; maximal rates (~55 pmol/min/mg protein for tyrosine; ~40 pmol/min/mg protein for phenylalanine) occurred at a medium concentration of ~10 M for either amino acid. The K_m for either amino acid was about 1 M (medium concentration). At tyrosine concentrations above 30 M, DOPA synthesis declined; inhibition was observed at higher concentrations for phenylalanine (300 M). These effects were most notable in the presence of 56 mM potassium. Measurements of intracellular phenylalanine and tyrosine suggested the K_m for either amino acid is 20–30 M; maximal synthesis occurred at 120–140 M. In the presence of both phenylalanine and tyrosine, DOPA synthesis was inhibited by phenylalanine only at a high medium concentration (1000 M), regardless of medium tyrosine concentration. The inhibition of DOPA synthesis by high medium tyrosine concentrations was antagonized by high medium phenylalanine concentrations (100, 1000 M). Together, the findings indicate that for PC12 cells, phenylalanine can be a significant substrate for tyrosine hydroxylase, is a relatively weak inhibitor of the enzyme, and at high concentrations can antagonize substrate inhibition by tyrosine.     

Properties of an enzymatic complex active in sulfite and thiosulfate oxidation by Rhodotorula sp.

An enzymatic complex from Rhodotorula was characterized and it was indicated that it possessed thiosulfate-oxidizing activity, forming tetrathionate as well as sulfite oxidase activity. Both activities coupled with ferricyanide and native cytochrome c but no with mammalian cytochrome c. Activities of these enzymes were inhibited by thiol inhibitors. Chelating agents did not affect thiosulfate oxidizing activity and only moderately inhibited sulfite oxidase. Both activities disappeared after treatment with proteolytic enzymes or sodium deoxycholate which indicates an essential role played not only by protein but also by phospholipids in the enzymatic activity of the complex. Thiosulfate oxidizing enzyme had a K _m for thiosulfate of 0.16 mM with ferricyanide as electron acceptor and of 14 M with native cytochrome c and of 0.34 mM for ferricyanide. Optimum pH for this activity was 7.8. Other properties of this enzyme were similar to those of thiobacilli and heterotrophic bacteria. The activity of sulfite oxidase was inhibited by 50% with 10 M AMP. The K _m values of this enzyme were 1 mM with ferricyanide as electron acceptor and 60 M with native cytochrome c for sulfite and 0.42 mM for ferricyanide. The enzyme did not show a specific optimum pH value with ferricyanide as electron acceptor. However, with native cytochrome c optimum pH was 7.8 for its activity. In many properties the sulfite oxidase from Rhodotorula was similar to the enzyme from Thiobacillus ferrooxidans, T. concretivorus, T. thioparus and T. novellus.Abbreviations CSH reduced glutathion - APS reductase, adenosine-S-phosphosulfate reductase - pHMB p-hydroxymercuribenzoate - NEM N-ethylmalcimide - TCA trichloroacetic acid - PPO 2,5-diphenyloxazole - POPOP 2,2-p-phenylen-bis 5-phenyloxazol     

Variations of the effect of insulin on neutrophil respiratory burst. The role of tyrosine kinases and phosphatases

The priming effect of insulin on the fMLP-induced respiratory burst of mouse neutrophils as well as the involvement of tyrosine protein kinases and phosphatases in this process have been studied. Peritoneal evoked neutrophils of NMRI strain mice were incubated with 0.01-100 nM insulin for 1-60 min at 22, 30, or 37°C and activated by 0.1-50 M N-formyl-methionyl-leucyl-phenylalanine (fMLP). The production of reactive oxygen species (ROS) by neutrophils was monitored by luminol-dependent chemiluminescence. We found that ¹²⁵I-labeled insulin binding by mouse neutrophils occurred with saturation and high affinity. Insulin itself did not change the basal level of the ROS production but could modulate fMLP-induced respiratory burst. The effect of insulin depended on temperature and duration of pretreatment of the neutrophils with insulin and the concentration combination of the insulin and fMLP. The tyrosine kinase inhibitor tyrphostin 51 decreased the fMLP-induced respiratory burst significantly. Insulin did not change the fMLP response of neutrophils pretreated with tyrphostin. However, the effect of tyrphostin on the response to 50 M fMLP was considerably decreased in neutrophils treated with insulin. There was no such effect during activation by 5 M fMLP, for which the priming effect of insulin was not observed. Insulin did not increase the fMLP-induced respiratory burst in neutrophils treated with the protein phosphatase inhibitors orthovanadate and pyrophosphate. If the inhibitors were added after insulin, the combined effect was nearly additive. It is possible that priming by insulin of the fMLP-induced respiratory burst is triggered by tyrosine phosphorylation, realized with its participation, and involves the signaling pathways initiated by tyrosine phosphorylation but subsequently is not dependent on the latter. The role of protein phosphatases in priming by insulin is of little importance. The data indirectly confirm the idea that priming of the neutrophil respiratory burst is a result of crosstalk of signaling pathways of the insulin and fMLP receptors with the participation of tyrosine phosphorylation.     

