Demonstration of Phosphoryl Group Transfer Indicates That the ATP-binding Cassette (ABC) Transporter Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Exhibits Adenylate Kinase Activity

Cystic fibrosis transmembrane conductance regulator (CFTR) is a membrane-spanning adenosine 5′-triphosphate (ATP)-binding cassette (ABC) transporter. ABC transporters and other nuclear and cytoplasmic ABC proteins have ATPase activity that is coupled to their biological function. Recent studies with CFTR and two nonmembrane-bound ABC proteins, the DNA repair enzyme Rad50 and a structural maintenance of chromosome (SMC) protein, challenge the model that the function of all ABC proteins depends solely on their associated ATPase activity. Patch clamp studies indicated that in the presence of physiologically relevant concentrations of adenosine 5′-monophosphate (AMP), CFTR Cl⁻ channel function is coupled to adenylate kinase activity (ATP+AMP ⇆ 2 ADP). Work with Rad50 and SMC showed that these enzymes catalyze both ATPase and adenylate kinase reactions. However, despite the supportive electrophysiological results with CFTR, there are no biochemical data demonstrating intrinsic adenylate kinase activity of a membrane-bound ABC transporter. We developed a biochemical assay for adenylate kinase activity, in which the radioactive γ-phosphate of a nucleotide triphosphate could transfer to a photoactivatable AMP analog. UV irradiation could then trap the ³²P on the adenylate kinase. With this assay, we discovered phosphoryl group transfer that labeled CFTR, thereby demonstrating its adenylate kinase activity. Our results also suggested that the interaction of nucleotide triphosphate with CFTR at ATP-binding site 2 is required for adenylate kinase activity. These biochemical data complement earlier biophysical studies of CFTR and indicate that the ABC transporter CFTR can function as an adenylate kinase.     

Purification of radioiodinated succinyl cyclic nucleotide tyrosine methyl esters by anion-exchange thin-layer chromatography

Tyrosine methyl esters of 2'-O-succinyl cyclic AMP and 2'-O-succinyl cyclic GMP were radioiodinated, and the products were purified by anion-exchange chromatography on polyethyleneimine-cellulose thin layers. Using 1.0 M LiCl for development, two major, immunologically active fractions (presumably the mono and diiodo products) were separated from unreacted iodide, ScAMP-TME, or ScGMP-TME, and immunologically inactive radiolabeled fractions. When used in a radioimmunoassay, both major fractions from each nucleotide showed essentially identical binding affinities; however, assays were more sensitive with the presumed diiodo product because of its higher content of 125I. Compared with other purification methods, this technique is faster and permits separation of defined, radioactive iodine-containing products from unreacted starting materials.     

Radioisotopic assay of femtomole quantities of total adenine nucleotides,ATP plus ADP,and AMP

AMP is converted to ATP by incubating overnight with pyruvate kinase, phosphoenolpyruvate and adenylate kinase in th prensence of endogenous ATP (ADP) as primer. In a subsequent incubation in the presence of pyruvate kinase, phosphoenolpyruvate, radioactive glucose and hexokinase. ATP and ADP are estimated together by coupling their recycling to the formation of glucose 6-phosphate. The latter is separated by precipitation using 76% (v/v) acetone for radioactivity measurement in the same Eppendorf tube. The sensitivity of these simple procedures matches or exceeds those of luciferase methods of nucleotide determination.     

Phosphorylation of histone catalyzed by a bovine brain protein kinase.

The use of polyethyleneimine-cellulose thin layer sheets to follow the phosphorylation of histone and decomposition of ATP catalyzed by an adenosine 3':5'-monophosphate (cyclic AMP)-stimulated protein kinase, protein kinase I, has made possible a more detailed analysis of the time course of these reactions than has been achieved previously be observing only recovered phosphorylated protein. When [gamma-32P] ATP was employed, significant error was introduced by the presence of 32Pi at the solvent front on these sheets, and this limited the accuracy of the available information. However, the analysis of assays performed with [U-14C] ATP was straightforward and appeared to have an accuracy comparable to that of the present standard assay. This appears to be the first use of [U-14C] ATP to assay protein kinases. Our physical characterization of protein kinase I showed it to be a homogeneous protein species by polyacrylamide gel electrophoresis, sodium dodecyl sulfate gel electrophoresis and analytical ultracentrifugation. Kinetic studies with protein kinase I indicated the absence of histone phosphatase and cyclic AMP phosphodiesterase activity. Furthermore, the ATPase activity seen is believed to be intimately associated with the protein kinase action, particularly in view of the observed dependence of the rate of Pi production on the presence of cyclic AMP. The kinetic data for the phosphorylation of histone catalyzed by protein kinase I under full stimulation by cyclic AMP are consistent with a double displacement mechanism.     

