PCR with 5-methyl-dCTP replacing dCTP

When dCTP is replaced by methyl5-dCTP in the polymerase chain reaction some templates cannot be efficiently amplified by Taq polymerase or Vent polymerase using standard cycling parameters. However, this phenomenon can be overcome by increasing the temperature of the denaturation steps to 100 degrees C, or by adding dITP to destabilize the m5dC:dG base pairs. Once the block to amplification of m5dC-substituted DNA was overcome, methylated DNA from the 'superpolylinker' of the plasmid pSL 1180 was used as a substrate to check the methyl-sensitivity of a variety of restriction endonucleases. The m5dC-substituted DNAs should also be valuable substrates for defining the specificity of methyl-dependent endonucleases.

Altered coupling states between calcium transport and (Ca2+, Mg2+)-ATPase in the AS-30D ascites hepatocarcinoma plasma membrane

Plasma membrane fractions from normal, regenerating liver and the AS-30D ascites hepatocarcinoma exhibited a high degree of enrichment when a set of plasma membrane enzyme markers were studied in comparison to the ones associated to the mitochondrial and cytosolic compartments. While the (Ca2+, Mg2+)-ATPase observed for the plasma membrane fraction isolated from normal liver showed an activity of 1.2 mumoles/mg/min, the regenerating liver and the AS-30D plasma membrane fractions presented a much lower ATPase activity (0.3 and 0.22 mumoles/mg/min respectively). Despite the differences in ATPase activity observed between models, the plasma membrane fraction from the AS-30D hepatocarcinoma presented a calcium transport activity similar to the value observed for the normal system (5.9 and 5.5 nmoles Ca2+/mg/10 min, respectively). Interestingly, the ATP in equilibrium with Pi exchange experiments carried out with the different plasma membrane fractions revealed that the (Ca2+, Mg2+)-ATPase contained in the plasma membrane from the AS-30D cells shows an exchange activity of 26 nmoles ATP in equilibrium with Pi/mg/min, similar to the one observed fo the enzyme from normal liver (30 nmoles ATP in equilibrium with Pi/mg/min). Our results suggest that the plasma membrane from the transformed model presents a more efficient mechanism to regulate the movement of calcium through the calcium pump, with an optimum expenditure of energy.

Modification of [3H]inositoltrisphosphate binding in kainic acid-lesioned and postischemic rat hippocampus

A quantitative autoradiographic study was made on the binding of the phosphatidylinositol system ligand [3H]inositol(1,4,5)-trisphosphate (IP3) to forebrain sections from rats decapitated various times after 10 min of forebrain ischemia. To investigate the effect of a deafferentation of the hippocampal CA1, kainic acid-induced CA3-lesioned rats with or without 10 min of cerebral ischemia, were also included. The highest binding was found in the hippocampal CA1. Ten min of cerebral ischemia did not change the binding significantly. Between 5 min and 1 h of recirculation there was a 25-35% binding decline in all regions. In the CA1, where the pyramidal cells became necrotic 6 days after ischemia, there was a further decline to 16% of control. In the cortex, where there is no necrosis in this model, binding did not return to control values until day 14. Four days after a selective CA3 lesion with kainic acid, there was a significant 25% decline in the cortex, dentate gyrus and CA1, whereas in the necrotic CA3 binding declined to 54% of control. Ten min of ischemia did not alter this binding significantly. This decrease in calcium mobilizing intracellular receptors after ischemia and seizures could be due to increased membrane degradation, or to a more specific down-regulation following increased intracellular concentration of calcium and IP3.

Selective cross-activation/inhibition of second messenger systems and the reduction of age-related deficits in the muscarinic control of dopamine release from perifused rat striata

Possible alterations in muscarinic cholinergic (mACh) signal transduction in senescence were studied in rat neostriata. Acetylcholine (ACh) activation of striatal muscarinic heteroreceptors by carbachol or oxotremorine enhances K(+)-evoked release of dopamine from perifused striata of 6- but not 24-month-old rats. Present experiments determined the effects of simultaneous activation or activation/inhibition of more than one second messenger on K(+)-evoked release of DA from perifused striatal slices from these age groups. Combinations of carbachol (500 microns), which stimulates inositol-1,4,5-bisphosphate (IP3) production and inhibits cyclic AMP production, with oxotremorine (500 microns), which inhibits cyclic AMP production, in the presence of 30 mM KCl (in a modified Krebs-Ringer medium) reduced the age-related reduction in mAChR enhancement of DA release (analyzed by HPLC coupled to electrochemical detection; 5 min fractions were collected on ice in perchloric acid; flow rate 120 microliters/min). Combinations of these agonists with the putative second messenger arachidonic acid (10 microM), also enhanced K(+)-evoked release of DA in the striatal tissue from the 24-month group. IP3 activation was lower in the striatal tissue from old animals than those from young under all conditions, but cross-activation/inhibition actually may have lowered the IP3 threshold necessary for enhanced DA release to occur. In a subsequent experiment, pre-loading striatal tissue from young animals with either carbachol or oxotremorine under basal release conditions reduced the responding when the basal release medium was switched to one containing 30 mM KCl and combinations of the agonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Altered Ca2+ homeostasis and functional correlates in hepatocytes and adipocytes in endotoxemia and sepsis

Decreased cytosolic [Ca2+] and impaired Ca2+ release in response to an IP3 challenge are among perturbations in hepatocyte Ca2+ homeostasis associated with endotoxemia and sepsis. These changes are consistent with the accompanying alterations in appropriate physiologic functions, e.g., activation of glycogen phosphorylase and gluconeogenesis, mediated by [Ca2+]c and defective phosphorylation of relevant enzymes. Attenuation of IP3 binding to the subcellular fractions that are imputed to be targets of IP3 and a decrease in the size of the IP3-sensitive pool of releasable Ca2+ are underlying components of the mechanism of the reduced Ca2+ release upon IP3 stimulation and its metabolic sequelae. ET treatment leads to a significant increase in Ca2+ associated with the cell surface compartment of adipocytes, a reduction in 45Ca2+ uptake by endoplasmic reticulum and higher cytosolic [Ca2+] under basal conditions and upon ACTH stimulation than that observed in cells of control rats. The reduced 45Ca2+ uptake is also manifest in adipocytes of septic rats. Alterations in adipocyte metabolism induced by ET include increased oxidation of glucose to CO2 (an insulin-like effect) and increased lipolysis upon NE and ACTH stimulation.

Bombesin receptor antagonists [corrected]. 1. Analogues with deleted, inverted or substituted amino acid residues

Bombesin is a 14-residue peptide hormone which serves a variety of biological functions, including cell growth control in Swiss 3T3 cultured fibroblasts. It has been also involved in an autocrine system regulating the growth of small cell lung carcinoma. A series of bombesin analogues were developed by amino acid deletion, inversion or substitution in the carboxy-terminal region, identified by the target cell receptor. Their properties were examined for: i) competitive binding assays; ii) mitogenic induction and inhibition assays; iii) effects on early cellular events (inositol phosphates production and protein tyrosine phosphorylation). Inversion of the Trp residue, or deletion of the C-terminal tripeptide amide, induced complete loss of receptor affinity and biological effects. Deletion of the His residue, or its derivatization as His (Dnp) in conjunction with Met deletion or modification, gave rise to compounds with reduced receptor affinity and weak antagonistic properties. Other modifications in the C-terminal tripeptide affected the potency but not the biological profile of the parent peptide. These results indicate the basis for the eventual design of improved, specific bombesin receptor antagonists and stimulate further studies on their possible application in the therapy of human small cell lung cancer.