Interaction between electron transport at the plasma membrane and nitrate uptake by maize (Zea mays L.) roots

Summary In the present study nitrate uptake by maize (Zea mays L.) roots was investigated in the presence or absence of ferricyanide (hexacyanoferrate III) or dicumarol. Nitrate uptake caused an alkalization of the medium. Nitrate uptake of intact maize seedlings was inhibited by ferricyanide while the effect of dicumarol was not very pronounced. Nitrite was not detected in the incubation medium, neither with dicumarol-treated nor with control plants after application of 100 M nitrate to the incubation solution. In a second set of experiments interactions between nitrate and ferricyanide were investigated in vivo and in vitro. Nitrate (1 or 3 mM) did neither influence ferricyanide reductase activity of intact maize roots nor NADH-ferricyanide oxidoreductase activity of isolated plasma membranes. Nitrate reductase activity of plasma-membrane-enriched fractions was slightly stimulated by 25 M dicumarol but was not altered by 100 M dicumarol, while NADH-ferricyanide oxidoreductase activity was inhibited in the presence of dicumarol. These data suggest that plasma-membrane-bound standard-ferricyanide reductase and nitrate reductase activities of maize roots may be different. A possible regulation of nitrate uptake by plasmalemma redox activity, as proposed by other groups, is discussed.Abbreviations ADH alcohol dehydrogenase - HCF III hexacyanoferrate III (ferricyanide) - ME NADP-dependent malic enzyme - NR nitrate reductase - PM plasma membrane - PM NR nitrate reductase copurifying with plasma membranes     

Insulin activation of NADH ferricyanide reductase in human erythrocytes is mediated by the insulin receptor tyrosine kinase: a comparative study in normal and diabetic states

Summary

We have previously reported that NADH ferricyanide reductase in human erythrocytes is stimulated by insulin. Hormone-stimulated activities are attenuated in the presence of glycolytic inhibitors like vanadate, indicating the involvement of glycolysis in the mechanism by which insulin stimulates ferricyanide reduction. Activation of erythrocyte metabolism in response to insulin could be a result of hormone binding to its receptor, inducing phosphorylation of band 3 (at a site for reversible association of glycolytic enzymes) and/or other membrane proteins like the Na⁺/H⁺ antiport. Activation of the antiporter protein by insulin can stimulate glycolysis by an increase in intracellular pH, an effect which is prevented by amiloride. Evidence for a role for tyrosine phosphorylation in triggering the reductase activation came from studies with protein kinase inhibitors. Genistein, sphingosine and acridine orange have been shown to prevent insulin-stimulated ferricyanide reduction, implicating tyrosine phosphorylation as an important signal for activation of the enzyme by insulin. To evaluate activation of the enzyme by insulin stimulated phosphorylation, a comparative study was done using erythrocytes from healthy and diabetic humans. We measured ferricyanide reductase activities in basal and insulin stimulated states. Basal activities were lower in diabetics than in normal humans. Nevertheless, hormone stimulated activities were similar, despite earlier reports of decreased receptor phosphorylation of exogenous substrates in type 2 diabetics. These observations, together with previous ones, suggest that insulin-receptor kinase interaction may mediate the action of insulin on human erythrocytes by phosphorylation of cellular proteins like band 3 and/or the Na⁺/H⁺ antiport.     

