Kinetic mechanism of mitochondrial adenosine triphosphatase. ADP-specific inhibition as revealed by the steady-state kinetics

1. A substantial increase of the initial rate of ATP hydrolysis was observed after preincubation of bovine heart submitochondrial particles with phosphoenolpyruvate and pyruvate kinase. 2. The activation was accompanied by an increase of Vmax, without change of Km for ATP. 3. The activated particles catalysed the biphasic hydrolysis of ATP in the presence of an ATP-regenerating system; the initial rapid phase was followed by a second, slower, phase in a time-dependent fashion. 4. The higher the ATP concentration used as a substrate, the higher is the rate of transition between these two phases. 5. The particles catalysed the hydrolysis of ITP with a lag phase; after preincubation with phosphoenolpyruvate and pyruvate kinase, ITP was hydrolysed at a constant rate. 6. Qualitatively the same phenomena were observed when soluble mitochondrial ATPase (F1-ATPase) prepared by the conventional method in the presence of ATP was used as nucleotide triphosphatase. 7. A kinetic scheme is proposed, in which the intermediate active enzyme-product complex (E.ADP) formed during ATP hydrolysis is in slow equilibrium with the inactive E*.ADP complex forming as a result of dislocation of ADP from the active site of ATPase to the other site, which is not in rapid equilibrium with the surrounding medium.

Calcium ion-insensitive contraction of glycerinated porcine cardiac muscle fibers by Mg-inosine triphosphate. ITP as a tool to dissociate the contraction mechanism from the regulatory mechanism

The contraction of cardiac muscle that has been treated with glycerol requires Ca2+ (pCa 8-5), when MgATP is used as a substrate. In contrast, this preparation contracts, even in the absence of Ca2+ (pCa 8-10), when ATP is replaced by ITP. Ca2+ dependency was not observed after increasing free Ca2+ concentrations from pCa 8.0 to 5.0, or after increasing MgITP concentration from 5 to 80 mM. On the other hand, rabbit skeletal muscle fiber treated by the same method as cardiac muscle demonstrates Ca2+ dependency in the presence of both MgITP and MgATP, although this Ca2+ regulation is less in the presence of MgITP. Loss of Ca2+ dependency was confirmed by the finding that, in contrast to ATPase, the ITPase activity of cardiac myofibrils was not dependent on Ca2+ concentrations. Furthermore, the very fast tension responses (quick phases) following quick stretch and quick release were missing in MgITP, and the contractions were similar to rigor. These were not rigor however, because phosphate liberation from ITP continued, and muscle shortening occurred in MgITP. These findings suggest that MgITP dissociates the contraction mechanism from the regulatory mechanism, modulating the regulatory properties of cardiac muscle fiber.

Activity of inosine triphosphate pyrophosphohydrolase in fresh and stored human erythrocytes

The accumulation of inosine triphosphate (ITP) in human erythrocytes incubated with inosine depends on the activity of inosine triphosphate pyrophosphohydrolase (ITPH). High activity of this enzyme is accompanied by a low concentration of ITP and conversely. We showed that ITPH activity decreases with the prolongation of blood preservation time. As a consequence there is a lower accumulation of ITP in fresh erythrocytes incubated in a medium containing high concentrations of inosine, pyruvate and phosphate (IPP) than in red blood cells preserved at 4 degrees C. Synthesis of ITP in erythrocytes incubated in IPP medium being so intensive, it seems possible that during incubation an intermediate accumulates which decreases ITPH activity.

Initiation of reovirus transcription by inosine 5'-triphosphate and properties of 7-methylinosine-capped, inosine-substituted messenger ribonucleic acids

Inosine 5'-triphosphate (ITP) can be utilized in place of guanosine 5'-triphosphate (GTP) for both the initiation and the elongation steps of reovirus transcription, resulting in the synthesis of mRNAs containing 5'-terminal m7IpppIm and internal pI. The apparent molecular weights of the I-substituted products were altered as a consequence of the absence of G-C base pairs and accompanying loss of ordered structure. The migration of I-substituted RNAs in agarose gels and glycerol gradients was similar to glyoxal-treated transcripts; i.e., it decreased 2-fold as compared to the corresponding untreated G-containing mRNAs. 7-Methylinosine-capped (m7I-capped), I-substituted transcripts readily attached to wheat germ 80S ribosomes. Unlike native G-containing mRNAs, they also formed heavier complexes that sedimented faster than 80S complexes even in the presence of the nonhydrolyzable ATP analogue AMPP(NH)P and elongation inhibitor sparsomycin. I-substituted molecules that were capped posttranscriptionally to form m7G-capped 5' ends yielded mostly 80S monosomes, consistent with a strong influence of 5'-terminal structure on initiation of translation. Under limited conditions of initiation, I-substituted RNAs outcompeted G-containing transcripts for ribosome attachment. Although the results are consistent with enhanced binding and freer movement of ribosomes on unstructured templates, synthesis of acid-precipitable polypeptides in wheat germ extract directed by I-substituted RNAs was 15-fold less than with G-containing mRNAs.