Ca2+ channel inhibition by a new dihydropyridine derivative, S11568, and its enantiomers S12967 and S12968

Biochemical and electrophysiological techniques were used to describe the Ca2+ channel blocking properties of a new dihydropyridine derivative, S11568 (+/-)- ([(amino-2-ethoxy)-2-ethoxy]methyl)-2-(dichloro-2',3'-phenyl)-4- ethoxy-carbonyl-3-methoxycarbonyl-5-methyl-6-dihydro-1,4-pyridine and its enantiomers S12967 ((+)-S11568) and S12968 ((-)-S11568). In binding studies, S11568 and S12968 displaced specifically bound [3H]PN 200-110 from cardiac and vascular smooth muscle preparations with potencies of 5.6-51 nM, respectively. S12967 was 6- to 18-fold less potent than S12968. A good correlation was found between the IC50 value for the inhibition of 45Ca2+ uptake by A7r5 aortic smooth muscle cells and binding data. Whole-cell patch clamp studies in both guinea-pig ventricular myocytes and A7r5 cells yielded similar results. At holding potential (VH) -50 mV, S12968 inhibited L-type Ca2+ current with an IC50 value near 70 nM, 2- to 3-fold more potently than S11568 and 30-fold more potently than S12967. With VH -100 mV, all three compounds were less potent, with IC50 values ranging from 500 nM to 3 microM. These results demonstrate conclusively that S12968 is the more active enantiomer. Furthermore, the pronounced voltage dependence of its actions in vitro suggests that in vivo it could exhibit good selectivity for vascular smooth muscle over cardiac muscle.

Influence of 2,3-diphosphoglycerate on the deformability of human erythrocytes

Effect of 2,3-diphosphoglycerate (2,3-DPG) on the deformability of human erythrocytes was examined with a rheoscope under shear stress of 8-82 dyn/cm2. With increasing 2,3-DPG in erythrocytes (from 5 to 15 mM/l cells) by incubating with inosine and pyruvate in isotonic 50 mM phosphate-buffered saline, erythrocyte deformability under uniform shear stress was remarkably impaired. But reduction of 2,3-DPG (from 5 to 2.2 mM/l cells) did not affect the deformability. In 2,3-DPG-enriched erythrocytes, increased intracellular hemoglobin concentration (MCHC), decreased intracellular pH, and increased contents of ATP and IMP (and ITP) were observed. (1) When the MCHC (i.e., the internal viscosity) was normalized by suspending in hypotonic medium, the deformability of 2,3-DPG-enriched erythrocytes was greatly improved, but still decreased. (2) The change of intracellular pH between 6.5 and 7.5 (as compared adjusting to same MCHC) did not alter the deformability. (3) The changes of purine nucleotides, ATP (0.6-2.1 mM/l cells), IMP (0-0.9 mM/l cells) and ITP (0-0.5 mM/l cells) did not alter the erythrocyte deformability. In conclusion, decreased deformability of erythrocytes induced by augmentation of 2,3-DPG is due mainly to the increased internal viscosity and due partly to the increased membrane viscoelasticity.

Comparison of the self-association properties of the 5'-triphosphates of inosine (ITP), guanosine (GTP), and adenosine (ATP). Further evidence for ionic interactions in the highly stable dimeric [H2(ATP)]2(4-) stack

The concentration dependence of the chemical shifts for the hydrogens H-2, H-8 and H-1' of ITP and for H-8 and H-1' of GTP has been measured in D2O at 25 degrees C under several degrees of protonation in the pD range 1.2-8.4. For reasons of comparison, inosine and guanosine have been included in the study The results are consistent with the isodesmic model of indefinite noncooperative stacking. The association constants for the nucleosides (Ns) inosine and guanosine decrease with increasing protonation: Ns greater than D(Ns)+/Ns in a 1:1 ratio greater than D(Ns)+. In contrast, a maximum is observed with ITP and GTP; the stacking tendency of GTP following the series: GTP4- less than or equal to D(GTP)3- (K approximately 0.7 M-1) less than D(GTP)3-/D2(GTP)2- in a 1:1 ratio (K approximately 2.9 M-1) greater than D2(GTP)2- greater than D3(GTP)- (K approximately 1.5 M-1). The order of the series with ITP corresponds to that with GTP, but the association constants are slightly smaller. At the maximum of the self-association tendency the triphosphate residue has only a minor influence; this follows from the fact that the association constants for the 1:1 ratios of Ino/D(Ino)+ and D(ITP)3-/D2(ITP)2- are identical within experimental error; this holds also for Guo/D(Guo)+ and D(GTP)3-/D2(GTP)2-; in all these pairs the K-7 site is 50% protonated. Comparison of the association constant for the deprotonated species shows that here charge effects, i.e. repulsion between the negatively charged triphosphate chains, are important: Ino (K approximately 3.3 M-1) greater than ITP4- (K approximately 0.4 M-1) and Guo (K approximately 8 M-1) greater than GTP4- (K approximately 0.8 M-1). In addition the series holds: Ado (K approximately 15 M-1) greater than Guo greater than Ino. However, most important is the comparison of the ITP and GTP series with previous data for ATP: ATP4- (K approximately 1.3 M-1) less than D(ATP)3- (2.1 M-1) less than 1:1 ratio of D(ATP)3-/D2(ATP)2- (6 M-1) much less than D2(ATP)2- (approximately 200 M-1) much greater than D3(ATP)- (K less than or equal to 17 M-1).(ABSTRACT TRUNCATED AT 400 WORDS)

Architecture of Purkinje cells of the reeler mutant mouse observed by immunohistochemistry for the inositol 1,4,5-trisphosphate receptor protein P400

P400 protein, which is identical to the inositol 1,4,5-trisphosphate receptor protein, is a glycoprotein closely associated with the membranes of Purkinje cells. Three types of monoclonal antibodies against P400 protein were employed for the immunohistochemical detection of Purkinje cells in the cerebellum and brainstem of the normal and reeler mouse. Purkinje cells in both types of mice were immunoreactive against anti-P400 antibodies, and the soma, dendrites, axon and even terminal boutons in the cerebellar and vestibular nuclei could be clearly visualized. In the cerebellum of the reeler mutant, the heterotopic Purkinje cells both within and below the granule cell layer were also immunopositive and could be clearly differentiated from the deep cerebellar nuclei, in which neurons were immunonegative. The molecular layer of the reeler cerebellum varied in thickness and certain parts were completely defective. The dendrites within the molecular layer extended from Purkinje cells whose cell bodies were located in the normal position, abnormally in the granule cell layer, or at the surface of the central mass. Outside the cortex of the cerebellum, ectopic Purkinje cells were demonstrated in 3 cerebellar nuclei, the cerebellar medulla and peduncle, and brainstem of the normal and reeler mouse.

