Calcium-dependent 3‘:5‘-cyclic nucleotide phosphodiesterase. Inhibition of basal activity at physiological levels of potassium ions

Role of Counterion in the Adsorption Behavior of 1:1 Ionic Surfactants at Fluid Interfaces-Adsorption Properties of Alkali Perfluoro-n-octanoates at the Air/Water Interface

Equilibrium surface tension (σ_e) versus bulk concentration (c) isotherms of aqueous, surface-chemically pure solutions of various alkali perfluoro-n-octanoates were measured at 295 K. These 1:1 ionic surfactant systems belong to the pseudo nonionic ones. Evaluating the different σ_e vs log c isotherms by basic adsorption equations reveals that they follow ideal surface behavior. The novelty of this investigation exists in the fact that the surface area demand per molecule adsorbed calculated from the experimental σ_e vs log c isotherms is identical to that of the hydrated alkali cation. Thus, as long as the counterion's cross-sectional area is greater than that of its amphiphilic anion, the amphiphile's total surface area demand will exclusively be governed by that of its alkali counterion. This, in turn, means that the counterion is nonrandomly bound to the amphiphilic anion in the adsorption layer. Furthermore, the size of the hydrated alkali counterion in the adsorption layer does not differ from that in the bulk phase.

Caffeine and theophylline analogues: correlation of behavioral effects with activity as adenosine receptor antagonists and as phosphodiesterase inhibitors

The behavioral stimulant effects of xanthines, such as caffeine and theophylline, appear to involve blockade of central adenosine receptors. However, 3-isobutyl-1-methylxanthine (IBMX), a potent phosphodiesterase (PDE) inhibitor, produces behavioral depression. The effects of caffeine analogs on open field behavior of mice and potencies as antagonists of adenosine receptors and as inhibitors of three classes of brain PDE have been compared. 1,7-Dimethyl-3-propargylxanthine, 1,3,7-tripropargylxanthine, and 3,7-dimethyl-1-propargylxanthine, which have high affinity for adenosine receptors and weaker activity as PDE inhibitors, all increase behavioral activity. In contrast, 1,3,7-tripropylxanthine, a more potent inhibitor of the brain calcium-independent (Ca-indep) PDEs than 1,3,7-tripropargylxanthine, produces behavioral depression, even though both analogues are potent adenosine receptor antagonists. 7-Benzyl-IBMX, an active receptor antagonist and selective inhibitor of a brain calcium-dependent (Ca-dep) PDE, produces a slight behavioral activation. Xanthines that are potent adenosine receptor antagonists and relatively weak inhibitors of the Ca-indep PDEs reverse the depressant effects of N6-cyclohexyladenosine, while xanthines, such as 1,3,7-tripropylxanthine, that are potent inhibitors of the Ca-indep PDEs, do not. The results suggest that the behavioral effects of xanthines may be determined primarily by relative activity as adenosine receptor antagonists and as inhibitors of brain Ca-indep PDEs.

Inhibition of Ca2+-dependent protein kinase and Ca2+/Mg2+-ATPase in cardiac sarcolemma by the anti-calmodulin drug calmidazolium

Sarcolemma (SL) vesicles, isolated from pig heart, contain both a Ca2+-calmodulin-dependent protein kinase (CaM-PK) and a Ca2+-dependent Mg2+-ATPase (Ca2+/Mg2+)-ATPase). Some of their properties have been compared: their affinity for Ca2+ ions, dependence on exogenous calmodulin (CaM) and sensitivity to the anti-CaM drug calmidazolium (R24571). The properties of Ca2+-CaM-dependent brain phosphodiesterase (PDE) have also been examined. R24571 appeared to be the most potent inhibitor from brain PDE. For the three enzymes studied, exogenously added CaM was able to antagonize the R24571 inhibition, although the efficiency to counteract was rather low in the case of the SL Ca2+/Mg2+-ATPase. R24571 decreased the affinity of the Ca2+/Mg2+-ATPase for Ca2+ ions (KCa 0.35 versus 0.75 microM) and exerted an inhibition non-competitive with Ca2+ ions on the other CaM-dependent enzymes. Membrane-bound CaM, which is involved in controlling the Ca2+/Mg2+-ATPase, appeared to be present in a stoichiometry varying from 1:1 to 1:4 compared to the 32P-intermediate of the ATPase. R24571 treatment of SL vesicles selectively solubilized a number of proteins in the molecular range of 13-20 kD, which may include CaM. The results suggest that different mechanisms are involved in the CaM control of the two SL enzymes studied.