Plant catechols prevent lipid peroxidation in human plasma and erythrocytes

The antioxidant activity of several plant catechol derivatives was tested in buffer, plasma, and human erythrocytes. In buffer, chlorogenic acid (CGA), caffeic acid (CA), and dihydrocaffeic acid (DCA) reduced ferric iron equally well in the ferric reducing antioxidant power (FRAP) assay. Low concentrations of the polyphenols enhanced the ability of plasma to reduce ferric iron by about 10%. In plasma, lipid hydroperoxide and F₂-isoprostane formation induced by a water-soluble free radical initiator were reduced by CGA at concentrations as low as 20 M. During incubation at 37°C, human erythrocytes took up DCA, but not CGA, and intracellular DCA enhanced the ability of erythrocytes to reduce extracellular ferricyanide. When intact erythrocytes were exposed to oxidant stress generated by liposomes containing small amounts of lipid hydroperoxides, extracellular CGA at a concentration of 5 M decreased both lipid peroxidation in the liposomes, and spared -tocopherol in erythrocyte membranes. These results suggest that the catechol structure of these compounds convey the antioxidant effect in plasma and in erythrocytes.     

Effect of anti-regucalcin antibody on neutral phosphatase activity in rat liver cytosol: Involvement of endogenous regucalcin

The effect of anti-regucalcin monoclonal antibody on neutral phoshatase activity in rat liver cytosol was investigated. Phosphotyrosine, phosphoserine, and phosphothreonine were used as the substrate toward phosphatase asssy. Liver cytosolic phosphatase activity with three phosphoaminoacids was significantly increased in the presence of anti-regucalcin antibody (100 and 200 ng/ml) in the enzyme reaction mixture with calcium chloride (0.1 mM) or EGTA (1.0 mM). The effect of anti-regucalcin antibody was completely abolished in the presence of exogenous regucalcin (1.0 M), indicating the involvement of endogenous regucalcin. The anti-regucalcin anti body- increased phosphatase activity was not significantly altered in the presence of trifluoperazine (20 M), an antagonist of calmodulin, or akadaic acid (10 M), an inhibitor of protein phosphatase, although these inihibitors caused a slight decrease in liver cytosolic phosphatase activity. The effect of endogenous regucalcin might be not related to calmodulin, and it was insensitive to okadaic acid. The present findings suggest that endogenous regucalcin is involved in the regulation of protein phasphatase in rat liver cytoplasm.     

Effects of polymyxin B on the sarcoplasmic reticulum membrane

Polymyxin B, a cyclic peptide antibiotic, inhibits Ca²⁺-ATPase, p-nitrophenyl phosphatase and phosphorylase kinase activities associated with rabbit skeletal muscle sarcoplasmic reticulum membranes; 50% inhibition is induced by 100 M, 130M and 550 M of polymyxin respectively. The fluorescence intensity of fluorescein isothiocyanate-labeled Ca²⁺-ATPase, decreases in the presence of polymyxin (50% of the total decrease at 70 M polymyxin). On the other hand, the polypeptide inhibits calmodulin-dependent endogenous phosphorylation of 60 kDa, 20 kDa and 14 kDa membrane proteins, while an increase of calmodulin-dependent phosphorylation is observed in 132 kDa and 86 kDa proteins.     

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.     

Pyridine nucleotide specificity and other properties of purified nitrate reductase from Chlorella variegata

Nitrate reductase (NR) (EC 1.6.6.2) from Chlorella variegata 211/10d has been purified by blue sepharose affinity chromatography. The enzyme can utilise NADH or NADPH for nitrate reduction with apparent K _m values of 11.5 M and 14.5 M, respectively. Apparent K _m values for nitrate are 0.13 mM (NADH-NR) and 0.14 mM (NADPH-NR). The diaphorase activity of the enzyme is inhibited strongly by parachloromercuribenzoic acid; NADH or NADPH protects the enzyme against this inhibition. NR proper activity of the enzyme is partially inactive after extraction and may be activated after the addition of ferricyanide. The addition of NAD(P)H and cyanide causes a reversible inactivation of the NR proper activity although preincubation with either NADH or NADH and ADP has no significant effect.Abbreviations NR Nitrate reductase - FAD Flavin-adenine dinucleotide - FMN Riboflavin 5-phosphate - p-CMB para-Chloromercuribenzoic - BV Benzyl viologen     