Properties of adenylate kinase after modification of Arg-97 by phenylglyoxal.

Adenylate kinase (ATP:AMP phosphotransferase, EC 2.7.4.3) isolated from porcine skeletal and heart muscle and from rabbit muscle are inactivated when a single arginine residue is modified. In adenylate kinase from pig the modified residue was identified as Arg-97 by peptide-mapping. In native adenylate kinase Arg-97 is located at the bottom of the active site cleft. The protein fluorescence of modified adenylate kinase is reduced. Whereas the addition of AMP, ADP and MgATP quench the fluorescence of native adenylate kinase, the fluorescence of phenylglyoxal-modified adenylate kinase is only affected by ADP and MgATP. This finding is discussed in connection with the structural isomerization observed in native adenylate kinase by X-ray diffraction analysis.     

Photokinetic microassay of adenylate kinase using the firefly luciferase reaction.

A new rapid photokinetic method is described for determining the activity of adenylate kinase (ATP:AMP phosphotranspherase, EC 2.7.4.3) in 0.1--5.0 micrograms of freeze-dried tissue. This represents a weight range far below that obtainable by fine-needle biopsy. The reaction 2 ADP in equilibrium with AMP + ATP was employed and the ATP formed assayed with firefly luciferase as light yielder. The light emission was recorded on a multi-channel scaler. The adenylate kinase activities found in tissues of mice were in the same range as previously described in a study using fluorometric microassay.     

ATP and AMP Mutually Influence Their Interaction with the ATP-binding Cassette (ABC) Adenylate Kinase Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) at Separate Binding Sites

Cystic fibrosis transmembrane conductance regulator (CFTR) is an anion channel in the ATP-binding cassette (ABC) transporter protein family. In the presence of ATP and physiologically relevant concentrations of AMP, CFTR exhibits adenylate kinase activity (ATP + AMP ⇆ 2 ADP). Previous studies suggested that the interaction of nucleotide triphosphate with CFTR at ATP-binding site 2 is required for this activity. Two other ABC proteins, Rad50 and a structural maintenance of chromosome protein, also have adenylate kinase activity. All three ABC adenylate kinases bind and hydrolyze ATP in the absence of other nucleotides. However, little is known about how an ABC adenylate kinase interacts with ATP and AMP when both are present. Based on data from non-ABC adenylate kinases, we hypothesized that ATP and AMP mutually influence their interaction with CFTR at separate binding sites. We further hypothesized that only one of the two CFTR ATP-binding sites is involved in the adenylate kinase reaction. We found that 8-azidoadenosine 5′-triphosphate (8-N₃-ATP) and 8-azidoadenosine 5′-monophosphate (8-N₃-AMP) photolabeled separate sites in CFTR. Labeling of the AMP-binding site with 8-N₃-AMP required the presence of ATP. Conversely, AMP enhanced photolabeling with 8-N₃-ATP at ATP-binding site 2. The adenylate kinase active center probe P¹,P⁵-di(adenosine-5′) pentaphosphate interacted simultaneously with an AMP-binding site and ATP-binding site 2. These results show that ATP and AMP interact with separate binding sites but mutually influence their interaction with the ABC adenylate kinase CFTR. They further indicate that the active center of the adenylate kinase comprises ATP-binding site 2.     

8-Azido-2'-O-dansyl-ATP. A fluorescent photoaffinity reagent for ATP-binding proteins and its application to adenylate kinase

