Kinetic co-operativity of wheat-germ RNA polymerase II with adenosine 5''-[beta gamma-imido]triphosphate as substrate.

A kinetic study of productive RNA chain elongation indicates that adenosine 5'-[beta gamma-imido]triphosphate can serve as substrate in reactions catalysed by purified wheat-germ RNA polymerase II on a poly[d(A-T)] template. However, in contrast with the results obtained with the natural substrate ATP, the double-reciprocal plots, 1/velocity versus 1/[nucleotide], are not linear but characteristic of apparent negative co-operativity. The extent of the kinetic co-operativity is modified when the reactions are conducted in the additional presence of fixed amounts of an alternative substrate such as ATP or inhibitors such as dATP or cordycepin triphosphate. Analogous results are obtained whether the reactions are carried out in the presence of Mg2+ or Mn2+ as the metal ion cofactor. However, the data show that with Mn2+ the RNA polymerase is less specific in substrate recognition than with Mg2+. Tentative kinetic models are proposed to account for the rate measurements.     

The inhibition of muscle contraction by adenosine 5' (beta, gamma-imido) triphosphate and by pyrophosphate.

We have studied the inhibition of the contraction of glycerinated rabbit psoas muscle caused by ligands that bind to the ATPase site of myosin. Two ligands, adenosine 5' (beta, gamma-imido) triphosphate (AMPPNP) and pyrophosphate (PPi), decreased the force and stiffness developed in isometric contractions and the velocity of shortening of isotonic contractions. The force exerted by isometric fibers was measured as a function of MgATP in the presence and absence of a constant concentration of the ligands. As the MgATP concentration decreased, the inhibition of tension caused by the ligand increased, reaching approximately 50% at 25 microM MgATP and either 2 mM MgPPi or 2 mM MgAMPPNP. The maximum velocity of shortening was also measured as a function of MgATP concentration in the presence of 1 and 2 mM MgPPi and 2.5 and 5 mM MgAMPPNP. Both ligands acted as pure competitive inhibitors with Ki = 3.0 mM for PPi and 5.1 mM for MgAMPPNP. These data show that both ligands are weak inhibitors of the contraction of fibers. The results provided information on the energetics of actin-myosin-ligand states that occur in the portion of the cross-bridge cycle where MgATP binds to myosin. A simple analysis of the inhibition of velocity suggests that MgAMPPNP binds to the actomyosin complex at this step of the cycle with an effective affinity constant of approximately 2 X 10(2) M-1.     

Effect of inosine 5' -(beta, gamma-imido) triphosphate and other nucleotides on beef heart mitochondrial ATPase.

The effects of various substrates and alternative substrates on the hydrolytic activity of beef heart mitochondrial ATPase was examined. It was found that ATP or ADP, ITP hydrolysis showed positive cooperativity. IDP inhibited ITP hydrolysis and caused positive cooperativity. When ITP was present during an ATP hydrolysis assay, the rate of ATP hydrolysis was stimulated. IDP had no effect on ATP hydrolysis rates. A nonhydrolyzable ITP analog, inosine 5'-(beta, gamma-imido)triphosphate (IMP-P(NH)P), was synthesized and purified. It was found to be a potent competitive inhibitor of ITP and GTP hydrolytic activity. However, this beta-gamma-imido-bridged ITP analog was found to change the ITP and GTP hydrolysis kinetics from linear to positively cooperative. This compound inhibited ATP hydrolysis at substrate concentrations of 100 muM and lower, and stimulated ATP hydrolysis at substrate concentrations between 100 muM and 2 mM. IMP-P(NH)P had no effect on ATP hydrolysis when the substrate concentration was above 2 mM. In the presence of the activating anion, bicarbonate, IMP-P(NH)P inhibited ATP hydrolysis competitively, and induced positive cooperativity. IMP-P(NH)P had no effect on the ATP equilibrium Pi exchange, the ITP equilibrium Pi exchange, or ATP synthesis catalyzed by beef heart submitochondrial particles.     