Classification of nucleotide binding sites on mitochondrial F1-ATPase from yeast

Methods are described to classify nucleotide binding sites of the mitochondrial coupling factor F1 from yeast on the basis of their affinities and stability properties. High affinity sites or states for ATP and related adenine analogs and low affinity sites or states which bind a broad range of different nucleotide triphosphates are found. The results are discussed in terms of a two site, two cycle scheme, where binding of nucleotide at one site facilitates the release of nucleotide at a second site.

Further kinetic characterization of the non-allosteric phosphofructokinase from Escherichia coli K-12

The labile non-allosteric form of phosphofructokinase (ATP:D-fructose-6-phosphate 1-phosphotransferase, EC 2.7.1.11) was purified to a specific activity of 107 U/mg (2078-fold) from aerobic cultures of Escherichia coli K-12. The enzyme has an isoelectric point (pI) of 5.1, a native molecular weight of 67 000 +/- 3000 and a subunit weight of 34 000 +/- 400. A number of divalent metal ions can substitute for Mg2+ in the enzyme reaction in decreasing order Mn2+ > Mg2+ > Co2+ > Ca2+. In the presence of excess Mg2+, nucleotides do not affect the Km for fructose 6-phosphate with a value of 0.042 mM. The order of efficiency for nucleotides to act as phosphoryl donors is ATP > ITP > GTP > UTP > CTP. This remains unchanged in the presence of excess Mn2+, but V is increased 2.4-fold with ATP. A 2 : 1 ratio of Mn2+/nucleotide 5'-triphosphate produced an equivalent dissociation constant of 1.1 mM for all nucleotides, which was markedly decreased at a high Mn2+ level. The rate of enzyme catalysis was found to be dependent on the concentration of MnATP2-. Mn2+ at non-limiting values does affect the binding of fructose 6-phosphate to the enzyme.

Glucagon and N6,O2'-dibutyryl adenosine 3':5'-monophosphate inhibition of lipogenesis and phosphofructokinase activity of hepatocytes from meal-fed rats

Glucagon and N6,O2'-dibutyryl adenosine 3':5'-monophosphate (dibutyryl cyclic AMP) inhibit net glucose utilization, lactate plus pyruvate accumulation and fatty acid synthesis by isolated hepatocytes prepared from meal-fed rats. A crossover in the metabolite profile of the glycolytic intermediates occurs between fructose-6-phosphate and fructose-1,6-bisphosphate, suggesting either inhibition of phosphofructokinase or activation of fructose diphosphatase, or both. Direct assay of the enzymes in cell-free extracts of the hepatocytes indicates that dibutyryl cyclic AMP inhibits phosphofructokinase but has no effect upon fructose diphosphatase. The assay for phosphofructokinase was modified by the use of ITP in place of ATP for the phosphate donor as the ATP-linked assay is complicated by an apparent time-dependent activation of the enzyme. These findings strongly suggest that cyclic AMP inhibition of phosphofructokinase explains in part cyclic AMP inhibition of aerobic glycolysis and lipogenesis by rat liver hepatocytes.

Individual variation of nucleoside triphosphate pyrophosphohydrolase activity in human erythrocytes, granulocytes, lymphocytes, and platelets

Analysis of the nucleoside triphosphate pyrophosphohydrolase specific activity of red cells obtained from a random Caucasian population indicated at least two subclasses. The specific activity of 18% of the population ranged from undetectable activity to 27.5 nmol ITP cleaved/20 min/mg hemoglobin. The remainder of the population had higher activity, 27.5-125 nmoles ITP cleaved/20 min/mg hemoglobin. The variation of NTPH activity evident in the red cells of an individual is reflected in granulocytes, lymphocytes, and platelets of that individual. Erythrocyte activity ranges from 0.7 to 21 units (nmol of ITP cleaved in 20 min)/10(7) cells, granulocytes have 17-201 units/10(7) cells, lymphocytes have 91-462 units/10(7) cells, and platelets have 1.1-7.1 units/10(7) platelets. These cell differences are discussed with respect to the hypothesis that NTPH prevents incorporation of ITP or dITP into nucleic acids.