Tyrosine alpha 244 is derivatized when the bovine heart mitochondrial F1-ATPase is inactivated with 5'-p-fluorosulfonylbenzoylethenoadenosine

The bovine heart mitochondrial F1-ATPase (MF1) is inactivated by 5'-p-fluorosulfonylbenzoylethenoadenosine (FSB epsilon A) with pseudo-first order kinetics. The dependence of the rate of inactivation on the concentration of FSB epsilon A revealed an apparent Kd of 0.25 mM. ATP and ADP, and to a lesser extent, ITP and IDP provide partial protection against inactivation by the reagent. Isolation and sequence analysis of major radioactive fragments in peptic or cyanogen bromide digests of MF1 inactivated with [3H]FSB epsilon A indicate that modification of Tyr-alpha 244 is associated with the loss of activity observed. Assessment of the amount of Tyr-alpha 244 derivatized with [3H]FSB epsilon A at specific points during inactivation of the ATPase indicates that maximal inactivation is achieved on modification of this residue in slightly greater than one copy of the alpha subunit. The following characteristics of inactivation of MF1 by FSB epsilon A have also been determined. (a) The rate of inactivation of ITPase activity by FSB epsilon A is 1.4 times greater than that observed for inactivation of ATPase activity under identical conditions. (b) After maximally inactivating the capacity of MF1 to hydrolyze saturating ATP with FSB epsilon A, the modified enzyme retained its capacity to hydrolyze substoichiometric ATP. (c) Inactivation of the ATPase by FSB epsilon A is accelerated by Pi. In each of the above characteristics, MF1 modified by FSB epsilon A resembles enzyme inactivated with 5'-p-fluorosulfonylbenzoyladenosine (FSBA) more than it does enzyme inactivated with 5'-p-fluorosulfonylbenzoylinosine (FSBI). Furthermore, prior inactivation of MF1 with FSBA completely prevents labeling of Tyr-alpha 244 with [3H]FSB epsilon A, whereas prior inactivation of the enzyme with FSBI does not. Since a single catalytic site is modified when FSBI inactivates MF1 whereas three noncatalytic sites are modified when it is maximally inactivated with FSBA, it is concluded that FSB epsilon A also modifies noncatalytic sites.

2,3-Diphosphoglycerate is a nonselective inhibitor of inositol 1,4,5-trisphosphate action and metabolism

Inositol 1,4,5-trisphosphate (InsP3) is an important second messenger generated from the hydrolysis of phosphatidylinositol 4,5-bisphosphate by phospholipase C in response to Ca2(+)-mobilizing stimuli. InsP3 interacts with specific intracellular receptors and triggers the release of sequestered Ca2+ from an intracellular store. We have looked at the influence of 2,3-diphosphoglycerate on the action and metabolism of InsP3 in the bovine adrenal cortex. 2,3-Diphosphoglycerate blocked InsP3 binding to adrenal cortex microsomes with a half-maximal efficiency of 0.5 mM. Scatchard analyses revealed that 2,3-diphosphoglycerate did not change the maximal capacity of the microsomes, but decreased their binding affinity for InsP3. The Ca2(+)-releasing activity of InsP3 on the same microsomal preparation was monitored with the fluorescent indicator, Fura-2. 2,3-Diphosphoglycerate blocked this activity with a half-maximal efficiency of 2 mM. The effect of 2,3-diphosphoglycerate could be overcome by supramaximal doses of InsP3, indicating a competitive inhibitory effect. The activity of InsP3 phosphatase from bovine adrenal cortex microsomes was also studied. 2,3-Diphosphoglycerate inhibited the activity of the phosphatase with a half-maximal efficiency of 0.3 mM. Lineweaver-Burke plots revealed that this effect was competitive. Finally, 2,3-diphosphoglycerate was also able to inhibit the activity of a partially purified preparation of InsP3 kinase from bovine adrenal cortex cytosol. The half-maximal dose was around 10 mM and the Lineweaver-Burke plot showed that the inhibition was competitive. These results show that 2,3-diphosphoglycerate can be considered as a structural analog of InsP3. Its inhibitory effects, however, are not selective enough to use it as an InsP3 protective agent in Ca2(+)-mobilization studies.

Mode of action of lithium in affective disorders. An influence on intracellular calcium functions

The inference that lithium acts by altering intracellular calcium functions is supported by the three areas considered above. First, recent work in other laboratories has broadened the range of lithium actions on calcium-dependent physiological functions. Second, a theoretical analysis of the coupling of calcium transport to the triphosphoinositide response presents a plausible mechanism by which lithium could limit the damage caused by deficient calcium transport. Third, we have recently reported that there is a direct enhancement of the calmodulin-activated membrane-bound calcium pump in lithium-treated bipolar subjects.

[Ability of nucleoside triphosphates to provide for Ca 2+ transport by sarcoplasmic reticulum fragments]

The Ca2+ transport by sarcoplasmic reticulum fragments was studied. ATP, CTP, ITP, GTP and UTP provided the same Ca-pump efficiency. When the NTP was exhausted, Ca2+ actively accumulated from the sarcoplasmic reticulum vesicles outflow, and with the higher rate of ATP was a substrate. The Ca-ATPase conformational transitions induced by ATP are discussed for their role in the provision of energy.

The cholinesterase inhibitor soman increases inositol trisphosphate in rat brain

Studies were conducted to determine the effect of the cholinesterase inhibitor soman on the amount of inositol trisphosphate in the neocortex, striatum, cerebellum, and medulla-pons regions of rat brain in vivo. The studies indicate that treatment with soman increases inositol trisphosphate in the neocortex and striatum, but not in the cerebellum or medulla-pons region. In the neocortex the most pronounced increases were observed in animals with severe poisoning symptoms, however inositol trisphosphate was also found to be elevated in animals with only mild poisoning symptoms.

The inhibitory effect of quinacrine on contractile responses to norepinephrine in isolated rabbit aorta

1. In rabbit aorta, quinacrine, but not indomethacin nor nordihydroguaiaretic acid, inhibited contractile responses to norepinephrine and KCl. Amiloride and nifedipine did not affect the effect of quinacrine. 2. In Ca2(+)-free medium, quinacrine (3 x 10(-6)-10(-4) M) inhibited the norpinephrine response less than that to a subsequent addition of Ca2+. 3. M&B 22, 948, nitroglycerin and forskolin inhibited the Ca2+ response. The effect of quinacrine was inhibited by M&B 22,948, but not by forskolin and potentiated by nitroglycerin. 4. Quinacrine and M&B 22,948 potentiated the nitroglycerin-relaxation. The effect of quinacrine plus M&B 22,948 was not different from that of quinacrine. 5. These results indicate that the effect of quinacrine may be different from that of nifedipine but is related to cGMP.

The effect of M&B 22948 on methacholine- and histamine-induced contraction and inositol 1,4,5-trisphosphate levels in guinea-pig tracheal tissue

The effect of a cyclic GMP phosphodiesterase inhibitor, M&B 22948, on methacholine- and histamine-induced contraction and inositol 1,4,5-trisphosphate (IP3) elevation was studied in guinea-pig tracheal rings. After addition of methacholine or histamine the rise in IP3 content was rapid and transient reaching a maximum after 5-15 s, which coincided with the maximum rate of tension development. Cyclic GMP levels of the tissue were elevated by M&B 22948 before agonist stimulation and further elevated by addition of methacholine or histamine. Cyclic AMP levels were not altered by any of these agents. M&B 22948 abolished IP3 generation induced by methacholine or histamine, but did not alter the rate or magnitude of tension development. Thus, IP3 generation does not appear to be responsible for the contractions induced by methacholine or histamine in this tissue.

