Location and properties of two isoenzymes of cardiac adenylate kinase

Proteomic Identification of Target Proteins of Thiodigalactoside in White Adipose Tissue from Diet-Induced Obese Rats

Previously, galectin-1 (GAL1) was found to be up-regulated in obesity-prone subjects, suggesting that use of a GAL1 inhibitor could be a novel therapeutic approach for treatment of obesity. We evaluated thiodigalactoside (TDG) as a potent inhibitor of GAL1 and identified target proteins of TDG by performing comparative proteome analysis of white adipose tissue (WAT) from control and TDG-treated rats fed a high fat diet (HFD) using two dimensional gel electrophoresis (2-DE) combined with MALDI-TOF-MS. Thirty-two spots from a total of 356 matched spots showed differential expression between control and TDG-treated rats, as identified by peptide mass fingerprinting. These proteins were categorized into groups such as carbohydrate metabolism, tricarboxylic acid (TCA) cycle, signal transduction, cytoskeletal, and mitochondrial proteins based on functional analysis using Protein Annotation Through Evolutionary Relationship (PANTHER) and Database for Annotation, Visualization, Integrated Discovery (DAVID) classification. One of the most striking findings of this study was significant changes in Carbonic anhydrase 3 (CA3), Voltage-dependent anion channel 1 (VDAC1), phosphatidylethanolamine-binding protein 1 (PEBP1), annexin A2 (ANXA2) and lactate dehydrogenase A chain (LDHA) protein levels between WAT from control and TDG-treated groups. In addition, we confirmed increased expression of thermogenic proteins as well as reduced expression of lipogenic proteins in response to TDG treatment. These results suggest that TDG may effectively prevent obesity, and TDG-responsive proteins can be used as novel target proteins for obesity treatment.

AMP promotes oxygen consumption and ATP synthesis in heart mitochondria through the adenylate kinase reaction: an NMR spectroscopy and polarography study

Adenylate kinase plays an important role in cellular energy homeostasis by catalysing the interconversion of adenine nucleotides. The goal of present study was to evaluate the contribution of the adenylate kinase reaction to oxidative ATP synthesis by direct measurements of ATP using (31) P NMR spectroscopy. Results show that AMP can stimulate ATP synthesis in the presence or absence of ADP. In particular, addition of 1 mM AMP to the 0.6 mM ADP superfusion system of isolated superfused mitochondria (contained and maintained in agarose beads) led to a 25% increase in ATP synthesis as measured by the increase in βATP signal. More importantly, we show that AMP can support ATP synthesis in the absence of ADP, demonstrated as follows. Superfusion of mitochondria without ADP led to the disappearance of ATP γ, α and β signals and the increase of Pi . Addition of AMP to the medium restored the production of ATP, as demonstrated by the reappearance of γ, α and β ATP signals, in conjunction with a decrease in Pi , which is being used for ATP synthesis. Polarographic studies showed Mg(2+) dependence of this process, confirming the specificity of the adenylate kinase reaction. Furthermore, data obtained from this study demonstrate, for the first time, that different aspects of the adenylate kinase reaction can be evaluated with (31) P NMR spectroscopy. SIGNIFICANCE OF RESEARCH PARAGRAPH: The data generated in the present study indicate that (31) P NMR spectroscopy can effectively be used to study the adenylate kinase reaction under a variety of conditions. This is important because understanding of adenylate kinase function and/or malfunction is essential to understanding its role in health and disease. The data obtained with (31) P NMR were confirmed by polarographic studies, which further strengthens the robustness of the NMR findings. In summary, (31) P NMR spectroscopy provides a sensitive tool to study adenylate kinase activity in different physiological and pathophysiological conditions, including but not exclusive of, cancer, ischemic injury, hemolytic anemia and neurological problems such as sensorineural deafness.