Partial characterization of a phosphoryl group transferring enzyme in the membrane of catecholamine storage vesicles

Phosphoryl group transfer from ATP to ADP occurred in the isolated membrane of catecholamine storage vesicles. The reaction was accelerated by extraction of the membranes with 50% (v/v) acetone and by treatment with 1% (v/v) Triton X-100. The phosphoryl group transfer reaction was activated by Mg2+ and by Mn2+. The activation profile differed from that obtained for the ATPase activity. The Michaelis-Menten kinetics of the phosphoryl transfer reaction were not entirely linear. From the linear parts of the double reciprocal plots KmATP approximately equal to 1 mM and KmADP approximately equal to 0.4 mM was obtained. All lines of the double reciprocal plots intersected indicating a sequential reaction mechanism. The reaction exhibited a narrow specificity for nucleoside diphospate and a broader one for nucleoside triphosphate indicating that ADP was the true substrate. The transfer reaction was slightly inhibited by AMP, orthophosphate and P1, P5-di(adenosine-5')pentaphosphate. The thiol reagents, N-ethylmaleimide and para-chloromercuribenzoate (PCMB), affected the ATPase activity and the phosphoryl transfer activity differently: with the blockade of 2.4 essential thiol equivalents by N-ethylmaleimide the ATPase was inhibited 50% and net uptake of catecholamine ceased, while the phosphoryl transfer remained unimpaired. PCMB affected both, the ATPase activity and phosphoryl transfer reaction. Treatment of the membranes with dithioerythritol prevented the PCMB-induced inhibition of the phosphoryl transfer, but was ineffective in protecting the ATPase activity, indicating that different thiol groups must be involved in the both enzymatic activities.

Induction by nucleotide triphosphate hydrolysis of a form of sarcoplasmic reticulum ATPase capable of medium phosphate-oxygen exchange in presence of calcium

Sarcoplasmic reticulum vesicles rendered leaky by exposure to alkaline pH, like intact vesicles, catalyze a rapid Mg2+-dependent exchange of oxygens of medium Pi with water. The exchange with 10 mM Pi is strongly inhibited by 0.15 mM Ca2+. Upon addition and hydrolysis of ITP or ATP, a rapid phosphate-oxygen exchange is observed even with 0.15 mM Ca2+ present and a definite but smaller exchange at 8 mM Ca2+. Oxygen exchange per Pi formed is greater with ITP than with ATP. When no Pi is initially present, the extent of oxygen exchange is increased with time of incubation as Pi is formed. With 18O-labeled Pi present, ATP hydrolysis accelerates 18O loss. The results show that much of the oxygen exchange occurs as a result of reversible binding of medium Pi. Thus the binding and cleavage of ITP or ATP overcomes the Ca2+ inhibition of the medium Pi in equilibrium HOH exchange. Such findings support the concept that the cleavage cycle includes a transient conformational form which can reversibly react with Pi to give a phosphoryl enzyme and resultant oxygen exchange or in a rate-limiting step decay to a form with high Ca2+ and NTP affinity.

Metabolism of the frog outer segments: a kinetic study

1. The reaction of the high-energy phosphate esters of the frog isolated rods, with the firefly lantern extract, has been studied by recording the luminescence in a stopped-flow apparatus. The onset of the reaction was determined by the rapid mixing of the firefly lantern extract with the high-energy phosphates released from the rods fragmented during the mixing.2. The time course of the reaction, i.e. of the luminescence, was not typical of adenosine-5'-triphosphate (ATP), indicating that the major part of the rod nucleotides is not ATP.3. The isolated rods were fragmented 1 sec-2 min after a flash of light. As soon as 1 sec after illumination, a substantial decrease of the luminescent yield has been detected in a range of light bleaching from 0.007 to 20% of the rhodopsin, indicating an early reduction of the high-energy phosphate esters.4. At longer times, flashes that bleached 7 or 20% of the rhodopsin induced a progressive decrease of the luminescent yield that was half completed in 6-9 sec, and was nearly complete in 20 sec, whereas a more or less complete recovery was observed after flashes bleaching from 0.7 to 0.007% of the rhodopsin, indicating the presence of buffering mechanisms.5. Also, the time course of the reaction was modified in a complex way after stimulation of the rods, suggesting that other nucleotides beside ATP are hydrolysed.6. The light-induced reduction of the high-energy phosphates has been observed also in broken rods, in the presence of the calcium chelating agent EGTA, suggesting that calcium is not needed for this effect.7. Cyclic guanosine monophosphate (10(-4)M) interfered with the photic effect.8. The rapid effect of light on the content of high-energy phosphate esters suggests a physiological role in the mechanism of excitation.