Nuclear magnetic resonance studies of the role of histidine residues at the active site of rabbit muscle creatine kinase

Mitochondrial Proteolipid Complexes of Creatine Kinase

Isoforms of creatine kinase (CK) generate and use phosphocreatine, a concentrated and highly diffusible cellular "high energy" intermediate, for the main purpose of energy buffering and transfer in order to maintain cellular energy homeostasis. The mitochondrial CK isoform (mtCK) localizes to the mitochondrial intermembrane and cristae space, where it assembles into peripherally membrane-bound, large cuboidal homooctamers. These are part of proteolipid complexes wherein mtCK directly interacts with cardiolipin and other anionic phospholipids, as well as with the VDAC channel in the outer membrane. This leads to a stabilization and cross-linking of inner and outer mitochondrial membrane, forming so-called contact sites. Also the adenine nucleotide translocator of the inner membrane can be recruited into these proteolipid complexes, probably mediated by cardiolipin. The complexes have functions mainly in energy transfer to the cytosol and stimulation of oxidative phosphorylation, but also in restraining formation of reactive oxygen species and apoptosis. In vitro evidence indicates a putative role of mtCK in mitochondrial phospholipid distribution, and most recently a role in thermogenesis has been proposed. This review summarizes the essential structural and functional data of these mtCK complexes and describes in more detail the more recent advances in phospholipid interaction, thermogenesis, cancer and evolution of mtCK.

Relating structure to mechanism in creatine kinase

Found in all vertebrates, creatine kinase catalyzes the reversible reaction of creatine and ATP forming phosphocreatine and ADP. Phosphocreatine may be viewed as a reservoir of "high-energy phosphate" which is able to supply ATP, the primary energy source in bioenergetics, on demand. Consequently, creatine kinase plays a significant role in energy homeostasis of cells with intermittently high energy requirements. The enzyme is of clinical importance and its levels are routinely used as an indicator of myocardial and skeletal muscle disorders and for the diagnosis of acute myocardial infarction. First identified in 1928, the enzyme has undergone intensive investigation for over 75 years. There are four major isozymes, two cytosolic and two mitochondrial, which form dimers and octamers, respectively. Depending on the pH, the enzyme operates by a random or an ordered bimolecular mechanism, with the equilibrium lying towards phosphocreatine production. Evidence suggests that conversion of creatine to phosphocreatine occurs via the in-line transfer of a phosphoryl group from ATP. A recent X-ray structure of creatine kinase bound to a transition state analog complex confirmed many of the predictions based on kinetic, spectroscopic, and mutagenesis studies. This review summarizes and correlates the more significant mechanistic and structural studies on creatine kinase.

Transition state structure of arginine kinase: implications for catalysis of bimolecular reactions

Arginine kinase belongs to the family of enzymes, including creatine kinase, that catalyze the buffering of ATP in cells with fluctuating energy requirements and that has been a paradigm for classical enzymological studies. The 1.86-A resolution structure of its transition-state analog complex, reported here, reveals its active site and offers direct evidence for the importance of precise substrate alignment in the catalysis of bimolecular reactions, in contrast to the unimolecular reactions studied previously. In the transition-state analog complex studied here, a nitrate mimics the planar gamma-phosphoryl during associative in-line transfer between ATP and arginine. The active site is unperturbed, and the reactants are not constrained covalently as in a bisubstrate complex, so it is possible to measure how precisely they are pre-aligned by the enzyme. Alignment is exquisite. Entropic effects may contribute to catalysis, but the lone-pair orbitals are also aligned close enough to their optimal trajectories for orbital steering to be a factor during nucleophilic attack. The structure suggests that polarization, strain toward the transition state, and acid-base catalysis also contribute, but, in contrast to unimolecular enzyme reactions, their role appears to be secondary to substrate alignment in this bimolecular reaction.