Anomeric preference of glucose utilization in rat erythrocytes

The -anomer of glucose relative to the -anomer was more rapidly metabolized into lactate by rat erythrocytes at 37° C (/ ratio = ca. 1.3): the amounts of - and -D-glucose metabolized into lactate during 3 min were 0.21 and 0.27 mol/gHb, respectively. Also, the transport of -D-glucose into erythrocytes was more rapid than that of -D-glucose: the amounts of - and -D-glucose transported into erythrocytes during 3 min were approximately 3.5 and 5.0 mol/gHb, respectively. Glucose phosphorylation by rat erythrocyte hexokinase (i.e., a possible rate-limiting step in glycolysis) occurred at higher velocities with the -anomer than with the a-anomer (/ ratio = 1.28). The Km value of hexokinase for either anomer of glucose was 53 M. The glucose concentrations in erythrocytes incubated with - and -D-glucose reached about 1 mM in 1 min, indicating that hexokinase is almost completely saturated with glucose within less than 1 min. The results suggest that glucose phosphorylation and glucose transport are major and minor determinants, respectively, for the anomeric preference of glucose utilization in rat erythrocytes.     

Identification of an autoinhibitory domain in the insulin receptor tyrosine kinase

We have tested the hypothesis that activation of the insulin receptor tyrosine kinase is due to autophosphorylation of tyrosines 1146, 1150 and 1151 within a putative autoinhibitory domain. A synthetic peptide corresponding to residues 1134–1162, with tyrosines substituted by alanine or phenylalanine, of the insulin receptor subunit was tested for its inhibitory potency and specificity towards the tyrosine kinase activity. This synthetic peptide gave inhibition of the insulin receptor tyrosine kinase autophosphorylation and phosphorylation of the exogenous substrate poly(Glu, Tyr) with an approximate IC₅₀ of 100 M. Inhibition appeared to be independent of the concentrations of insulin or the substrate poly(Glu, Tyr) but was decreased by increasing concentrations of ATP. This same peptide also inhibited the EGF receptor tyrosine kinase but not a serine/threonine protein kinase. These results are consistent with the hypothesis that this autophosphorylation domain contains an autoinhibitory sequence. (Mol Cell Biochem120: 103–110, 1993)Abbreviations IR Insulin Receptor - SDS/PAGE Sodium Dodecyl Sulfate Polyacrylamide Gel Electrophoresis - CaM Calmodulin - HEPES 4-(2-Hydroxyethyl)-Piperazineethane-Sulfonic Acid - DMEM Dulbecco's Modified Eagle' Medium - PMSF Phenylmethyl-Sulfonyl Fluoride - HPLC High Performance Liquid Chromatography - PKC Protein Kinase C - PKI Inhibitory Peptide for cAMP-Kinase - CaMK II Ca²⁺/Calmodulin-Dependent Protein Kinase II - CaN A A Subunit of Calcineurin     

Metabolism, not autoxidation, plays a role in α-oxoaldehyde- and reducing sugar-induced erythrocyte GSH depletion: Relevance for diabetes mellitus

Erythrocyte and lens reduced glutathione (GSH) levels are often lower in patients with diabetes whereas erythrocyte dicarbonyl levels are often higher. We hypothesise that high plasma carbohydrates may be metabolised by glycolytic and pentose phosphate pathways to form -oxoaldehydes, which deplete cellular GSH. Our aims were: (1) to compare the effectiveness of various carbohydrates or metabolites at depleting erythrocyte GSH, (2) to determine if GSH loss is related to the autoxidation or metabolism of carbohydrates. It was found that erythrocyte GSH was depleted by 50% (ED-50) at t = 2.5 h when erythrocytes were incubated with the following: methylglyoxal (MG) 23 M, glyoxal 75 M, DL-glyceraldehyde 299 M, deoxyribose 606 M, xylitol 626 M, and ribose 2 mM. The glycolytic inhibitors, sodium arsenate and KF prevented ribose, deoxyribose, xylitol and MG-induced GSH depletion in erythrocytes over 2 h. However, the antioxidant trolox and the ferric chelator detapac did not affect MG-induced GSH depletion. These data suggest that the carbohydrates or glyceraldehyde were metabolised to form carbonyls such as MG which depleted erythrocyte GSH as a result of catalysis by glyoxalase I. None of the carbohydrates were autoxidised to carbonyls over this time period. We speculate that as a result of GSH depletion, subsequent glycoxidative stress affects erythrocyte function and contributes to diabetic complications.     