Molecular and functional characterization of a Trypanosoma cruzi nuclear adenylate kinase isoform

Trypanosoma cruzi, the etiological agent of Chagas' disease, is an early divergent eukaryote in which control of gene expression relies mainly in post-transcriptional mechanisms. Transcription levels are globally up and down regulated during the transition between proliferating and non-proliferating life-cycle stages. In this work we characterized a nuclear adenylate kinase isoform (TcADKn) that is involved in ribosome biogenesis. Nuclear adenylate kinases have been recently described in a few organisms, being all related to RNA metabolism. Depending on active transcription and translation, TcADKn localizes in the nucleolus or the cytoplasm. A non-canonical nuclear localization signal was mapped towards the N-terminal of the protein, being the phosphate-binding loop essential for its localization. In addition, TcADKn nuclear exportation depends on the nuclear exportation adapter CRM1. TcADKn nuclear shuttling is governed by nutrient availability, oxidative stress and by the equivalent in T. cruzi of the mammalian TOR (Target of Rapamycin) pathway. One of the biological functions of TcADKn is ribosomal 18S RNA processing by direct interaction with ribosomal protein TcRps14. Finally, TcADKn expression is regulated by its 3′ UTR mRNA. Depending on extracellular conditions, cells modulate protein translation rates regulating ribosome biogenesis and nuclear adenylate kinases are probably key components in these processes.

Infection with Trypanosoma cruzi produces a condition known as Chagas disease which affects at least 17 million people. Adenylate kinases, so called myokinases, are involved in a wide variety of processes, mainly related to their role in nucleotide interconversion and energy management. Recently, nuclear isoforms have been described in several organisms. This “atypical” isoform in terms of primary structure was associated to ribosomes biogenesis in yeast and to Cajal body organization in humans. Moreover nuclear adenylate kinases are essential for maintaining cellular homeostasis. In this manuscript we characterized T. cruzi nuclear adenylate kinase (TcADKn). TcADKn localizes in the nucleolus or cell cytoplasm. Nuclear shuttling mechanisms were also studied for the first time, being dependent on nutrient availability, oxidative stress and by the equivalent of the mammalian TOR pathway in T. cruzi. Furthermore we characterized the signals involved in nuclear importation and exportation processes. In addition, TcADKn expression levels are regulated at an mRNA level, being its 3′UTR involved in this process. These findings are the first steps in the understanding of ribosome processing in trypanosomatids.

Adenylate kinase and AMP signaling networks: metabolic monitoring, signal communication and body energy sensing

Adenylate kinase and downstream AMP signaling is an integrated metabolic monitoring system which reads the cellular energy state in order to tune and report signals to metabolic sensors. A network of adenylate kinase isoforms (AK1-AK7) are distributed throughout intracellular compartments, interstitial space and body fluids to regulate energetic and metabolic signaling circuits, securing efficient cell energy economy, signal communication and stress response. The dynamics of adenylate kinase-catalyzed phosphotransfer regulates multiple intracellular and extracellular energy-dependent and nucleotide signaling processes, including excitation-contraction coupling, hormone secretion, cell and ciliary motility, nuclear transport, energetics of cell cycle, DNA synthesis and repair, and developmental programming. Metabolomic analyses indicate that cellular, interstitial and blood AMP levels are potential metabolic signals associated with vital functions including body energy sensing, sleep, hibernation and food intake. Either low or excess AMP signaling has been linked to human disease such as diabetes, obesity and hypertrophic cardiomyopathy. Recent studies indicate that derangements in adenylate kinase-mediated energetic signaling due to mutations in AK1, AK2 or AK7 isoforms are associated with hemolytic anemia, reticular dysgenesis and ciliary dyskinesia. Moreover, hormonal, food and antidiabetic drug actions are frequently coupled to alterations of cellular AMP levels and associated signaling. Thus, by monitoring energy state and generating and distributing AMP metabolic signals adenylate kinase represents a unique hub within the cellular homeostatic network.