Evolution of phosphagen kinase. VI. Isolation, characterization and cDNA-derived amino acid sequence of lombricine kinase from the earthworm Eisenia foetida, and identification of a possible candidate for the guanidine substrate recognition site

Lombricine kinase (LK) from the body wall muscle of the earthworm Eisenia foetida was purified to homogeneity. The enzyme was shown to be a dimer consisting of 40 kDa subunits. The cDNA-derived amino acid sequence of 370 residues of Eisenia LK was determined. The validity of the sequence was supported by chemical sequencing of internal tryptic peptides. This is the first reported lombricine kinase amino acid sequence. Alignment of Eisenia LK with those of creatine kinases (CKs), arginine kinases (AKs) and glycocyamine kinase (GK) suggested a region displaying remarkable amino acid deletions (referred to GS region), as a possible candidate for guanidine substrate recognition site. A phylogenetic analysis using amino acid sequences of all four phosphagen kinases indicates that CK, GK and LK probably evolved from a common immediate ancestor protein.

Mitochondrial creatine kinase--a square protein

The recently determined structure of octameric mitochondrial creatine kinase has provided new insights into the functioning of this enzyme and its role in channelling energy from the mitochondria to the cytoplasm. Creatine kinase, a member of the family of guanidino kinases, is structurally similar to glutamine synthetase, suggesting a possible evolutionary link between both protein families.

Evolution of phosphagen kinase. Isolation, characterization and cDNA-derived amino acid sequence of two-domain arginine kinase from the sea anemone Anthopleura japonicus

Arginine kinase (AK) was isolated from the body wall muscle of the primitive sea anemone Anthopleura japonicus by Ultrogel AcA34 gel filtration, DEAE-32 chromatography and elution on a Cosmogel-SP column. The denatured molecular mass as determined with SDS/PAGE was 80 kDa, twice that of the usual AK subunit, indicating that this AK has an unusual two-domain structure. The native form was eluted on a Superose 12 column with the same retention time as that of rabbit homodimeric creatine kinase, indicating that Anthopleura AK is a monomer of 80 kDa. The isolated enzyme gave a specific activity of 100-120 micromol of Pi/min per mg of protein in the pH range 7.9-9.1 for the forward reaction. The enzyme is fully activated by Ca2+, as it is with Mg2+. The cDNA-derived amino acid sequence of 715 residues of Anthopleura AK was determined. The validity of the sequence was supported by chemical sequencing of internal tryptic peptides. A bridge intron of 686 bp, which separates the two domains of Anthopleura AK, is present between the second and third nucleotide in the codon of Ala-364. This is the first two-domain AK to be sequenced. Anthopleura AK shows 48-54% amino acid sequence identity with known invertebrate AKs, and also shows a lower, but significant, similarity (39-46%) to marine worm glycocyamine kinase and rabbit creatine kinase.

ADP-binding and ATP-binding sites in native and proteinase-K-digested creatine kinase, probed by reaction-induced difference infrared spectroscopy

Conformational changes induced by nucleotide binding to native creatine kinase (CK) from rabbit muscle and to proteinase-K-digested (nicked) CK, were investigated by infrared spectroscopy. Photochemical release of ATP from ATP[Et(PhNO2)] in the presence of creatine and native CK produced reaction-induced difference infrared spectra (RIDS) of CK related to structural changes of the enzyme that paralleled the reversible phosphoryl transfer from ATP to creatine. Similarly the photochemical release of ADP from ADP[Et(PhNO2)] in the presence of phosphocreatine and native CK allowed us to follow the backward reaction and its corresponding RIDS. Infrared spectra of native CK indicated that carboxylate groups of Asp or Glu, and some carbonyl groups of the peptide backbone are involved in the enzymatic reaction. Native and proteinase nicked CK have similar Stokes' radii, tryptophan fluorescence, fluorescence fraction accessible to iodide, and far-ultraviolet CD spectra, indicating that native and modified enzymes have the same quaternary structures. However, infrared data showed that the binding site of the gamma-phosphate group of the nucleotide was affected in nicked CK compared with that of the native CK. Furthermore, the infrared absorptions associated with ionized carboxylate groups of Asp or Glu amino acid residues were different in nicked CK and in native CK.