The photoaffinity reagent 8-azido-2'-O-[14C]dansyl-ATP (AD-ATP) has been synthesized for labeling and monitoring the active sites of ATPases and kinases. In its first application, the reagent is used to explore the active site of adenylate kinase from rabbit muscle. In the dark, AD-ATP inhibits adenylate kinase reversibly and competitively with KI = 0.25 +/- 0.01 microM. Under weak UV illumination, AD-ATP labels adenylate kinase irreversibly. The photoinactivation data also show KI = 0.25 +/- 0.02 microM. The ratio (r) of the specific activity of AD-ATP-labeled adenylate kinase to that of the unlabeled enzyme has been determined as a function of the number (n) of label/enzyme. The linear plot of r versus n with slope equal to -1 shows that the labeling is very specific, i.e. each label completely inactivates an enzyme molecule. After the labeled enzyme was partially hydrolyzed and the radioactive peptides analyzed and sequenced, it was found that Leu-115, Cys-25, and probably His-36 were labeled, in agreement with previous conclusions on the structure of the active site of this enzyme based on amino acid sequence, x-ray diffraction, and NMR studies. The environment-sensitive fluorescent dansyl group of AD-ATP can function as an in situ probe for monitoring ligand or conformation changes at the active site. The fluorescence of AD-ATP-labeled enzyme with n = 0.9 is not affected by ATP but increases with the concentration of AMP in solution. This observation is also in agreement with the previous conclusion that ATP does not bind to the AMP site of adenylate kinase. The observed enhancement of fluorescence indicates that binding of AMP by this enzyme causes environmental change at its ATP site. The possible usefulness of AD-ATP as an effective biological inhibitor or as a molecular probe for studying the structure and regulation of ATP-binding proteins is discussed.     

Evidence for compartmentalized adenylate kinase catalysis serving a high energy phosphoryl transfer function in rat skeletal muscle

The first characterization of the kinetics and subcellular compartmentation of adenylate kinase activity in intact muscle has been accomplished using rat diaphragm equilibrated with [18O]water. Rates of adenylate kinase-catalyzed phosphoryl transfer were measured by appearance of 18O-labeled beta-phosphoryls in ADP and ATP resulting from the transfer to AMP of newly synthesized 18O-labeled gamma-ATP. Unique features of adenylate kinase catalysis were uncovered in the intact cell not predictable from cell free analysis. This enzyme activity, which in non-contracting muscle is limited to 1/1000 of the estimated Vmax (cell free) apparently because of restricted ADP availability, is localized in subcellular compartments that increase in size and/or number with contractile frequency. Contraction also causes frequency-dependent increments in adenylate kinase velocity (22-fold at 4 Hz) as does oxygen deprivation (35-fold). These enhanced rates of adenylate kinase activity, equivalent to processing all the cellular ATP and ADP in approximately 1 min, occur when levels of ATP, ADP, and AMP are maintained very near their basal steady state. These characteristics of the dynamics of adenylate kinase catalysis in the intact cell demonstrate that rapid rates of AMP production from ADP are balanced by equally rapid rates of AMP phosphorylation with no net synthesis or accumulation of any adenine nucleotide. This rapid processing of nucleotide phosphoryls conforms to a proposed scheme whereby the adenylate kinase system provides the unique function of transferring, as beta-ADP, high energy phosphoryls generated by glycolytic metabolism to ATP-utilizing components in muscle.     

Adenylate kinase of plants. Properties of adenylate kinase of pea leaves]

The properties of adenylate kinase in 2 ADP in equilibrium ATP + AMP reaction have been studied. The dependence of the enzyme activity on medium pH, protein concentration, substrates, Mg++ ions, AMP, adenine and adenosine has been also investigated. pH optimum is found to be 8.5 for forward reaction and 8-9--for the reverse one. The Michaelis constants are as follows: for ADP--1.17-10(-4) M, for ATP--3.33-10(-4) M at 24 degrees C, in 50 mM tris-HCl pH 7.6. The optimal ratio, Mg++ ions/substrates (ADP, ATP + AMP), is 1:2. The chelates of adenine nucleotides with Mg++ ions are proved to be "true" reaction substrates. Unlike adenine and adenosine, the product of AMP reaction inhibits adenylate kinase activity. It is concluded that the properties of adenylate kinase in plants are similar to those of animals and humans (moikinase).     

Characteristics of modulators and substrates binding to rat liver adenylate kinase.

The binding of adenine nucleotides to liver adenylate kinase was dependent on Mg2+ ions. Citric acid enhanced the binding of all metal-chelated radioactive nucleotides and indicated two observable binding sites for Mg3H-ADP and Mg3-ATP and one-half binding site for Mg3H-AMP. Two binding sites of Mg3H-ADP and one binding site for Mg3H-ATP were also observed in the absence of citric acid. Stoichiometric binding of 14C-citric acid to liver adenylate kinase varied with additions of different nucleotides. AMP prevented whereas ADP and ATP enhanced the binding of 14C-citric acid.     