Acyclic phosphonate nucleotides and human adenylate kinases: impact of a borano group on alpha-P position

Adenylate kinases are involved in the activation of antiviral drugs such as the acyclic phosphonates analogs PMEA and (R)PMPA. We examine the in vitro phosphorylation of PMEA and PMPA bearing a borano- or a H- group on the phosphorus atom. The alpha-borano or alpha-H on PMEA and PMPA were detrimental to the activity of recombinant human AMP kinases 1 and 2. Docking PMEA to the active site of AMP kinase 1 indicated that the borano group may prevent two conserved critical Arg interactions with the alpha-phosphate, resulting in substrate bad positioning.

Modeling of spatial metabolite distributions in the cardiac sarcomere

Although a high ATP diffusion rate implies homogeneous distribution of the principal energetic currency in the cytosol, local diffusion barriers represented by macromolecular structures can render ATP concentrations to be inhomogeneous. A method is presented here that provides apparent diffusion coefficient values in local intracellular regions and allows the estimation of spatial metabolite distribution. The apparent local diffusion coefficient for ATP in cardiac myofibrils was determined from the analysis of diffusion-dependent rightward shift of the substrate dependence for actomyosin ATPase activity using the reaction-diffusion model, which accounted for the properties of phosphotransfer reactions. This functional analysis, which took into account the local diffusional ATP delivery to the active sites, provided an apparent value that was three orders of magnitude lower than that defined by direct methods for the cytosol. The low value of the diffusion coefficient was shown to define unusual properties of the intracellular space in working heart, where small reductions in ATP levels in the surrounding cytosol result in a large drop in [ATP] inside myofibrils. This drop is critical for vital cellular functions, and the analysis presented here defines its physical basis. The diffusion barriers thus defined explain the coexistence of pathological energy deficit with almost normal average ATP levels.

Ca2+-activated myosin-ATPases, creatine and adenylate kinases regulate mitochondrial function according to myofibre type in rabbit

Mitochondrial respiration rates and their regulation by ADP, AMP and creatine, were studied at different free Ca(2+) concentrations (0.1 versus 0.4 microm) on permeabilized fibre bundles of rabbit skeletal muscles differing in their myosin heavy chain profiles. Four fibre bundle types were obtained: pure types I and IIx, and mixed types IIax (approximately 50% IIa and 50% IIx fibres) and IIb+ (60% IIb fibres, plus IIx and IIa). At rest, pure type I fibres displayed a much higher apparent K(m) for ADP (212 microm) than IIx fibres (8 microm). Within the IIax and IIb+ mixed fibre bundle types, two K(ADP)(m) values were observed (70 microm and 5 microm). Comparison between pure IIx and mixed types indicates that the intermediate K(m) of 70 microm most probably corresponds to the mitochondrial affinity for ADP in IIa fibres, the lowest K(m) for ADP (5 microm) corresponding to IIx and IIb types. Activation of mitochondrial creatine and adenylate kinase reactions stimulated mitochondrial respiration only in type I and IIax fibre bundles, indicating an efficient coupling between both kinases and ADP rephosphorylation in type I and, likely, IIa fibres, since no effect was observed in pure IIx fibres. Following Ca(2+)-induced activation of myosin-ATPase, an increase in mitochondrial sensitivity to ADP of 45% and 250% was observed in type IIax and I bundles, respectively, an effect mostly prevented by addition of vanadate, an inhibitor of myosin-ATPase. Ca(2+)-induced activation of myosin-ATPase also prevented the stimulation of respiration rates by creatine and AMP in I and IIax bundles. In addition to differential regulation of mitochondrial respiration and energy transfer systems at rest in I and IIa versus IIx and IIb muscle fibres, our results indicate a regulation of phosphotransfer systems by Ca(2+) via the stimulation of myosin-ATPases in type I and IIa fibres of rabbit muscles.

High K(m) of oxidative phosphorylation for ADP in skinned muscle fibers: where does it stem from?