Control of erythrocyte metabolism by redox-regulated tyrosine phosphatases and kinases

Summary We wish to elaborate a novel mechanism of metabolic regulation mediated by cytoplasmic tyrosine phosphatases and kinases. Briefly we propose that phosphofructokinase, aldolase, and glyceraldehyde-3-phosphate dehydrogenase (G3PDH) bind reversibly to the N-terminus of the cytoplasmic domain of band 3. Once the enzymes are bound, they are inhibited; however, upon release they are restored to full activity. We demonstrate that control of enzyme binding and consequently control of substrate flow down the pathway is executed by phosphorylation of Tyr 8 and Tyr 21 within the glycolytic enzyme binding site at the N-terminus of band 3. This phosphorylation results in obstruction of enzyme binding, leading to enzyme activation. Importantly, the tyrosine kinase that phosphorylates band 3 is activated by oxidation, while the tyrosine phosphatase that dephosphorylates band 3 is inhibited by the same redox changes. Consequently, treatment of red cells wih oxidants such as H₂O₂ and ferricyanide can enhance both tyrosine phosphorylation of the N-terminus of band 3 and glycolysis in a coordinate manner. Because oxidant entry into the cell is not essential, a plasma membrane electron transport pathway is believed to mediate the oxidant's effects.     

Insulin-stimulated tyrosine phosphorylation of a 43 kDa protein in rat liver membranes

The insulin receptor (IR) tyrosine kinase is essential for the regulation of different cellular functions by insulin. This may occur by a direct phosphorylation of membrane and/or cytoplasmic proteins by the IR tyrosine kinase. Hence it is important to identify putative physiological substrates for the IR tyrosine kinase. In this study we found that the glycoprotein fraction from rat liver membranes contain a 43 kDa protein (pp43) which, like the -subunit of IR, is phosphorylated in an insulin-dependent manner. A 25-fold enhancement of ³²P incorporation into pp43 by insulin was found under optimal conditions. Half-maximal phosphorylation of pp43 and the -subunit of IR were attained at 66 nM and 60 nM insulin, respectively. Mn²⁺ (K_a = 1.0 mM) was much better than Mg²⁺ (K_a = 6.3 mM) in supporting pp43 phosphorylation. Insulin-stimulated phosphorylation of pp43 (t_1/2 = 3.6 min) proceeded at a much slower rate compared to that of the -subunit of IR (t_1/2 = 1.2 min). Phosphoamino acid analysis of pp43 revealed that both tyrosine and serine are phosphorylated in the ratio 4 : 1. Tyrosine, but not serine, phosphorylation was increased 12-fold by insulin. Phosphorylation of pp43 occurred on 4 major tryptic peptides. Comparison to the tryptic phosphopeptides from IR -subunit suggest that pp43 was not derived from IR -subunit by proteolysis. Our results suggest that pp43 may be an endogenous substrate for the IR tyrosine kinase.     

Protein tyrosine phosphorylation in cardiovascular system

Treatment of rat parotid acinar cells with sodium orthovanadate (an inhibitor of protein tyrosine phosphatase) caused a dose-dependent inhibition of phosphatase activity as measured by the hydrolysis of para nitrophenylphosphate (pNPP). Inclusion of 50 M sodium orthovanadate inin vitro gland cultures prevented the amylase secretion from both untreated control and isoproterenol-stimulated parotid acinar cells. Four different tyrosine-phosphorylated proteins with M_r 40, 45, 70 and 95 kDa, respectively, were identified in secretory granule preparations from rat parotid glands by immunoblot using a monospecific antibody for phosphotyrosine. An increase in the phosphorylation levels of these phosphoproteins was noted in the presence of 50 M sodium orthovanadate, suggesting that a protein tyrosine phosphatase (PTPase) is involved in parotid gland protein dephosphorylation reactions. Using antibody to Syp (a PTPase belonging to class 1D), a major fraction of subcellular activity was found to be associated with secretory granule membranes. These results suggest the possible involvement of a PTPase (Syp) in parotid gland secretory mechanisms.     