Three-dimensional structure of the complex between the mitochondrial matrix adenylate kinase and its substrate AMP

Crystals of adenylate kinase from beef heart mitochondrial matrix (EC 2.7.4.10) complexed with its substrate AMP were analyzed by X-ray diffraction. The crystal structure was solved by multiple isomorphous replacement and solvent flattening at a resolution of 3.0 A. There are two enzyme-substrate molecules in the asymmetric unit. The resolution was extended to 1.9 A by model building and refinement using simulated annealing. The current R-factor is 28.4%. The model is given as a backbone tracing for residues 5-218. The enzyme can be subdivided into three domains, the relative arrangements of which differ slightly but significantly between the two crystallographically independent molecules. When compared with other adenylate kinase structures, the chain fold is similar but the observed domain arrangement differs grossly, suggesting that large parts of the enzyme move during catalysis. The observed binding site of AMP is described. Its location in conjunction with data from homologous proteins clarifies the nucleotide-binding sites of the adenylate kinases. Previous assignments of these sites derived from X-ray crystallographic and nuclear magnetic resonance analyses are discussed.     

Thrombin exerts a dual effect on stimulated adenylate cyclase in hamster fibroblasts, an inhibition via a GTP-binding protein and a potentiation via activation of protein kinase C.

Previous studies in Chinese-hamster fibroblasts (CCL39 line) indicate that an important signalling pathway involved in thrombin's mitogenicity is the activation of a phosphoinositide-specific phospholipase C, mediated by a pertussis-toxin-sensitive GTP-binding protein (Gp). The present studies examine the effects of thrombin on the adenylate cyclase system and the interactions between the two signal transduction pathways. We report that thrombin exerts two opposite effects on cyclic AMP accumulation stimulated by cholera toxin, forskolin or prostaglandin E1. (1) Low thrombin concentrations (below 0.1 nM) decrease cyclic AMP formation. A similar inhibition is induced by A1F4-, and both thrombin- and A1F4- -induced inhibitions are abolished by pertussis toxin. (2) Increasing thrombin concentration from 0.1 to 10 nM results in a progressive suppression of adenylate cyclase inhibition and in a marked enhancement of cyclic AMP formation in pertussis-toxin-treated cells. A similar stimulation is induced by an active phorbol ester, and thrombin-induced potentiation of adenylate cyclase is suppressed by down-regulation of protein kinase C. Therefore, we conclude that (1) the inhibitory effect of thrombin on adenylate cyclase is the direct consequence of the activation of a pertussis-toxin-sensitive inhibitory GTP-binding protein (Gi) possibly identical with Gp, and (2) the potentiating effect of thrombin on cyclic AMP formation is due to stimulation of protein kinase C, as an indirect consequence of Gp activation. Our results suggest that the target of protein kinase C is an element of the adenylate cyclase-stimulatory GTP-binding protein (Gs) complex. At low thrombin concentrations, activation of phospholipase C is greatly attenuated by increased cyclic AMP, leading to predominance of the Gi-mediated inhibition.     

Mutating the Conserved Q-loop Glutamine 1291 Selectively Disrupts Adenylate Kinase-dependent Channel Gating of the ATP-binding Cassette (ABC) Adenylate Kinase Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) and Reduces Channel Function in Primary Human Airway Epithelia

The ATP-binding cassette (ABC) transporter cystic fibrosis transmembrane conductance regulator (CFTR) and two other non-membrane-bound ABC proteins, Rad50 and a structural maintenance of chromosome (SMC) protein, exhibit adenylate kinase activity in the presence of physiologic concentrations of ATP and AMP or ADP (ATP + AMP ⇆ 2 ADP). The crystal structure of the nucleotide-binding domain of an SMC protein in complex with the adenylate kinase bisubstrate inhibitor P¹,P⁵-di(adenosine-5′) pentaphosphate (Ap₅A) suggests that AMP binds to the conserved Q-loop glutamine during the adenylate kinase reaction. Therefore, we hypothesized that mutating the corresponding residue in CFTR, Gln-1291, selectively disrupts adenylate kinase-dependent channel gating at physiologic nucleotide concentrations. We found that substituting Gln-1291 with bulky side-chain amino acids abolished the effects of Ap₅A, AMP, and adenosine 5′-monophosphoramidate on CFTR channel function. 8-Azidoadenosine 5′-monophosphate photolabeling of the AMP-binding site and adenylate kinase activity were disrupted in Q1291F CFTR. The Gln-1291 mutations did not alter the potency of ATP at stimulating current or ATP-dependent gating when ATP was the only nucleotide present. However, when physiologic concentrations of ADP and AMP were added, adenylate kinase-deficient Q1291F channels opened significantly less than wild type. Consistent with this result, we found that Q1291F CFTR displayed significantly reduced Cl⁻ channel function in well differentiated primary human airway epithelia. These results indicate that a highly conserved residue of an ABC transporter plays an important role in adenylate kinase-dependent CFTR gating. Furthermore, the results suggest that adenylate kinase activity is important for normal CFTR channel function in airway epithelia.     