Mitochondria in saponin-skinned cardiac fiber bundles were reported to have an order of magnitude lower apparent affinity to ADP than isolated mitochondria. Although ADP was measured outside the bundles, it was thought that the low affinity was not caused by diffusion gradients because of relatively short diffusion distances. Here we test the hypothesis that considerable ADP diffusion gradients exist and can be diminished by increasing the intrafiber ADP production rate. We increased the ADP-producing activity in rat heart skinned fiber bundles by incubating with 100 IU/ml yeast hexokinase and glucose. Consequently, we observed a significant decrease of the apparent Michaelis constant (K(m)) to ADP of the respiration rate of bundles from 216 +/- 59 to 50 +/- 9 microM. Fitting the results with a mathematical model, we estimated the K(m) of mitochondria in the bundles to be 25 microM. We conclude that the affinity to ADP of in situ mitochondria in heart is of the same order of magnitude as that of isolated mitochondria.

Human deoxycytidine kinase is located in the cell nucleus

Human deoxyribonucleoside kinases are required for the pharmacological activity of several clinically important anticancer and antiviral nucleoside analogs. Human deoxycytidine kinase and thymidine kinase 1 are described as cytosolic enzymes in the literature, whereas human deoxyguanosine kinase and thymidine kinase 2 are believed to be located in the mitochondria. We expressed the four human deoxyribonucleoside kinases as fusion proteins with the green fluorescent protein to study their intracellular locations in vivo. Our data showed that the human deoxycytidine kinase is located in the cell nucleus and the human deoxyguanosine kinase is located in the mitochondria. The fusion proteins between green fluorescent protein and thymidine kinases 1 and 2 were both predominantly located in the cytosol. Site-directed mutagenesis of a putative nuclear targeting signal, identified in the primary structure of deoxycytidine kinase, completely abolished nuclear import of the protein. Reconstitution of a deoxycytidine kinase-deficient cell line with the wild-type nuclear or the mutant cytosolic enzymes both restored sensitivity toward anticancer nucleoside analogs. This paper reports that a deoxyribonucleoside kinase is located in the cell nucleus and we discuss the implications for deoxyribonucleotide synthesis and phosphorylation of nucleoside analogs.

Reversal of the ATP-liganded state of ATP-sensitive K+ channels by adenylate kinase activity

The mechanism that promotes transition from the ATP- to the ADP-liganded state of ATP-sensitive K+ (KATP) channels and consequent channel opening in a cytosolic environment of high ATP concentration has yet to be understood. A mechanism examined here that could reverse the ATP-inhibited state is based on the action of adenylate kinase to catalyze phosphoryl transfer between ATP and AMP, resulting in transformation of ATP into ADP. In membrane patches excised from guinea pig cardiomyocytes, AMP alone did not affect channel behavior but increased the open probability of ATP-inhibited KATP channels. This required MgCl2 and a hydrolyzable form of ATP and was prevented by P1,P5-di-adenosine-5'-pentaphosphate, an inhibitor of adenylate kinase. The single channel amplitude and kinetics of channel openings induced by the ADP-generating substrates of adenylate kinase, AMP and MgATP, were indistinguishable from the biophysical properties of the KATP channel exhibited after addition of MgADP. In whole cell voltage-clamped cardiomyocytes, introduction of exogenous adenylate kinase along with millimolar MgATP and AMP induced a K+ current that was suppressed by a sulfonylurea blocker of KATP channels. Enriched sarcolemmal membrane preparations were found to possess ATP.AMP phosphotransferase activity with properties attributable to an extramitochondrial isoform of adenylate kinase. These results indicate that adenylate kinase is a naturally occurring component of sarcolemmal membranes that could provide dynamic governance of KATP channel opening through its phosphoryl transfer catalytic action in the microenvironment of the channel.

Biochemical properties of rice adenylate kinase and subcellular location in plant cells

Previously, we characterized nucleotide sequences of two cDNAs encoding adenylate kinase from rice plants (Oryza sativa L.). Each cDNA (Adk-a or Adk-b) was cloned into the expression vector pET 11d-GST to produce GST-AK fusion proteins in Escherichia coli. Recombinant proteins were cleaved by thrombin, and GST-free adenylate kinase proteins were obtained. Enzyme activity profiles of different pH and inhibition effects to the enzyme by Ap5A (adenosine-5'-pentaphospho-5'-adenosine) indicates that both adenylate kinase proteins have similar biochemical characteristics. Among the nucleoside monophosphates (AMP, CMP, GMP and UMP) investigated, only AMP reacted with ATP. Furthermore, using the antiserum against the rice adenylate kinase proteins, the cellular location of adenylate kinase proteins was examined by immunomicroscopic analysis in combination with a subcellular fractionation method. The results indicated that adenylate kinase proteins were distributed largely in cytosol of rice cells.