DNA-synthesis regulation and correlation with inositol trisphosphate levels in cultured bovine retinal capillary pericytes

Inositol phosphate (IP), inositol bisphosphate (IP2) and inositol trisphosphate (IP3) in cultured bovine retinal capillary pericytes (BRCP) were quantitated by an ion-pair reverse-phase HPLC. BRCP were grown in media with standard (5 mM) or high (30 mM) glucose, and were either labeled with myo-[2-3H]inositol (20 microCi ml-1) for 60 hr or with dual isotopes (20 microCi ml-1 myo-[2-3H]inositol and 2 microCi ml-1 [14C]glycerol) for 8 hr. In parallel, BRCP in different glucose-media were incubated with 1 microCi ml-1 [3H]thymidine for 4 hr. High glucose significantly suppressed the accumulation of [3H]label in IP, IP2 and IP3, and specifically reduced the incorporation of [14C]glycerol into inositol phospholipids, but not that of neutral lipids and other types of phospholipids. The reduced IP3 level correlated with the decrease in the incorporation of [3H]thymidine into DNA. Both the reduced IP3 formation and DNA synthesis which were induced by high glucose were significantly reversed by adding either myo-inositol or AL1576, an aldose reductase inhibitor (ARI). However, the addition of neither myo-inositol nor ARI stimulated IP3 formation and/or DNA synthesis when BRCP were grown in the standard medium (5 mM glucose). These findings indicate that myo-inositol metabolism and the polyol pathway affect inositol phospholipid-mediated pericyte division in vitro only under the high-glucose condition. These data are compatible with the hypothesis that altered inositol phospholipid metabolism accounts for the loss of pericytes in diabetic retinopathy.

DNA sequencing artifacts in dITP reactions containing gene 32 protein

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Stimulation by phaclofen of inositol 1,4,5-triphosphate production in cultured neurons from chick tectum

The influence of gamma-aminobutyric acid (GABA) receptor agonists and antagonists on the intracellular concentration of inositol 1,4,5-triphosphate (IP3) was examined in neuronal cultures of embryonic chick tectum. GABA and selective agonists of its receptors were inactive, whereas phaclofen, a GABA-B receptor antagonist, increased IP3 levels in a concentration-dependent manner. This effect occurred at phaclofen concentrations much lower than those required to affect GABA-B receptors. It is concluded that phaclofen, in addition to its known effect at GABA-B receptors, is also active at an as-yet undefined site.

Effect of citreoviridin and isocitreoviridin on beef heart mitochondrial ATPase

Citreoviridin is a toxic metabolite from fungus that has been shown to be an inhibitor of mitochondrial F1-ATPases. Studies of citreoviridin, however, have been compromised by the light-dependent isomerization that it undergoes. The isomerization is a potential source of extensive variability in the studies, if citreoviridin and isocitreoviridin have different kinetic effects and binding properties. Both citreoviridin and isocitreoviridin recently have been purified and have been shown to be stable in the dark. Using the purified isomers, the effects of both citreoviridin and isocitreoviridin on soluble and membrane-bound beef heart mitochondrial F1-ATPase activity were investigated. It was found that citreoviridin was an uncompetitive inhibitor of ATP hydrolysis, and a non-competitive inhibitor of ITP hydrolysis catalyzed by soluble F1-ATPase. Isocitreoviridin had no effect on the hydrolysis of either of the triphosphates catalyzed by soluble F1-ATPase. The inhibition constant, Ki for citreoviridin was determined as 4.5 microM for ATP hydrolysis. The inhibition constants Kii and Kis for ITP hydrolysis were determined as 4.3 and 1.03 microM, respectively. Citreoviridin was an uncompetitive inhibitor of ATP hydrolysis and a noncompetitive inhibitor of ATP synthesis catalyzed by membrane-bound F1-ATPase. The inhibition constant, Ki, for ATP hydrolysis was around 4 microM. For ATP synthesis the inhibition constants were determined as 0.12 and 0.16 microM for Kis and Kii, respectively, when ADP concentration was kept saturating. Isocitreoviridin had no effect on either activity of the membrane-bound enzyme.

[Calcium release from vesicles of heavy sarcoplasmic reticulum of rabbit skeletal muscles]

The release of Ca2+ from vesicles of heavy sarcoplasmic reticulum after its accumulation due to hydrolysis of ATP, GTP, CTP, UTP or ITP has been studied using Antipyrylazo III, a metal-chromic Ca-indicator. All the studied substrates of the Ca-pump provide Ca2+ accumulation inside the heavy sarcoplasmic reticulum vesicles, the spontaneous Ca2+ outflux rate being different for different nucleoside triphosphates. It is only ATP that provides Ca-(caffeine)-induced Ca2+ release, however AMP, ADP, beta, gamma-methylene-ATP induce Ca2+ ejection in the presence of nonadenylic nucleotides. The ruthenium red (10(-7M) inhibits the induced ejection of Ca2+ from vesicles of the heavy sarcoplasmic reticulum, but does not prevent the spontaneous release of Ca2+ in the same concentrations. A conclusion is drawn that besides Ca-channels sensitive to Ca2+ and caffeine in the presence of ATP (or to AMP, ADP, beta, gamma-methylene-ATP in the presence of nonadenylic nucleotides) and possessing high sensitivity to the ruthenium red there is another pathway for Ca2+ in the heavy reticulum membranes along which its spontaneous release occurs after the substrate exhaustion. It is supposed that this release is provided by the presence of the Ca-ATPase protein.

Hormonal modulation of guanyl nucleotide binding to rat luteal membranes

Guanyl nucleotides are known to play a dual role in the activation of the adenylate cyclase system of the rat corpus luteum, being required for human choriogonadotropin (hCG) stimulation of the enzyme and modulating hCG binding to some hormone receptors. Current models of adenylate cyclase activation require that guanyl nucleotide binding be enhanced by hormones, and we have examined this binding in rat luteal membrane preparations known to contain guanyl nucleotide-modulated hCG receptors. [3H] Guanylyl-imidodiphosphate (GMPPnP), a nonhydrolyzable analog of guanosine triphosphate (GTP), was used to investigate binding to urea-washed, heavy rat luteal membranes. Binding was found to be linear, with respect to the amount of membranes added, in the range of 2-10 mg wet wt. tissue equivalents, and equilibrium was reached after a 30-min incubation at 30 degrees C. Analysis of equilibrium binding experiments gave a Ka of 1.2.10(7) +/- 0.9.10(7) M-1, with 460 +/- 430 fmol binding sites per mg tissue in the absence of hormone, Kinetic experiments showed an association rate constant of 2.6.10(5) +/- 0.5.10(5) M-1 min-1 and a dissociation rate constant of 1.8.10(-2) +/- 0.9.10(-2) min-1. In the presence of hCG, the Ka was unchanged; however, the number of binding sites increased by 50-120%. Competitive binding assays utilizing other nucleotides revealed that a hierarchy of GMPPnP = GTP greater than guanosine diphosphate (GDP) greater than inosine triphosphate (ITP) in displacing labeled GMPPnP. A similar hierarchy was also found for hCG-stimulated adenylate cyclase activity (GMPPnP = GTP greater than ITP) and for modulation of hCG binding (GMPPnP greater than GTP greater than ITP).(ABSTRACT TRUNCATED AT 250 WORDS)

Factor-dependent transcription termination by vaccinia virus RNA polymerase. Evidence that the cis-acting termination signal is in nascent RNA