Affinity chromatography of brain cyclic nucleotide phosphodiesterase using 3-(2-pyridyldithio)propionyl-substituted calmodulin linked to thiol-sepharose

[3-(2-Pyridylthio)propionyl]calmodulin (PDP-CaM), an activated thiol derivative of calmodulin (CaM), was synthesized. Preparations of this derivative containing an average of 2.8 mol of substituent/mol of protein activated purified cyclic nucleotide phosphodiesterase in a manner indistinguishable from that of native CaM. PDP-CaM was covalently coupled to free thiol-Sepharose 4B through formation of a stable mixed disulfide bond for use in affinity chromatography. The binding capacity of the disulfide-linked CaM-Sepharose for phosphodiesterase activity was proportional to substituent level up to 4 mg of CaM/mL of gel; the total capacity of the gel for binding phosphodiesterase was 4 times that of CNBr-coupled CaM-Sepharose. Quantitative recovery was achieved by desorption of both ligand and bound proteins with a reducing agent. The thiolated CaM derivative was then separated from phosphodiesterase by rapid gel filtration; the overall recovery of phosphodiesterase activity was greater than 70%. Preparations of homogeneous enzyme in good yield were obtained after a second chromatography step on CaM-Sepharose. Binding and recovery of phosphodiesterase activity were entirely reproducible, since each preparation of affinity gel was used only once. As it permits separation of interacting species in free solution, this general method may be useful with other ligands for increasing yields from affinity chromatography, particularly when dissociation of molecules in their matrix-bound conformation may be difficult to achieve.

Selective alteration of Ca2+-dependent and Ca2+-independent cyclic nucleotide phosphodiesterase activity in rat cerebral cortex by cyclic nucleotides and their analogs

The effects of various cyclic nucleotides and cyclic nucleotide analogs on the activity of Ca2+-independent and Ca2+-dependent phosphodiesterases purified from rat cerebral cortex were examined. The order of potency for inhibition of the Ca2+-dependent enzyme by the various agents was the same using either cAMP or cGMP as substrate with 2'-O-monobutyryl cGMP, cIMP and 2-deoxy cGMP being the most potent. The inhibition of deoxy cGMP using cAMP or cGMP as substrate was competitive, with Ki values of 11 and 13 microM, respectively. In marked contrast, hydrolysis of cAMP or cGMp by the Ca2+-independent enzyme was stimulated 50-75% by cIMP, deoxy cGMP and N2-monobutyryl cGMP with EC50 values of 7, 20 and 30 microM, respectively. cGMP (EC50, 1.5 microM) produced quantitatively the same degree of stimulation of cAMP hydrolysis by the Ca2+-independent phosphodiesterase and the activation was not additive with that of deoxy cGMP. Of the other derivatives examined, 2'-O-monobutyryl cAMP and 2'-deoxy cAMP were the most potent inhibitors of cAMP hydrolysis by the Ca2+-independent enzyme and were 30-60 times more effective in inhibiting cAMP hydrolysis as compared to cGMP hydrolysis. The specificity of K+ in inhibiting the activity of the Ca2+-dependent phosphodiesterase and deoxy cAMP in inhibiting the Ca2+-independent enzyme may provide convenient means to examine specifically these activities in crude extracts from rat cerebral cortex.