Interaction of creatine kinase and adenylate kinase systems in muscle cells

Elsewhere in this book the important role of creatine kinase and its metabolites in high energy phosphate metabolism and transport in muscle cells has been reviewed. The emphasis of this review article is mainly on the compartmentalized catalytic activity of adenylate kinase in relation to creatine kinase isoenzymes, and other enzymes of energy production and utilization processes in muscle cells. At present the role of adenylate kinase is considered simply to equilibrate the stores of adenine nucleotides. Recent studies by us and others, however, suggest an entirely new view of the metabolic importance of adenylate kinase in muscle function. This view offers a closer interaction between adenylate kinase and creatine kinase, in the process of energy production (at mitochondrial and glycolytic sites), and energy utilization (at myofibrillar sites and perhaps other sites such as sarcoplasmic reticular, sarcolemmal membrane, etc.), thus being an integral part of the high energy phosphate transport system. This review article opens up the opportunity to further examine the metabolism of adenine nucleotides and their fluxes through the adenylate kinase system in intact muscle cells. Using an intact system, having a preserved integrity of their compartmentalized enzymes and substrates, is essential in clarifying the exact role of adenylate kinase in high energy phosphate metabolism in muscle cells.

Nuclear location of phosphoglycerate mutase BB isozyme in rat tissues

We have previously reported (Ureña et al. Eur. J. Cell Biol. 1990) that in skeletal muscle, type MM phosphoglycerate mutase isozyme is present in the nucleus as well as in the cytosol. To determine whether type BB phosphoglycerate mutase isozyme is also present in nucleus, the subcellular location of this isozyme was studied in different rat tissues by cell fractionation and immunogold techniques. With the aid of high affinity-purified anti-phosphoglycerate mutase BB isozyme antibodies, the isozyme was located in the nucleus of neuronal, astroglial and liver cells but not in the nucleus of oligodendroglial and endothelial cells. Biochemical studies on purified nuclear fractions also demonstrated the presence of phosphoglycerate mutase activity in the nucleus. Both immunocytochemical and biochemical techniques showed that nuclear phosphoglycerate mutase-specific activity depended on the type of cell.

Postanoxic recovery of spontaneously beating isolated atria: pH related role of adenylate kinase

The functional activity, adenine nucleotides, and creatine phosphate content of spontaneously beating isolated rabbit atria were measured prior to anoxia, after 1 hr anoxia, and at the end of 1 hr reoxygenation at pH 6.7 and 7.2 During anoxia at pH 7.2 there was 13.3% loss of adenine nucleotides pool, 35.2% loss of ATP, 36.2% increase in ADP, 200% increase in AMP, and a decrease to 8.8% of CP assayed to the beating atria in oxygen. At pH 6.7 there was almost the same decrease in CP, about 10% decrease in ATP, no change in total adenine nucleotides, no change in AMP and a higher increase in ADP (88.7%). The postanoxic recovery was much more complete when the pH was 6.7 during anoxia, and the first 40 min of reoxygenation. The extent of recovery of functional activity correlated well with the level of ATP in all cases not CP. Since the adenylate kinase and ATPase activity both decrease at acidic pH, their combined diminution would tend to preserve the adenine nucleotide pool and thus the better recovery at pH 6.7, because of a decrease in energy demand and unavailability of AMP for the degradation process. This study also supports the notion of compartmented adenine nucleotides connected by the creatine phosphate-creatine energy shuttle.