Transcription termination in vitro by vaccinia RNA polymerase is dependent on a trans-acting factor, VTF, that is associated with, if not identical to, the vaccinia mRNA capping enzyme. VTF-induced termination occurs approximately 50 nucleotides downstream of a signal sequence TTTTTNT in the non-transcribed templated strand; thus the cognate sequence UUUUUNU is expressed in the nascent RNA. To address the role of the nascent RNA in chain termination, the effects of nucleotide base analog substitutions were studied. Incorporation of bromo- (Br) UMP or iodo- (I) UMP into RNA abrogated factor-dependent termination without preventing the synthesis of read-through transcripts. Substitution of either ITP or 7'-methylguanosine for GTP did not inhibit factor-dependent termination, nor did the substitution of BrCTP or ICTP for CTP. The early transcripts synthesized in vitro were sensitive to RNase T2 but resistant to RNase H, indicating an absence of extensive hybridization of RNA product to the DNA template. Substitution of BrUTP for UTP did not alter the nuclease sensitivity of the transcripts, suggesting that increased stability of RNA:DNA hybrid structures did not account for the analog effects. These results are consistent with a model in which recognition of the primary sequence UUUUUNU in nascent RNA by the polymerase and/or VTF is required for transcription termination.

An unusual phosphatidylinositol turnover pathway in noradrenaline-perfused rat hearts

1. The phosphatidylinositol turnover pathway has been studied in noradrenaline-perfused rat hearts using anion-exchange high performance liquid chromatography. 2. The active calcium-releasing compound inositol-(1,4,5)trisphosphate was detected together with its degradation products inositol-(1,4)bisphosphate and inositol monophosphate. All these products increased in response to noradrenaline stimulation. 3. At noradrenaline perfusion times from 5 s to 20 min there was no appearance of inositol-(1,3,4,5)tetrakisphosphate or its degradation products: inositol-(1,3,4)trisphosphate or inositol-(1,3) and (3,4)bisphosphates. 4. These data suggest the absence of the inositol-(1,4,5)trisphosphate phosphorylation/dephosphorylation pathway in heart.

Translational requirement for La Crosse virus S-mRNA synthesis: a possible mechanism

Ongoing protein synthesis is required for La Crosse S-mRNA synthesis in vivo, and complete S-mRNA can be made in vitro only in the presence of an active rabbit reticulocyte lysate. Using in vitro systems based on the polymerase activity of purified virions, we further support the notion that it is translation of the nascent mRNA that is required for complete transcription. Since replacement of guanosine with inosine in the nascent mRNA substitutes for the translational requirement, it appears that translation is required to prevent interactions of the nascent chain from taking place, which, if not prevented, lead to premature termination. These interactions appear to be between the nascent mRNA chain and its nucleocapsid template. A model for the translational requirement for complete S-mRNA synthesis is presented.

The substrate specificity of dynein from Tetrahymena cilia

The substrate specificity of the 22S dynein ATPase from Tetrahymena cilia was investigated. The 22S dynein exhibited a high specificity for ATP in terms of both apparent Km and Vmax: naturally occurring nucleoside triphosphates other than ATP were hydrolyzed slowly with an apparent Km of 0.25-1 mM, a sharp contrast to that of ATP hydrolysis (1-4 microM). Pyrophosphate was a poor inhibitor for the dynein ATPase, indicating weak affinity. Since dynein binds ATP tightly and hydrolyzes it at a high rate, a method to determine a trace amount of ATP in the presence of other nucleoside triphosphates has been developed by taking advantage of this enzymatic characteristic of dynein. The effect of P1,P5-di(adenosine-5'-)-pentaphosphate (Ap5A) on the 22S dynein ATPase was also investigated. Ap5A acted as a weak competitive inhibitor of the ciliary 22S dynein ATPase and the nonlinearity of the double-reciprocal plot of the ATPase was confirmed in the presence of Ap5A.

Effects of arsenate on the Ca2+ ATPase of sarcoplasmic reticulum

The effect of arsenate on the partial reactions of the catalytic cycle of the Ca2+ ATPase of skeletal muscle of sarcoplasmic reticulum was studied. With the use of native vesicles it was found that arsenate accelerates the rate of ITP hydrolysis and inhibits both Ca2+ or Sr2+ uptake. These effects were not observed when ATP was used as substrate or, with the use of ITP, when leaky vesicles were assayed. Activation of ITP hydrolysis is related to an increase of the enzyme's apparent affinity for ITP. Arsenate increases the steady-state level of the phosphoenzyme formed from ITP. This depends on the concentration of both Pi and Ca2+, in the medium. Ca2+ and Sr2+ efflux were accelerated by arsenate. The fast Ca2+ efflux promoted by arsenate is impaired by external Ca2+. Arsenate competes with Pi for the phosphorylating site of the enzyme.

Kinetics of hexokinase D ('glucokinase') with inosine triphosphate as phosphate donor. Loss of kinetic co-operativity with respect to glucose

When ATP, the normal phosphate donor for hexokinase D ('glucokinase'), is replaced by ITP, the positive co-operativity with respect to glucose disappears. This may be rationalized in relation to kinetic models for hexokinase D co-operativity, which assume that with the normal substrates the chemical reaction and subsequent release of products occur so rapidly that binding of substrates cannot approach equilibrium and is therefore not constrained by the thermodynamic requirement that the Hill coefficient for substrate binding cannot exceed the number of binding sites. ITP is a much poorer substrate than ATP, however: its Km value at high glucose concentrations is 24 times the value for ATP, whereas the value of the limiting rate V is decreased about 8-fold. Consequently it is no longer possible for the ternary complex to be converted into products rapidly enough to generate kinetic co-operativity. The negative co-operativity with respect to glucose observed in 2H2O with ATP as phosphate donor also disappears when ITP is used instead of ATP.

Mapping and characterization of transcriptional pause sites in the early genetic region of bacteriophage T7

During transcription of DNA templates in vitro, Escherichia coli RNA polymerase pauses at certain sequences before resuming elongation. Previous studies have established that some pausing events are brought about by the formation of RNA hairpin structures in the nascent transcript; however, it is not known whether this is an invariant and causal relationship. We have mapped and characterized almost 200 distinct pause sites located within the early region of bacteriophage T7 DNA using a collection of T7 deletion mutant DNAs and taking advantage of a procedure that permits synchronous transcription from the T7 A1 promoter. The pausing pattern is sensitive both to the overall concentration of nucleotide substrates and to the relative concentrations of the four nucleotides. The apparent Ks value for a particular nucleoside triphosphate can vary over a 500-fold range depending on the nucleotide sequence, and pausing at some sites can be induced by modest reductions in substrate concentrations. However, pausing is not solely a consequence of substrate limitation. Pausing at certain sites is caused by some feature of the template or of the transcript itself. Substitution of inosine triphosphate (ITP) for GTP during transcription strongly affects the pattern and strength of pausing events, suggesting that base-pairing interactions involving the RNA strand are important for some pausing events. Other pauses are determined by sequences downstream from the elongation site that have not yet been transcribed, and pausing at these sites is generally insensitive to substitution of IMP for GMP in the nascent transcript. Pausing at one particular site on T7 DNA is strongly enhanced by the presence of E. coli gene nusA protein. These results confirm that there are multiple classes of sites that lead to transcriptional pausing, and provide a collection of sites for further study. Using selected pause sites in the early region of T7 DNA, we have tried to evaluate the possible roles of primary sequence, base composition and secondary structure in pausing. Computer analysis was used to compare primary sequences and potential RNA hairpin structures in transcripts for pauses known to share similar biochemical properties. We see no correlation of pause sites with regions of particular base composition or with specific primary sequences. While some pauses are correlated with the potential to form stable RNA hairpins just upstream from the growing point of the RNA chain, there is not a strict one-to-one relationship between predicted RNA hairpins and the location of pause sites.(ABSTRACT TRUNCATED AT 400 WORDS)