Carrier-free 8-azidoadenosine 5'-[gamma-32P]triphosphate

We found 8-azidoadenosine 5'-diphosphate to be a phosphoryl acceptor in the enzymatic conversion of 1,3-diphosphoglyceric acid to 3-phosphoglycerate. This has allowed us to synthesize in a single-step procedure carrier-free 8-azidoadenosine 5'-[gamma-32P]triphosphate, requiring no further purification of the end product. The synthesized 8-azidoadenosine 5'-[gamma-32P]triphosphate has been characterized and shown to meet all the criteria for a specific photoreactive ATP analogue.     

Opiate receptor binding affected by guanine nucleotide. Partial loss of sensitivity to guanosine-5'-(beta,gamma-imido)triphosphate evident in vitro

The sensitivity of the receptor dissociation rate of the opiate agonist [3H]etorphine to the effects of guanosine-5'-(beta, gamma-imido)triphosphate and Na+ was measured in washed rat brain membrane homogenates after in vivo labeling. Comparison to the previously measured rapid in vivo dissociation curve (t1/2 approximately 50 s) (Perry, D. C., Rosenbaum, J. S., Kurowski, M., and Sadée, W. (1982) Mol. Pharmacol. 21, 272-279) revealed that brain homogenization and membrane washing procedures significantly prolonged the dissociation rate, even when measured in the presence of Na+ and guanyl nucleotide. The in vivo dissociation rate could only be reproduced in vitro when labeling occurred in vivo and brain homogenization occurred in the presence of these regulatory factors. The prolonged in vitro [3H]etorphine dissociation curve was predominately a result of a decreased sensitivity to guanine nucleotide rather than to Na+. These data suggest that partial functional uncoupling of the opiate receptor-effector system may occur in vitro.     

A calcium ion-dependent adenosine triphosphate pyrophosphohydrolase in plasma membrane from rat liver. Demonstration that the adenosine triphosphate analogues adenosine 5''-[betagamma-imido]triphosphate and adenosine 5''-[betagamma-methylene]-triphosphate are substrates for the enzyme.

An ATP pyrophosphohydrolase in a rat liver plasma-membrane subfraction was studied with respect to specific Ca2+ activation of the beta-phosphate bond hydrolysis. ATP and, in addition, adenosine 5'-[betagamma-imido]triphosphate and adenosine 5'-[betagamma-methlylene]triphosphate were substrates for Ca2+-stimulated enzymic hydrolysis of the beta-phosphate bond. A 15-fold activation was observed by raising the free Ca2+ concentration from 10(-7) to 10(-5) M. Mg2+ had little effect. Solubilization in 1% deoxycholate and partial purification on a sucrose density gradient resulted in a 5-fold increase in specific activity with unaltered Ca2+-stimulation pattern. The possible importance of the enzyme in Ca2+ transport is discussed.     

Activation of signal-transducing guanine-nucleotide-binding regulatory proteins by guanosine 5'-[gamma-thio]triphosphate. Information transfer by intermediately thiophosphorylated beta gamma subunits

Signal-transducing guanine-nucleotide-binding regulatory proteins (G proteins) are heterotrimers, composed of the nucleotide-binding alpha subunit and a beta gamma dimer. The influence of beta gamma dimer preparations of the retinal G protein transducin (TD) was studied on formylpeptide-receptor--G-protein interactions in membranes of differentiated HL 60 cells. For this, TD was prepared from bovine rod outer segment (ROS) membranes with either GTP or its analogs, guanosine 5'-[gamma-thio]triphosphate (GTP[S]) and guanosine 5'-[beta gamma-imino]triphosphate (Gpp[NH]p). After removal of free nucleotides, TD beta gamma was separated from TD alpha and its function analyzed. Addition of TD beta gamma isolated from TD prepared with GTP[S] (TD beta gamma GTP[S]) to HL 60 membranes abolished high-affinity binding of fMet-Leu-[3H]Phe (fMet, N-formylmethionine) to its receptor. In contrast, TD beta gamma isolated from TD prepared with GTP (TD beta gamma GTP), boiled TD beta gamma GTP[S] and TD alpha prepared with GTP[S] had no or only slight effects. The inhibitory effect of TD beta gamma GTP[S] on fMet-Leu-[3H]Phe receptor binding was potentiated by GDP at low concentrations but not by GTP[S]. Furthermore, TD beta gamma GTP[S], but not TD beta gamma GTP or TD beta gamma isolated from TD prepared with Gpp[NH]p (TD beta gamma Gpp[NH]p), prevented fMet-Leu-Phe-stimulated binding of [35S]GTP[S] to G proteins in HL 60 membranes, measured in the presence of GDP. When TD beta gamma GTP was incubated with GTP [S] and TD-depleted illuminated ROS membranes, and subsequently separated from the membranes and free GTP[S], this TD beta gamma GTP, similar to TD beta gamma GTP[S], abolished high-affinity binding of fMet-Leu-[3H]Phe to its receptor, fMet-Leu-Phe-stimulated binding of [35S]GTP[S], and fMet-Leu-Phe-stimulated GTP hydrolysis in HL 60 membranes. Inhibition of [35S]GTP[S] binding by TD beta gamma was not seen in the presence of the metabolically stable GDP analog, guanosine 5'-[beta-thio]diphosphate. In order to obtain an insight into the modification of TD beta gamma apparently caused by GTP[S], and into its mechanism of action in HL 60 membranes, TD, TD alpha and TD beta gamma, all prepared in the presence of GTP, were incubated with [35S]GTP[S] and TD-depleted illuminated ROS membranes. Fluorographic analysis of the supernatant proteins revealed 35S labelling of the beta band of the G protein. When apparently thiophosphorylated TD beta gamma was incubated with [3H]GDP in the presence of HL 60 membranes, [3H]GTP[S] was rapidly formed.(ABSTRACT TRUNCATED AT 400 WORDS)     