Purification and characterization of ATP:AMP phosphotransferase from Mycobacterium marinum

ATP:AMP phosphotransferase (EC 2.7.4.3) (adenylate kinase) has been purified 1746-fold from Mycobacterium marinum (ATCC 927) by successive column chromatography on DEAE-cellulose (DE-53), Reactive Blue agarose, Sephadex G-75, hydroxyapatite and, finally, DEAE-Sephadex A-50. The final enzyme preparation had a specific activity of 576 mumol/min per mg protein with an overall yield of 51%. The preparation was homogeneous by sodium dodecyl sulfate polyacrylamide gel electrophoresis. The enzyme was estimated to have an Mr of 29500 and an isoelectric point of 6.7, properties which generally resemble those of the mitochondrial enzyme. Indeed, the two enzymes failed to separate when subjected to polyacrylamide gel electrophoresis under denaturing conditions. The extinction coefficient (at 276 nm) was calculated to be 3.114 X 10(4) M-1 X cm-1 and E1%1cm = 10.556. Adenylate kinase was present at a concentration of 0.06 mg/g (wet weight) bacteria. Enzyme was stable for months in 60% glycerol in the freezer; at 4 degrees C, less than 5% of the activity was lost over a 7 day period.

Mitochondrial adenylate kinase (AK2) from bovine heart. The complete primary structure

The sequence analysis of adenylate kinase isoenzyme 2 (AK2) was completed using a gas-phase sequencer constructed in our laboratory. The enzyme contains 238 amino acid residues in the following order: (sequence; see text) The four cysteine residues of AK2 were reinvestigated. Cys-41 and Cys-233 contain free thiols, which can be carboxymethylated in the intact protein without loss of enzymic activity. Chemical and model-building studies suggest that the pair Cys-43/Cys-93 forms a disulfide in native AK2. The relative molecular mass of AK2, as deduced from the sequence, is 26104. Other methods, including titration of -SH groups, sedimentation equilibrium ultracentrifugation and gel filtration yielded Mr values in the range from 26 000 to 31 500, each value depending on the respective method of determination. Bovine heart AK2 contains 44 residues more than the homologous isoenzyme AK1 (myokinase). As all but one single insertions and deletions cancel, the higher Mr of AK2 is due to 9 residues preceding the N terminus of AK1, a stretch of 30 residues in the middle of the molecule and 6 residues at the end. AK2 and AK1 are similar in their active-site geometry. In contrast, AK2 does not possess any of the three antigenic sites of AK1, which is consistent with the lack of immunological cross-reactivity between AK1 and AK2.

Accumulation and salvage of adenosine and inosine by isolated mature cardiac myocytes

Reoxygenation of ischaemic, energy-depleted heart does not result in sufficiently rapid regeneration of normal adenine nucleotide concentrations for preservation of cardiac function and structure. Salvage of nucleoside as a mechanism for restoration of ATP in the post-ischaemic myocardium is limited by efflux of adenosine during ischaemia. Isolated cardiac myocytes have been used to establish the kinetics of uptake and salvage of adenosine and inosine, measuring the distribution of radioactive nucleoside incorporated into ATP, ADP and AMP. Maximum rates of catalysis of reactions on the salvage pathway, and of enzymes competing for substrates on the pathway, have been established in myocyte extracts. Myocytes have little capacity to salvage or catabolise inosine. Enzyme measurements indicate that salvage of adenosine should proceed at 7-8-times the rate exhibited by intact myocytes dependent upon extracellular adenosine as substrate. The data indicate that the rate of transport of adenosine is not determined by its metabolic utilization, but is the rate-limiting step in the salvage of adenosine.

Metabolism of glucose 1,6-P2--II. Glucose 1,6-P2 phosphatase in pig muscle

Most of the glucose 1,6-P2 phosphatase activity of pig skeletal muscle is present in the cytosolic fraction. Four peaks of glucose 1,6-P2 phosphatase activity are obtained when the cytosolic fraction from pig muscle is subjected to DE-cellulose chromatography. All the peaks hydrolyze other phosphocompounds in addition to glucose 1,6-P2. The glucose 1,6-P2 phosphatase activity of the main peak shows an optimal neutral pH. It is activated by divalent cations, Mg2+ being more effective than Mn2+. The addition of Ca2+ or EGTA does not affect the enzymatic activity. IMP does not possess any effect. It is concluded that this enzyme is different from the glucose 1,6-P2 phosphatases found in mouse brain cytosol and rat skeletal muscle.