Synthesis of bound adenosine triphosphate from bound adenosine diphosphate by the purified coupling factor 1 of chloroplasts. Evidence for direct involvement of the coupling factor in this "adenylate kinase-like" reaction.

Electrophoretically homogeneous coupling factor 1 from spinach chloroplasts binds ADP and converts the bound ADP to bound ATP and AMP. That this transphosphorylation of enzyme-bound ADP is catalyzed by the coupling factor itself, and not be a conventional adenylate kinase which might possibly contaminate preparations of the coupling factor, is supported by the following evidence. 1. The procedure for isolatio of the coupling factor is designed to separate this large (approximately 13 S) enzyme from the smaller (4.2 S) conventional adenylate kinase of spinach chloroplasts. The conventional adenylate kinase cannot be detected in purified preparations of the coupling factor by biochemical assay or by polyacrylamide gel electrophoresis. 2. The activity of spinach adenylate kinase is completely dependent upon magnesium ions. However, the production of bound ATP and AMP from bound ADP by the coupling factor can be assayed in the total absence of added magnesium ions or even in the presence of added EDTA. 3. Comparative studies with inhibitors show that the coupling factor can produce bound ATP from ADP under conditions where the activity of adenylate kinase is strongly inhibited. Conversely, the coupling factor is prevented from synthesizing bound ATP from ADP under other conditions where the conventional adenylate kinase has high levels of activity. 4. AMP, when added in solution to the coupling factor, does not bind to this enzyme, even in the presence of APT. Thus, it is unlikely that the appearance of AMP bound to the coupling factor after its incubation with ADP is due to the production of free AMP by contaminating adenylate kinase. These results demonstrate that the isolated, homogeneous coupling factor from spinach chloroplasts has the intrinsic capacity to perform a phosphoryl group transfer between two bound ADP molecules and thus to synthesize ATP. This reaction may have an important role in the photosynthetic production of ATP by the chloroplast, as is discussed in this communication.     

Adenine nucleotides and the adenylate kinase equilibrium in livers of foetal and newborn rats

1. Measurements of ATP, ADP and AMP concentrations in livers of rats that had been delivered by Caesarian section indicate a rapid shift from a low to a high [ATP]/[AMP] ratio. This change is consistent with the cessation of glycolysis and the initiation of gluconeogenesis at birth. 2. When newborn animals are exposed to a 100% nitrogen atmosphere the hepatic ATP concentration falls and AMP increases. 3. Calculations of the [ATP][AMP]/[ADP](2) ratio give values that are close to the equilibrium constant of adenylate kinase except when the ATP concentration is high. 4. This difference cannot be accounted for by the preferential binding of available Mg(2+) to ATP(4-) rather than ADP(3-). It is concluded that the relative proportions of adenine nucleotides at any level of phosphorylation are only partly regulated by adenylate kinase.     

Purification and properties of squid mantle adenylate kinase. Role of NADH in control of the enzyme.

Adenylate kinase (ATP:AMP phosphotransferase, EC 2.7.4.3) from the mantle muscle of the squid, Loligo pealeii, was purified over 170-fold to homogeneity as judged by polyacrylamide and starch gel electrophoresis. The tissue contains a single isozyme of adenylate kinase, the enzyme from cytoplasmic and mitochondrial compartments (90 and 10% of total activity, respectively) being identical in physical and kinetic properties. Molecular weight was found to be 27,000 +/- 400. The enzyme shows a pH optimum of 8.2 in the forward (APD utilizing) and 7.4 in the reverse direction. Michaelis constants for ADP, ATP, and AMP are 0.70, 0.13, and 0.15 mM, respectively, with optimal Mg2+:adenylate ratios being 1:2 for ADP and 1:1 for ATP. A comparison of mass action ratios with the equilibrium constant indicated that squid adenylate kinase is held out of equilibrium in resting, but not active, muscle. A search for metabolic modulators of adenylate kinase revealed that NADH (Ki of 0.1 mM) was the only modulator which exerted a significant effect within its in vivo concentration range. The data presented indicate that NADH inhibition is the factor maintaining adenylate kinase in a nonequilibrium state in resting muscle and that release of this inhibition can serve to integrate adenylate kinase into the known scheme of intermediary metabolism in this tissue. A sharp drop in NADH levels at the onset on muscular work co-ordinates that activation of aerobic metabolism in this tissue and allows adenylate kinase to return to equilibrium function. At equilibrium, the enzyme can function to ampligy the concentration of AMP, a potent activator and deinhibitor of key glycolytic and Krebs cycle enzymes. The effect of modulators of adenylate kinase in preventing denaturation by heat or proteolysis revealed that NADH and substrates induced conformational changes in the enzyme which rendered it less susceptible to denaturation. The conformation state induced by NADH differed from that induced by substrate.     