Kinetic studies on the reaction catalysed by phosphofructokinase from Trypanosoma brucei

The steady-state kinetics of the reaction catalysed by the bloodstream form of Trypanosoma brucei were studied at pH 6.7. In the presence of 50 mM-potassium phosphate buffer, the apparent co-operativity with respect to fructose 6-phosphate and the non-linear relationship between initial velocity and enzyme concentration, which were found when the enzyme was assayed in 50 mM-imidazole buffer [Cronin & Tipton (1985) Biochem. J. 227, 113-124], are not evident. Studies on the variations of the initial rate with changing concentrations of MgATP and fructose 6-phosphate, the product inhibition by fructose 1,6-bisphosphate and the effects of the alternative substrate ITP were consistent with an ordered reaction pathway, in which MgATP binds to the enzyme before fructose 6-phosphate, and fructose 1,6-bisphosphate is the first product to dissociate from the ternary complex.

Regulation of inositol trisphosphate accumulation by muscarinic cholinergic and H1-histamine receptors on human astrocytoma cells. Differential induction of desensitization by agonists

Although activation of muscarinic cholinergic receptors on 1321N1 human astrocytoma cells results in a linear accumulation of inositol phosphates for up to 60 min in the presence of LiCl [Masters, Quinn & Brown (1985) Mol. Pharmacol. 27, 325-332], activation of H1-histamine receptors resulted in an increase in total inositol phosphate formation that was maintained for less than 5 min. The effects of stimulation of these two receptors on accumulation of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], inositol 1,3,4-trisphosphate [Ins(1,3,4)P3] and inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] were also examined. Incubation of 1321N1 cells with carbachol resulted in a rapid accumulation of all three inositol phosphates, reaching a maximum within 30 s; this elevated value was maintained for up to 60 min. The rate of disappearance of Ins(1,3,4)P3 from carbachol-treated cells after the addition of atropine paralleled or exceeded the rate of disappearance of Ins(1,4,5)P3. Although the initial rates of accumulation of Ins(1,4,5)P3, Ins(1,3,4)P3 and Ins(1,3,4,5)P4 in the presence of histamine were similar to that observed with carbachol, the amounts of these inositol phosphates had returned to control values within 5 min after the addition of histamine. The results indicate that, although the acute effects of muscarinic receptor and H1-histamine receptor stimulation on phosphoinositide hydrolysis are very similar, the histamine receptor is desensitized rapidly, whereas the muscarinic receptor is not. This effect on histamine-receptor function is apparently homologous, since preincubation of 1321N1 cells with histamine did not decrease the subsequent response to carbachol.

Purification and characterization of phosphoenolpyruvate carboxykinase from the parasitic helminth Ascaris suum

Phosphoenolpyruvate carboxykinase has been purified from homogenates of Ascaris suum muscle strips to apparent homogeneity as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The purification is a three-step procedure which yields pure enzyme in milligram quantities with good yield. The subunit molecular weight of the Ascaris enzyme is between 75,000 and 80,000. The native molecular weight is 83,000 as determined by gel filtration. The kinetic constants for substrates of the carboxylation reaction were determined and compared to those measured for the avian liver enzyme. From kinetic studies it appears likely that two separate roles for divalent metal ions exist in the catalytic process. Studies conducted with Mn2+ or with micromolar concentrations of Mn2+, in the presence of millimolar concentrations of Mg2+ suggest that Mn2+ but not Mg2+ binds directly to and activates the enzyme while either Mn2+ or Mg2+ may bind to the nucleotide resulting in the metal-nucleotide complex. The metal-nucleotide is the active form of the substrate for the reaction. In the presence of Mg2+, an increase in the Mn2+ concentration results in a decrease in the Km for P-enolpyruvate suggesting a direct role for Mn2+ stimulation and regulation of activity. The concentrations of Mn2+ and Mg2+ in Ascaris muscle strips were determined by atomic absorption spectroscopy and support the proposed hypothesis of a specific Mn2+ activation of the enzyme. The nucleotides ATP and ITP act as competitive inhibitors against GTP with KI values of 0.50 and 0.75 mM, respectively. ITP is a competitive inhibitor against both IDP and P-enolpyruvate, suggesting overlapping binding sites for the two substrates on the enzyme.

Stereochemical course of thiophosphoryl transfer catalyzed by cytosolic phosphoenolpyruvate carboxykinase

Rat liver cytosolic phosphoenolpyruvate carboxykinase (PEPCK) utilizes inosine 5'-(3-thiotriphosphate) (ITP gamma S) as an excellent substrate, with Km and V values of 0.08 mM and 37 mumol min-1 (mg of protein)-1, respectively, compared with the corresponding values of 0.168 mM and 76 mumol min-1 (mg of protein)-1 for ITP. Thus, the V/Km values for the two substrates are the same. Reaction of (RP)-[gamma-18O2]ITP gamma S with oxalacetate catalyzed by cytosolic PEPCK produces (SP)-thio[18O]phosphoenolpyruvate. Therefore, thiophosphoryl transfer catalyzed by this enzyme proceeds with overall inversion of configuration at P. The reaction mechanism involves an uneven number of phosphotransfer steps, most likely a single step transfer between bound substrates. The results do not support the involvement of a phosphoryl enzyme intermediate in the mechanism.

Isolation of ATPase I, the proton pump of chromaffin-granule membranes

Chromaffin-granule membranes contain two ATPases, which can be separated by (NH4)2SO4 fractionation after solubilization with detergents, or by phase segregation in Triton X-114. ATPase I (Mr 400000) is inhibited by trialkyltin, quercetin and alkylating agents, and hydrolyses both ATP and ITP. It contains up to five types of subunit, including a low-Mr hydrophobic polypeptide that reacts with dicyclohexylcarbodi-imide; these subunits are unrelated to those of mitochondrial F1F0-ATPase, as judged by size and reaction with antibodies. ATPase II (Mr 140000) is inhibited by vanadate, and is specific for ATP; it has not been extensively purified. Proton translocation by resealed chromaffin-granule 'ghosts', measured by uptake of methylamine or by quenching of the fluorescence of 9-amino-6-chloro-2-methoxyacridine, is supported by the hydrolysis of ATP or ITP, and inhibited by quercetin or alkylating agents, but not by vanadate. ATPase I must therefore be the proton translocator involved in the uptake of catecholamines and possibly of other components of the chromaffin-granule matrix, whereas ATPase II does not translocate protons.

Quantitative analysis of intermediate phosphate-oxygen exchange during hydrolyses by ATPases

A simple mathematical model is presented that predicts the paradoxical effect of substrate concentration on the extent of oxygen exchange into the phosphate produced during hydrolyses of nucleotide triphosphates by ATPases in labelled water. The same model also predicts the deviations from Michaelis-Menten kinetics observed at low salt concentrations.