Adenine nucleotide metabolism in pigeon liver and heart: diurnal changes and correlations between indices

The adenine nucleotide and Pi content of pigeon liver and heart were determined, and the energy charge, phosphoryl potential and mass action ratio of adenylate kinase were calculated over the 24 hr period. All the indices of adenine nucleotide metabolism were shown to vary a 2- to 4-fold extent, both in liver and heart. The correlation coefficients for each of the adenine nucleotides and each of the calculated indices were computed and shown to be different for liver and heart. The difference between pigeon liver, pigeon heart and rat liver in the diurnal variation of adenine nucleotide metabolism, in the regulatory mechanism, and in metabolism on the whole is discussed.

Properties and subcellular localization of adenosine diphosphatase in rat heart

Some properties and subcellular localization of adenosine diphosphatase (ADPase) activity from rat heart have been investigated. The pH optimum was 7.4, maximal activity was found with 5 mM MgCl2, and the apparent Km was 20 microM. ADPase activity was strongly inhibited by NaF and AppNHp, and to a lesser extent by AMP and GppNHp. The enzyme was not inhibited by p-nitrophenylphosphate, beta-glycerophosphate, or pyridoxal phosphate. The distribution of ADPase activity in subcellular fractions obtained by differential centrifugation parallel ouabain-sensitive (Na+-K+)ATPase and 5'-nucleotidase activities, suggesting a plasma membrane-bound localization. The functional significance of ADPase in adenosine production and hemostasis is discussed.

Mitochondrial adenylate kinase (AK2) from bovine heart. Homology with the cytosolic isoenzyme in the catalytic region

The adenylate kinase isoenzyme located in the intermembrane space of mitochondria, AK2, is a monomeric protein of Mr 30000 which catalyzes the reaction ATP + AMP + AMP in equilibrium 2 ADP. The protein was reduced and S-carboxymethylated with iodo[14C2]acetate. Using a Laursen sequenator, the N-terminal sequence of S-carboxymethylated AK2 was determined as Ala-Pro-Asn-; in some batches of the isolated protein the N-terminal dipeptide portion was missing. The C-terminus of AK2 was found to be Met. Cleavage with CNBr yielded eight fragments which could be isolated in one step using high-performance size-exclusion chromatography. They ranged in size over 4-88 amino acid residues, the total being approximately 270 residues. All CNBr fragments were overlapped with Met-containing tryptic peptides of AK2. The N-terminal 111 residues of AK2 were sequenced. Except for an N-terminal extension of nine residues, this segment of AK2 could be aligned with the sequence 1-104 of cytosolic AK1. Allowing for two deletions in AK2, 43 of the 102 aligned residues are identical. Since this section contains the catalytic residues such as His-36 and Asp-93, we conclude that AK1 can serve as a three-dimensional model of AK2 in mechanistic and drug-designing studies. preliminary sequence results on AK2 beyond position 104 show that AK2 here contains a wing of approximately 50 residues which has no counterpart in AK1. The chain folds of the adenylate kinase isoenzymes are similar again from a position corresponding to residue 115 of AK1 onwards. The additional structural motifs of AK2 are probably related to the location of this isoenzyme in the mitochondrion.

Preliminary X-ray studies on the GTP: AMP phosphotransferase from beef heart mitochondria

Crystals of GTP: AMP phosphotransferase from beef heart mitochondria suitable for X-ray analysis have been grown. They belong to space group I4 with unit cell dimensions: a = b = 88.2 A, c = 147.8 A. The asymmetric unit contains two molecules each of Mr = 26,000. So far, two heavy-atom derivatives have been obtained.