Activation of rat liver adenylate cyclase by guanosine 5''-[beta,gamma-imido]triphosphate and glucagon. Existence of reversibly and irreversibly activated states of the stimulatory GTP-binding protein.

The effects of guanosine 5'-[beta-thio]diphosphate (GDP[S]) on the kinetics of activation of rat liver membrane adenylate cyclase by guanosine 5'-[beta,gamma-imido]triphosphate (p[NH]ppG) were examined. GDP[S] caused immediate inhibition of the activation by p[NH]ppG at all time points tested. Substantial inhibition by GDP[S] was observed even after the time required for the enzyme to reach its steady-state activity, but the extent of inhibition became progressively smaller as the preincubation time with p[NH]ppG increased. The rate at which adenylate cyclase became quasi-irreversibly activated was a strictly first-order process. In the presence of glucagon, the formation of the irreversibly activated state was much slower. A combination of GDP[S] and glucagon could partially reverse the quasi-irreversible activation by p[NH]ppG. Glucagon decreased the lag time required for p[NH]ppG to activate adenylate cyclase and increased the extent of activation by p[NH]ppG. This stimulatory effect of the hormone on top of guanine nucleotide decreased on preincubation with p[NH]ppG, but not with GTP. Our results suggest that the activation of adenylate cyclase by non-hydrolysable GTP analogues is a two-stage process: the formation of a reversibly activated form (G rev) is a rapid process, followed by a much slower formation of the quasi-irreversibly activated form (G irr). Glucagon can stimulate G rev but not G irr, and can partially facilitate the formation of the G rev from the G irr state.     

Activation of adenylate cyclase in human platelet membranes by guanosine 5''-[beta gamma-imido]triphosphate is inhibited by cyclic-AMP-dependent phosphorylation. Slow activation occurs in the absence of ATP.

Incubation of platelet membranes with guanosine 5'-[beta gamma-imido]triphosphate causes a slow increase in GS (stimulatory GTP-binding protein) activation of adenylate cyclase. Mg2+ is necessary for this slow activation. This process is inhibited in the presence of ATP, and inhibition is greater if cyclic AMP is also included in the incubation. Adenosine 5'-[beta gamma-imido]triphosphate instead of ATP in the incubation facilitates the slow activation in the presence of cyclic AMP, and incubation of membranes with cyclic-AMP-dependent protein kinase inhibitor decreased inhibition of the slow activation of adenylate cyclase by ATP and cyclic AMP. A protein of 45 kDa in platelet membranes is phosphorylated in a cyclic-AMP-dependent manner. The transition from a reversibly activated form of GS to an irreversibly activated form is substantially slower in the presence of ATP and cyclic AMP. We propose that guanosine 5'-[beta gamma-imido]triphosphate-activated GS may exist in phosphorylated or non-phosphorylated forms, and that the non-phosphorylated form is the more active of the two species. The non-phosphorylated form of GS may correspond to the irreversibly activated state.     