Evolution of phosphagen kinase V. cDNA-derived amino acid sequences of two molluscan arginine kinases from the chiton Liolophura japonica and the turbanshell Battilus cornutus

The cDNAs of arginine kinases from the chiton Liolophura japonica (Polyplacophora) and the turbanshell Battilus cornutus (Gastropoda) were amplified by polymerase chain reaction (PCR), and the complete nucleotide sequences of 1669 and 1624 bp, respectively, were determined. The open reading frame for Liolophura arginine kinase is 1050 nucleotides in length and encodes a protein with 349 amino acid residues, and that for Battilus is 1077 nucleotides and 358 residues. The validity of the cDNA-derived amino acid sequence was supported by chemical sequencing of internal tryptic peptides. The molecular masses were calculated to be 39,057 and 39,795 Da, respectively. The amino acid sequence of Liolophura arginine kinase showed 65-68% identity with those of Battilus and Nordotis (abalone) arginine kinases, and the homology between Battilus and Nordotis was 79%. Molluscan arginine kinases also show lower, but significant homology (38-43%) with rabbit creatine kinase. The sequences of arginine kinases could be used as a molecular clock to elucidate the phylogeny of Mollusca, one of the most diverse animal phyla.

Adenosine conformations of nucleotides bound to methionyl tRNA synthetase by transferred nuclear Overhauser effect spectroscopy

The conformations of MgATP and AMP bound to a monomeric tryptic fragment of methionyl tRNA synthetase have been investigated by two-dimensional proton transferred nuclear Overhauser effect spectroscopy (TRNOESY). The sample protocol was chosen to minimize contributions from adventitious binding of the nucleotides to the observed NOE. The experiments were performed at 500 MHz on three different complexes, E.MgATP, E.MgATP.L-methioninol, and E.AMP.L-methioninol. A starter set of distances obtained by fitting NOE build-up curves (not involving H5' and H5") were used to determine a CHARMm energy-minimized structure. The positioning of the H5' and H5" protons was determined on the basis of a conformational search of the torsion angle to obtain the best fit with the observed NOEs for their superposed resonance. Using this structure, a relaxation matrix was set up to calculate theoretical build-up curves for all of the NOEs and compare them with the observed curves. The final structures deduced for the adenosine moieties in the three complexes are very similar, and are described by a glycosidic torsion angle (chi) of 56 degrees +/- 5 degrees and a phase angle of pseudorotation (P) in the range of 47 degrees to 52 degrees, describing a 3(4)T-4E sugar pucker. The glycosidic torsion angle, chi, deduced here for this adenylyl transfer enzyme and those determined previously for three phosphoryl transfer enzymes (creatine kinase, arginine kinase, and pyruvate kinase), and one pyrophosphoryl enzyme (PRibPP synthetase), are all in the range 52 degrees +/- 8 degrees. The narrow range of values suggests a possible common motif for the recognition and binding of the adenosine moiety at the active sites of ATP-utilizing enzymes, irrespective of the point of cleavage on the phosphate chain.

The active site histidines of creatine kinase. A critical role of His 61 situated on a flexible loop

A histidine residue with a pKa of 7 has been inferred to act as a general acid-base catalyst for the reaction of creatine kinase (CK), catalyzing the reversible phosphorylation of creatine by ATP. The chicken sarcomeric muscle mitochondrial isoenzyme Mib-CK contains several histidine residues that are conserved throughout the family of creatine kinases. By X-ray crystal structure analysis, three of them (His 61, His 92, and His 186) were recently shown to be located close to the active site of the enzyme. These residues were exchanged against alanine or aspartate by in vitro mutagenesis, and the six mutant proteins were expressed in E. coli and purified. Structural integrity of the mutant proteins was checked by small-angle X-ray scattering. Kinetic analysis showed the mutant His 61 Asp to be completely inactive in the direction of ATP consumption while exhibiting a residual activity of 1.7% of the wild-type (wt) activity in the reverse direction. The respective His to Ala mutant of residue 61 showed approximately 1% wt activity in the forward and 10% wt activity in the reverse reaction. All other mutants showed near wt activities. Changes in the kinetic parameters K(m) or Vmax, as well as a significant loss of synergism in substrate binding, could be observed with all active mutants. These effects were most pronounced for the binding of creatine and phosphocreatine, whereas ATP or ADP binding were less severely affected. Based on our results, we assume that His 92 and His 186 are involved in the binding of creatine and ATP in the active site, whereas His 61 is of importance for the catalytic reaction but does not serve as an acid-base catalyst in the transphosphorylation of creatine and ATP. In addition, our data support the idea that the flexible loop bearing His 61 is able to move towards the active site and to participate in catalysis.