Functions of chloroplastic adenylate kinases in Arabidopsis

Adenosine monophosphate kinase (AMK; adenylate kinase) catalyses the reversible formation of ADP by the transfer of one phosphate group from ATP to AMP, thus equilibrating adenylates. The Arabidopsis (Arabidopsis thaliana) genome contains 10 genes with an adenylate/cytidylate kinase signature; seven of these are identified as putative adenylate kinases. Encoded proteins of at least two members of this Arabidopsis adenylate kinase gene family are targeted to plastids. However, when the individual genes are disrupted, the phenotypes of both mutants are strikingly different. Although absence of AMK2 causes only 30% reduction of total adenylate kinase activity in leaves, there is loss of chloroplast integrity leading to small, pale-looking plantlets from embryo to seedling development. In contrast, no phenotype for disruption of the second plastid adenylate kinase was found. From this analysis, we conclude that AMK2 is the major activity for equilibration of adenylates and de novo synthesis of ADP in the plastid stroma.     

Defective metabolic signaling in adenylate kinase AK1 gene knock-out hearts compromises post-ischemic coronary reflow

Matching blood flow to myocardial energy demand is vital for heart performance and recovery following ischemia. The molecular mechanisms responsible for transduction of myocardial energetic signals into reactive vasodilatation are, however, elusive. Adenylate kinase, associated with AMP signaling, is a sensitive reporter of the cellular energy state, yet the contribution of this phosphotransfer system in coupling myocardial metabolism with coronary flow has not been explored. Here, knock out of the major adenylate kinase isoform, AK1, disrupted the synchrony between inorganic phosphate P(i) turnover at ATP-consuming sites and gamma-ATP exchange at ATP synthesis sites, as revealed by (18)O-assisted (31)P NMR. This reduced energetic signal communication in the post-ischemic heart. AK1 gene deletion blunted vascular adenylate kinase phosphotransfer, compromised the contractility-coronary flow relationship, and precipitated inadequate coronary reflow following ischemia-reperfusion. Deficit in adenylate kinase activity abrogated AMP signal generation and reduced the vascular adenylate kinase/creatine kinase activity ratio essential for the response of metabolic sensors. The sarcolemma-associated splice variant AK1beta facilitated adenosine production, a function lost in the absence of adenylate kinase activity. Adenosine treatment bypassed AK1 deficiency and restored post-ischemic flow to wild-type levels, achieving phenotype rescue. AK1 phosphotransfer thus transduces stress signals into adequate vascular response, providing linkage between cell bioenergetics and coronary flow.     

Adenylate kinase activity in ejaculated bovine sperm flagella

Adenylate kinase (ATP:AMP phosphotransferase, EC 2.7.4.3) activity was detected in the flagella of ejaculated bovine spermatozoa. This activity provided sufficient ATP to produce normal motility in cells permeabilized with digitonin and treated with 0.5 mM MgADP. In the presence of ADP, adenylate kinase activity was inhibited by P1,P5-di(adenosine 5')-pentaphosphate (Ap5A), an adenylate kinase-specific inhibitor, and motility was stopped. ATP-supported motility was not affected by Ap5A. Mitochondrial adenylate kinase activity allowed AMP to stimulate respiration in permeabilized sperm. Adenylate kinase activity in tail fragments was most active in a pH range from 7.6 to 8.4, and a similar pH sensitivity was observed for this enzyme activity in a hypotonic extract of whole sperm. The apparent km of adenylate kinase activity in permeabilized tail fragments was about 1.0 mM ADP in the direction of ATP synthesis. The fluctuation of nucleotide concentrations in normal and metabolically stimulated sperm suggested that adenylate kinase was most active when the cell was highly motile, although adenylate kinase activity did not appear to be coupled strictly with motility.