Effects of guanosine 5'-[gamma-thio]triphosphate and thrombin on the phosphoinositide metabolism of electropermeabilized human platelets

Incubation of human platelets with myo-[3H]inositol in a low-glucose Tyrode's solution containing MnCl2 enhanced the labelling of phosphoinositides about sevenfold and greatly facilitated the measurement of [3H]inositol phosphates formed by the activation of phospholipase C. Labelled platelets were permeabilized by high-voltage electric discharges and equilibrated at 0 degree C with ATP, Ca2+ buffers and guanine nucleotides, before incubation in the absence or presence of thrombin. Incubation of these platelets with ATP in the presence or absence of Ca2+ ions led to the conversion of [3H]phosphatidylinositol to [3H]phosphatidylinositol 4-phosphate and [3H]phosphatidylinositol 4,5-bisphosphate ([3H]PtdInsP2). At a pCa of 6, addition of 100 microM GTP[gamma S] both prevented this accumulation of [3H]PtdInsP2 and stimulated its breakdown; the formation of [3H]inositol phosphates was increased ninefold. After 5 min these comprised 70% [3H]inositol monophosphate ([3H]InsP), 28% [3H]inositol bisphosphate ([3H]InsP2) and 2% [3H]inositol trisphosphate ([3H]InsP3). In shorter incubations higher percentages of [3H]InsP2 and [3H]InsP3 were found. In the absence of added Ca2+, the formation of [3H]inositol phosphates was decreased by over 90%. Incubation of permeabilized platelets with GTP[gamma S] in the presence of 10 mM Li+ decreased the accumulation of [3H]InsP and increased that of [3H]InsP2, without affecting [3H]InsP3 levels. Addition of unlabelled InsP3 decreased the intracellular hydrolysis of exogenous [32P]InsP3 but did not trap additional [3H]InsP3. These results and the time course of [3H]inositol phosphate formation suggest that GTP[gamma S] stimulated the action of phospholipase C on a pool of [3H]phosphatidylinositol 4-phosphate that was otherwise converted to [3H]PtdInsP2 and that much less hydrolysis of [3H]phosphatidylinositol to [3H]InsP or of [3H]PtdInsP2 to [3H]InsP3 occurred. At a pCa of 6, addition of thrombin (2 units/ml) to permeabilized platelets caused small increases in the formation of [3H]InsP and [3H]InsP2. This action of thrombin was enhanced twofold by 10-100 microM GTP and much more potently by 4-40 microM GTP[gamma S]. In the presence of the latter, thrombin also increased [3H]InsP3. The total formation of [3H]inositol phosphates by permeabilized platelets incubated with thrombin and GTP[gamma S] was comparable with that observed on addition of thrombin alone to intact platelets. However, HPLC of the [3H]inositol phosphates formed indicated that about 75% of the [3H]InsP accumulating in permeabilized platelets was the 4-phosphate, whereas in intact platelets stimulated by thrombin, up to 80% was the 1-phosphate.(ABSTRACT TRUNCATED AT 400 WORDS)     

Ca-releasing action of beta, gamma-methylene adenosine triphosphate on fragmented sarcoplasmic reticulum.

beta,gamma-Methylene adenosine triphosphate (AMPOPCP) has two effects on fragmented sarcoplasmic reticulum (FSR), i.e., inhibition of the rate of Ca uptake and the induction of Ca release from FSR filled with Ca. The Ca release brought about by AMPOPCP has many features in common with the mechanism of Ca-induced Ca release: i) it is inhibited by 10 mM procaine; ii) the amount of Ca release increases with increase in the extent of saturation of FSR with Ca; iii) increase of the Ca concentration in the extent of saturation of FSR with Ca; iii) increase of the Ca concentration in the medium facilitates the release of Ca. However, no facilitation of Ca release upon decrease of Mg concentration in the medium is observable. AMPOPCP and caffeine potentiate each other remarkably in their Ca-releasing action, irrespective of the kind of substrate. From the mode of action of AMPOPCP on the rate of Ca uptake, the amount of phosphorylated intermediate (EP), and the effect on Sr release, it is suggested that the state of the FSR-ATP complex is crucial for Ca-induced Ca release.