Changes of creatine kinase secondary structure induced by the release of nucleotides from caged compounds. An infrared difference-spectroscopy study

Light-induced release of ADP and ATP from their respective caged nucleotides produced small distinct difference infrared spectra of creatine kinase (CK), indicating that ADP and ATP binding to CK promoted different structural alteration. The positive band at 1638-1640 cm-1 and the negative band at about 1650-1652 cm-1 on the reaction-induced infrared difference spectra in the amide I region were insensitive to the deuteration effects. They were assigned to the peptide backbone of the ADP/ATP-binding site. In addition Pi or ATP binding produced another positive band at 1657-1659 cm-1 corresponding to the C = O (amide I band) associated with the gamma-phosphate of ATP. This site was also affected when ADP was added, indicating coupling interactions between both sites. No additional structural changes were observed when creatine and ADP were added, suggesting that the creatine-binding site was uncoupled from the ADP-binding site. The infrared difference spectra of a transition-state-analog complex formed by the addition of ADP, creatine and NO3- (a planar-phosphate-mimicking group) lacked the 1657-1659-cm-1 band indicating that the binding site of gamma-phosphate within CK, was not affected. Infrared changes in the 1560-1590-cm-1 region suggested that carboxylate groups of Asp or Glu were involved in the binding of Pi, ADP and ATP.

Structure of mitochondrial creatine kinase

Creatine kinase (CK, EC 2.7.3.2), an enzyme important for energy metabolism in cells of high and fluctuating energy requirements, catalyses the reversible transfer of a phosphoryl goup from phosphocreatine to ADP. We have solved the structure of the octameric mitochondrial isoform, Mib-CK, which is located in the intermembrane compartment and along the cristae membranes. Mib-CK consumes ATP produced in the mitochondria for the production of phosphocreatine, which is then exported into the cytosol for fast regeneration of ATP by the cytosolic CK isoforms. The octamer has 422 point-group symmetry, and appears as a cube of side length 93 angstrom with a channel 20 angstrom wide extending along the four-fold axis. Positively charged amino acids at the four-fold faces of the octamer possibly interact with negatively charged mitochondrial membranes. Each monomer consists of a small alpha-helical domain and a large domain containing an eight-stranded antiparallel beta-sheet flanked by seven alpha-helices. The conserved residues of the CK family form a compact cluster that covers the active site between the domains.

Photoaffinity labeling of creatine kinase with 2-azido- and 8-azidoadenosine triphosphate: identification of two peptides from the ATP-binding domain

Two different analogs of ATP, [gamma-32P]2N3ATP and and [gamma-32P]8N3ATP, were used to photoaffinity label the MM and BB isoforms of rabbit cytosolic creatine kinase. Evidence that photoinsertion was within the ATP-binding domain was as follows: (1) Assays for creatine phosphate production demonstrated that [gamma-32]2N3ATP and [gamma-32P]8N3ATP are substrates for creatine kinase. (2) Enzymatic activity was inhibited by photolabeling with either analog. (3) Saturation of photoinsertion was observed for both analogs. Half-maximal saturation was observed at 5 microM [gamma-32P]2N3ATP or 12 microM (gamma-32P]8N3ATP. (4) Photoinsertion of both probes could be decreased by micromolar levels of ATP. Immobilized Al3+ affinity chromatography and HPLC were used to isolate the peptides modified by these probes. Overlapping sequence analysis of the isolated peptides from the tryptic and chymotryptic digests of the photolabeled MM isoform revealed that [gamma-32P]8N3ATP photoinserted into the peptide region corresponding to Val279-Arg291, whereas [gamma-32P]2N3-ATP photoinserted into Val236-Lys241. The corresponding peptide (Ile279-Arg291 and Val236-Lys241) from the BB isoform were shown to be selectively modified. We conclude that amino acid residues within the peptide regions 236-241 and 279-291 of rabbit cytosolic creatine kinase are localized within the binding domain for the adenine moiety of ATP. The results also demonstrate the effectiveness and selectivity of Al3+ as the chelating agent in immobilized metal affinity chromatography for the isolation of photolabeled peptides as well as its potential to enhance retention of radiolabel during HPLC.