Inosine analogs. Their metabolism in mouse L cells and in Leishmania donovani

The growth of Leishmania donovani promastigotes and mouse L cells is differentially inhibited by several inosine analogs with modifications in the imidazole ring. The protozoal and mammalian cells also demonstrate differential metabolism of these analogs. 7-Deazainosine, 7-thia-7,9-dideazainosine, and formycin B were converted to their respective ATP analogs by both cell types. 8-Azainosine was converted to a GTP analog by mouse L cells; L. donovani did not metabolize this nucleoside. 9-Deazainosine and allopurinol riboside were metabolized only to their respective IMP analogs by L cells. L. donovani metabolized 9-deazainosine and allopurinol riboside to their ATP analogs and also metabolized 9-deazainosine to its GTP analog. All nucleosides studied were resistant to cleavage by either organism. From metabolism studies in the presence of a specific enzyme inhibitor, it was deduced that allopurinol riboside, formycin B, and 9-deazainosine were phosphorylated by at least two different routes in the mouse L cells. The metabolism of formycin B was inhibited 65% by the adenosine kinase inhibitor, 5-iodotubercidin, whereas the metabolism of allopurinol riboside (14% inhibition) and 9-deazainosine (0% inhibition) was only slightly affected by this inhibitor. The metabolism of allopurinol riboside and 9-deazainosine by L. donovani was not affected by 5-iodotubercidin. In contrast to the results of L cells, the metabolism of formycin B by L. donovani was also not affected by 5-iodotubercidin. The abilities of mouse L cells and L. donovani to metabolize these inosine analogs to the corresponding nucleotide analogs of ATP or GTP may be considered to be an activating step and correlates well with the respective cytotoxic effects of these compounds.

Electrogenic calcium transport in plasma membrane of rat pancreatic acinar cells

ATP-dependent 45Ca2+ uptake was investigated in purified plasma membranes from rat pancreatic acinar cells. Plasma membranes were purified by four subsequent precipitations with MgCl2 and characterized by marker enzyme distribution. When compared to the total homogenate, typical marker enzymes for the plasma membrane, (Na+,K+)-ATPase, basal adenylate cyclase and CCK-OP-stimulated adenylate cyclase were enriched by 43-fold, 44-fold, and 45-fold, respectively. The marker for the rough endoplasmic reticulum was decreased by fourfold compared to the total homogenate. Comparing plasma membranes with rough endoplasmic reticulum, Ca2+ uptake was maximal with 10 and 2 mumol/liter free Ca2+, and half-maximal with 0.9 and 0.5 mumol/liter free Ca2+. It was maximal at 3 and 0.2 mmol/liter free Mg2+ concentration, at an ATP concentration of 5 and 1 mmol/liter, respectively, and at pH 7 for both preparations. When Mg2+ was replaced by Mn2+ or Zn2+ ATP-dependent Ca2+ uptake was 63 and 11%, respectively, in plasma membranes; in rough endoplasmic reticulum only Mn2+ could replace Mg2+ for Ca2+ uptake by 20%. Other divalent cations such as Ba2+ and Sr2+ could not replace Mg2+ in Ca2+ uptake. Ca2+ uptake into plasma membranes was not enhanced by oxalate in contrast to Ca2+ uptake in rough endoplasmic reticulum which was stimulated by 7.3-fold. Both plasma membranes and rough endoplasmic reticulum showed cation and anion dependencies of Ca2+ uptake. The sequence was K+ greater than Rb+ greater than Na+ greater than Li+ greater than choline+ in plasma membranes and Rb+ greater than or equal to K+ greater than or equal to Na+ greater than Li+ greater than choline+ for rough endoplasmic reticulum. The anion sequence was Cl greater than or equal to Br greater than or equal to 1 greater than SCN greater than NO3 greater than isethionate greater than cyclamate greater than gluconate greater than SO2(4) greater than or equal to glutarate and Cl- greater than Br greater than gluconate greater than SO2(4) greater than NO3 greater than 1 greater than cyclamate greater than or equal to SCN, respectively. Ca2+ uptake into plasma membranes appeared to be electrogenic since it was stimulated by an inside-negative K+ and SCN diffusion potential and inhibited by an inside-positive diffusion potential. Ca2+ uptake into rough endoplasmic reticulum was not affected by diffusion potentials. We assume that the Ca2+ transport mechanism in plasma membranes as characterized in this study represents the extrusion system for Ca2+ from the cell that might be involved in the regulation of the cytosolic Ca2+ level.

alpha, beta-Bidentate CrADP abolishes the negative cooperativity of yeast mitochondrial F1-ATPase

The hydrolysis of MgATP and MgITP by mitochondrial F1-ATPase from Saccharomyces cerevisiae is competitively inhibited by alpha, beta-CrADP, alpha, beta, gamma-CrATP and beta, gamma-CrATP. The apparent K1 values of the three complexes are in the range of the half-saturating MgATP concentration. The negative cooperativity (nH = 0.7) of MgATP hydrolysis is totally abolished by alpha, beta-CrADP (nH = 1.0), while it is not affected by the CrATP. It is concluded that alpha, beta-CrADP binds exclusively at the regulatory site and that CrATP binds exclusively to the catalytic site.

Relationships of inosine triphosphate and bicarbonate effects on F1 ATPase to the binding change mechanism

Two interesting previously reported properties of mitochondrial F1 ATPase have been confirmed and have been examined by 18O exchange measurements to assess if they are consistent with sequential participation of catalytic sites during ATP hydrolysis. These are the ability of HCO3- to increase reaction rate with apparent loss of cooperative interaction between subunits and the ability of ITP to accelerate the hydrolysis of a low concentration of ATP. The effect of HCO3- was tested at concentrations of ATP lower than previous measurements. The activation disappeared when ATP was reduced to 0.1 microM. The HCO3- activation at higher ATP concentrations did not change the extent of reversal of the cleavage of tightly bound ATP at the catalytic site, as measured by the average number of water oxygens incorporated with each Pi formed when 5 or 10 microM ATP is hydrolyzed. The data are consistent with sequential site participation with HCO3- acceleration of ADP departure after a binding change that stops 18O exchange and loosens ADP binding. When ITP concentration was lowered during net ITP hydrolysis by F1 ATPase an increase in water oxygen incorporation into Pi formed is observed, as noted previously for ATP hydrolysis. The acceleration of the cleavage of a constant low concentration of [gamma-18O]ATP by concomitant hydrolysis of increasing concentrations of ITP was accompanied by a decrease in water oxygen incorporation with each Pi formed from the ATP. These results add to evidence for the binding change mechanism for F1 ATPase with sequential participation of catalytic sites.

[Several peculiarities of the purine-base fixation of the substrate in the active sites of myosin Ca2+-ATPase]

A comparative study of kinetic parameters (Km and V) of hydrolysis by heavy meromyosin of several synthetic ATP analogs with substituents at positions N(1) and N(C6) of the purine ring was carried out. Analysis of changes in the Km values suggests that the purine base of ATP is fixed in the active center due to the formation of a hydrogen bond between N1 and the proton donor group of the protein as well as between the 6-NH2-amino group of the nucleotide and the proton acceptor group of the protein. It was shown that the rate of catalytic conversion of the substrate is determined by the mode of binding of its purine ring. Depending on the properties of the substituent radical, the latter either prevents the binding by causing little or no increase in the rate of hydrolysis or causes the displacement of the whole substrate molecule in the active center, which leads to the deceleration of hydrolysis.