Phosphorylation of low-molecular-weight proteins in preparations of rat heart sarcolemma and sarcoplasmic reticulum

Two substrate proteins for cAMP-dependent protein kinase detected in a rat heart sarcolemma preparation displayed molecular weights of 24,000 and 9000 in sodium dodecyl sulfate gels and were shown to be interconvertible. The 9000-dalton protein could readily be separated from other low molecular weight phosphoproteins (mol. wt. 14,000 and 7000) by the use of 15% polyacrylamide gels. In addition to an endogenous cAMP-dependent protein kinase the membrane preparation also contained a protein-phosphorylation system that required Ca2+ and calmodulin. It appeared that both 24,000- and 55,000-dalton proteins were substrates for the endogenous Ca2+- and calmodulin-dependent protein kinase. Contaminating sarcoplasmic reticulum vesicles, first loaded with calcium oxalate, could be separated from the enriched sarcolemma preparation by sucrose gradient centrifugation. The separation was confirmed by comparative analysis of 5'-nucleotidase, Na+ -Ca2+ antiporter, and (Ca2+ + Mg2+)-dependent ATPase activities and by determination of gel electrophoretic (phospho)protein composition, sialic acid, cholesterol, and phospholipid contents. The 24,000-dalton phosphoprotein complex was equally distributed between sarcolemmal and sarcoplasmic reticulum fractions, whereas the 55,000- and 7000-dalton proteins were predominantly found in the sarcolemmal fraction. The 24,000-dalton protein was most likely phospholamban, because no other phosphoprotein was found in the 20,000 molecular weight range.

The role of Ca2+ and cyclic AMP in the phosphorylation of rat-liver soluble proteins by endogenous protein kinases

Rat liver soluble proteins were phosphorylated by endogenous protein kinase with [gamma-32P]ATP. Proteins were separated in dodecyl sulphate slab gels and detected with the aid of autoradiography. The relative role of cAMP-dependent, cAMP-independent and Ca2+-activated protein kinases in the phosphorylation of soluble proteins was investigated. Heat-stable inhibitor of cAMP-dependent protein kinase inhibits nearly completed the phosphorylation of seven proteins, including L-type pyruvate kinase. The phosphorylation of eight proteins is not influenced by protein kinase inhibitor. The phosphorylation of six proteins, including phosphorylase, is partially inhibited by protein kinase inhibitor. These results indicate that phosphoproteins of rat liver can be subdivided into three groups: phosphoproteins that are phosphorylated by (a) cAMP-dependent protein kinase or (b) cAMP-independent protein kinase; (c) phosphoproteins in which both cAMP-dependent and cAMP-independent protein kinase play a role in the phosphorylation. The relative phosphorylation rate of substrates for cAMP-dependent protein kinase is about 15-fold the phosphorylation rate of substrates for cAMP-independent protein kinase. The Km for ATP of cAMP-dependent protein kinase and phosphorylase kinase is 8 microM and 38 microM, respectively. Ca2+ in the micromolare range stimulates the phosphorylation of (a) phosphorylase, (b) a protein with molecular weight of 130 000 and (c) a protein with molecular weight of 15 000. The phosphate incorporation into a protein with molecular weight of 115 000 is inhibited by Ca2+. Phosphorylation of phosphorylase and the 15 000-Mr protein in the presence of 100 microM Ca2+ could be completely inhibited by trifluoperazine. It can be concluded that calmodulin is involved in the phosphorylation of at least two soluble proteins. No evidence for Ca2+-stimulated phosphorylation of subunits of glycolytic or gluconeogenic enzymes, including pyruvate kinase, was found. This indicates that it is unlikely that direct phosphorylation by Ca2+-dependent protein kinases is involved in the stimulation of gluconeogenesis by hormones that act through a cAMP-independent, Ca2+-dependent mechanism.