In vitro propagation of Anacardium occidentale L.

Multiple buds have been induced from cotyledonary nodes of cashew, Anacardium occidentale L. Maximum number of buds were initiated on Murashige & Skoog's (MS) medium supplemented with 117 mM sucrose, 14.6 mM maltose and 22.2 M BA. The buds were harvested at each subculture after elongation on MS medium supplemented with 100 ml l^-1 coconut water, 14.6 mM maltose, increased sucrose (146 mM) and decreased BA (4.4 M). Excised microshoots were rooted in vitro on MS medium in the presence of 117 mM sucrose, 2.9 M IAA and 4.9 M IBA. Plantlets have been successfully transferred to soil and have been established in the field.     

Adenylylsulfate reductase in some new sulfate-reducing bacteria

Four recently described species of new genera of sulfate-reducing bacteria, Desulfobulbus propionicus, Desulfobacter postgatei, Desulfococcus multivorans and Desulfosarcina variabilis were examined with respect to adenylylsulfate reductase. All of the species examined contained the enzyme in sufficient concentrations to account for dissimilatory sulfate reduction.Adenylylsulfate reductase was enriched 17.1-fold from Desulfobulbus propionicus by ammonium sulfate fractionation, ion exchange chromatography and gel filtration. The molecular weight was 175,000 and the enzyme contained 1 mol of flavin, 8 mol of non heme iron and 8 mol of labile sulfide per mol enzyme. Either ferricyanide or cytochrome c could be used as electron acceptors; the pH optimum was 7.7 with ferricyanide and 8.8 with cytochrome c. K _m values for AMP and sulfite were 90 M and 1.3 M with ferricyanide and 91 M and 71 M with cytochrome c as electron acceptor. K _m values for ferricyanide and cytochrome c were 89 M and 21 M, respectively. The properties of the enzyme were compared with those of purified adenylylsulfate reductases from other microorganisms.Non-common abbreviation APS adenylylsulfate     

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.     

Simultaneous photoreduction and photooxidation of cytochrome b-559 in Photosystem II treated with carbonylcyanide-m-chlorophenylhydrazone

The possibility of a Photosystem II (PS II) cyclic electron flow via Cyt b-559 catalyzed by carbonylcyanide m-chlorophenylhydrazone (CCCP) was further examined by studying the effects of the PS II electron acceptor 2,6-dichloro-p-benzoquinone (DCBQ) on the light-induced changes of the redox states of Cyt b-559. Addition to barley thylakoids of micromolar concentrations of DCBQ completely inhibited the changes of the absorbance difference corresponding to the photoreduction of Cyt b-559 observed either in the presence of 10 M ferricyanide or after Cyt b-559 photooxidation in the presence of 2 M CCCP. In CCCP-treated thylakoids, the concentration of photooxidized Cyt b-559 decreased as the irradiance of actinic light increased from 2 to 80 W m^-2 but remained close to the maximal concentration (0.53 photooxidized Cyt b-559 per photoactive Photosystem II) in the presence of 50 M DCBQ. The stimulation of Cyt b-559 photooxidation in parallel with the inhibition of its photoreduction caused by DCBQ demonstrate that the extent of the light-induced changes of the redox state of Cyt b-559 in the presence of CCCP is determined by the difference between the rates of photooxidation and photoreduction of Cyt b-559 occuring simultaneously in a cyclic electron flow around PS II.We also observed that the Photosystem I electron acceptor methyl viologen (MV) at a concentration of 1 mM barely affected the rate and extent of the light-induced redox changes of Cyt b-559 in the presence of either FeCN or CCCP. Under similar experimental conditions, MV strongly quenched Chl-a fluorescence, suggesting that Cyt b-559 is reduced directly on the reducing side of Photosystem II.Abbreviations ADRY acceleration of the deactivation reactions of the water-splitting system Y - ANT-2p 2-(3-chloro-4-trifluoromethyl)anilino-3,5-dinitrothiophene - CCCP carbonylcyanide-m-chlorophenylhydrazone - DCBQ 2,6-dichloro-p-benzoquinone - FeCN ferricyanide - MV methyl viologen - P₆₈₀ Photosystem II reaction center Chl-a dimer CIW-DPB publication No. 1118.