Protective role of adenine nucleotide translocase in O2-deficient hearts

At subsaturating concentrations of palmitoyl-CoA, the carnitine-dependent oxidation of the palmitoyl portion by uncoupled rat heart mitochondria was stimulated by ADP or ATP. This effect was traced to the prevention of acyl-CoA binding to adenine nucleotide translocase and the consequent sparing of acyl-CoA for acylcarnitine formation. Palmitoyl-CoA oxidation was stimulated by ITP also although ITP served neither as a transportable substrate nor as an inhibitor of ADP transport. ITP and other nontransportable nucleoside di(tri)phosphates prevented octanoyl-CoA binding to mitochondria. ITP was bound to mitochondria, and this binding was reversed by ADP, octanoyl-CoA, and carboxyatractyloside. Thus, besides a substrate site, there is a site on the translocase that binds nucleoside di(tri)phosphates, CoA and its esters, and atractylosides; inhibition of the translocase results, however, only from the binding of CoA esters of fatty acids and of atractylosides. We suggest that in O2-deficient hearts, when nucleotides decline and fatty acyl-CoA rises, the binding of the latter to the translocase becomes operational to slow fatty acylcarnitine production. By retarding the rise in amphipathic burden, this mechanism could protect heart against irreversible damage during brief periods of ischemia or hypoxia.

Effects of deuterium on the kinetics of beef heart mitochondrial ATPase

A study was done examining the steady-state kinetics of F1-catalyzed ATP and ITP hydrolyses in the presence or absence of D2O as a function of temperature. The steady-state kinetic parameters kcat and kcat/Km were obtained. For ATP hydrolysis, kcat/Km was independent of temperature in the presence or absence of D2O, while kcat/Km for ITP hydrolysis increased in both cases. The relative magnitudes of change of kcat and kcat/Km in the presence and absence of D2O over the temperature range studied were much different for the cases of ATP and ITP hydrolysis. A normal isotope effect was observed in plots of kcat H2O/kcat D2O versus temperature for ATP hydrolysis, which increased then leveled off as temperature increased. An inverse isotope effect at low temperatures changed to a normal isotope effect and increased dramatically as temperature increased during ITP hydrolysis. The results are discussed in terms of the nature and location of the rate-limiting steps in the reaction mechanisms.

In vitro synthesis of genome length complementary RNA of vesicular stomatitis virus in the presence of inosine 5'-triphosphate

The virion-associated RNA polymerase of vesicular stomatitis virus (VSV) synthesizes RNA in vitro when GTP is replaced by inosine 5'-triphosphate (ITP). The synthesis is optimal at an ITP concentration of 200 microM and the extent of synthesis is between 15 to 20% compared to normal transcription in the presence of GTP. Analyses of the RNA products revealed that approximately 10% of the product RNA represented plus-strand complement of the genome RNA. Defective interfering particles of VSV were also capable of synthesizing complementary RNA in the presence of ITP, in addition to 46-base RNA, although in lesser amount (2%). Since I substitution facilitates read-through of the genome RNA, these results suggest that interaction of the product RNA with the genome template or ITP-mediated modification of a viral protein may be involved in the read-through process in vitro.

ATPase of bovine heart mitochondria. Modulation of ITPase activity by ATP, ADP, acetyl ATP and acetyl AMP

(1) Mitochondrial ATPase (F1) is influenced by specific nucleotides in its kinetic behavior towards its substrates. In this work, initial hydrolysis rates, as well as continuous reaction progress, were measured by recording proton production (equivalent to triphosphate hydrolysis). (2) After preincubation with ATP, F1 hydrolyzes MgITP partly as if it were MgATP, with respect to temperature dependence and 2,4-dinitrophenol inhibition/stimulation. (3) Acetyl ATP is a competitive inhibitor versus ATP on the F1-ATPase. With F1 which has been freed of ambient ATP by repeated precipitations with ammonium sulfate the Ki of acetyl ATP is 400 nM. (4) F1-ATPase which was depleted of bound nucleotides in the presence of glycerol (Garret, N.E. and Penefsky, H.S. (1975) J. Biol. Chem. 250, 6640-6647) was preincubated with ADP and acetyl ATP. These preparations were assayed for hydrolytic activity with MgITP as substrate. Compared to a nonpreincubated control enzyme, the hydrolysis with these preparations was first stimulated, then inhibited. This stimulation/inhibition effect is most pronounced at 10 degrees C, but is also observed at 20 degrees C. (5) When nucleotide-depleted enzyme is preincubated with acetyl AMP, its ability to hydrolyze MgITP slowly decreases to approx. 50% after 60 min. This effect is reversed by further preincubation with acetyl ATP. It is speculated that under appropriate conditions AMP may exist or arise in a buried position on F1-ATPase, and act there as an inhibitor of MgITP hydrolysis.

Hydrolysis of ITP generates a membrane potential in submitochondrial particles

ITP hydrolysis catalysed by the ATPase of submitochondrial particles from both bovine heart and rat liver is shown to be linked to the generation of a membrane potential, and therefore also to proton translocation. The magnitude of the membrane potential is similar to that observed during ATP hydrolysis at equivalent concentrations of phosphate and nucleoside tri- and diphosphates. An explanation is suggested for why in other reports ITP was found to be a poor substrate for supporting energy-linked reactions that are driven by the membrane potential.

Metabolism of dITP in HeLa cell extracts, incorporation into DNA by isolated nuclei and release of hypoxanthine from DNA by a hypoxanthine-DNA glycosylase activity

dITP may be generated from dATP by a slow, nonenzymatic hydrolysis. While [3H]dITP was degraded rapidly to [3H]deoxyinosine by HeLa cell nuclear extracts, no net degradation of [3H]dITP was observed in the presence of physiological concentrations of ATP, apparently because the extract contained deoxynucleoside diphosphate kinase activity that regenerated [3H]dITP from [3H]dIDP. Isolated HeLa cell nuclei, as well as partially purified DNA polymerase alpha, incorporated [3H]dITP into DNA at 50-60% of the rate of [3H]dGTP incorporation. No rapid release of the incorporated radioactivity was observed. The molecular weight of nascent DNA containing dIMP residues, however, decreased slightly after prolonged incubation in the presence of EDTA, suggesting that a repair process is initiated in dIMP-containing chromatin. Furthermore, release of free [3H]hypoxanthine from [3H]dIMP-containing DNA was detected after incubation with nuclear extracts in the presence of EDTA, suggesting the presence of hypoxanthine-DNA glycosylase activity in HeLa cell nuclei.

Abortive termination of bioBFCD RNA synthesized in vitro from the bioABFCD operon of Escherichia coli K-12

The nature of divergent transcription from the bioABFCD gene cluster of Escherichia coli K-12 has been studied in vitro by using DNA restriction fragments as templates. The in vitro RNA transcript initiated at the promoter pa was found to be similar to the in vivo bioA transcript, whereas the transcription initiated at pB produced a small piece of RNA less than 200 nucleotides long. Substituting ITP for GTP or using an S100 cell-free extract as a source of antiterminator permitted transcription of the bioBFCD gene cluster initiating at the promoter pB to be read through. The possible site for in vitro transcription termination in the bioB region is discussed.