Two-dimensional transferred nuclear Overhauser effect spectroscopy study of the confirmation of MgATP bound at the active and ancillary sites of rabbit muscle pyruvate kinase

Pyruvate kinase binds one adenosine 5'-triphosphate (ATP) molecule at its active site and another at an ancillary site on each subunit. In order to determine the conformation of ATP bound at these sites, proton transferred two-dimensional nuclear Overhauser effect spectroscopy (TRNOESY) measurements were made at 500 MHz and 10 degrees C for several mixing times in the range 40-200 ms. The NOE values for the proton pair H1'-H2' of ribose (which are 2.9 +/- 0.2 A apart, irrespective of nucleotide conformation) as a function of ligand concentration (1-10 mM ATP), with the ratio of ligand to enzyme being kept constant, indicate that at higher ligand concentrations adventitious binding of ATP at nonspecific site(s) makes a major contribution to the observed NOEs. When the ligand concentration is < 2 mM, site-specific NOEs can be measured. Furthermore, addition of phosphoenolpyruvate (PEP) to the enzyme-MgATP sample results in competitive displacement of MgATP from the active site and reduces the observed NOE to that arising exclusively at the ancillary site, thus allowing the measurement of site-specific NOEs. The interproton distances determined from such site-specific NOE buildup curves were used as constraints in CHARMm to obtain the structure of MgATP. At the active site, MgATP has a glycosidic torsion chi = 44 +/- 5 degrees and the phase angle of pseudorotation for ribose P = 42.4 degrees. At the ancillary site chi = 46 +/- 5 degrees and P = 127.6 degrees. Thus the orientation of the adenine with respect to the sugar moiety is the same at both sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Resonance energy transfer between the active sites of creatine kinase from rabbit brain

Resonance energy transfer was measured between the active site domains of the brain isozyme of creatine kinase (CK-BB). The reactive thiol near the active sites, one on each subunit of the dimeric protein, was derivatized using 5-[2-[iodoacetyl)amino)ethyl]aminonaphthalene-1-sulfonic acid (AED), 2-[4'-iodoacetamidoanilino]naphthalene-6-sulfonic acid (AANS) and 5-iodoacetamidofluorescein (AF). Suitable donor/acceptor protein conjugated hybrids were prepared by controlled kinetics producing CK-BB-AED/AF and CK-BB-AANS/AF. Transfer efficiencies, measured from the quenching of the donor lifetime and steady-state sensitized acceptor emission, ranged from 0.10 to 0.17. From determination of the donor/acceptor overlap integrals, donor quantum yields and attempts to delimit the orientation factor using steady-state and phase-resolved anisotropy measurements, it was found that a suitable estimate of the range between the active sites was between 45 and 57 A. This range is similar to that reported previously for the muscle isozyme of creatine kinase (Grossman, S.H. (1989) Biochemistry 28, 4894-4902) but is a significantly greater distance than detected for the hybrid, myocardial specific isozyme (Grossman, S.H. (1983) Biochemistry 22, 5369-5375).

31P NMR studies of the thermodynamics and kinetics of the creatine kinase reaction

The thermodynamic and kinetic parameters of the reaction catalyzed by creatine kinase (CK) were measured in vitro in the temperature range 13 to 35 degrees C, using 31P NMR spectroscopy, including magnetization transfer methods. The apparent equilibrium constant of the reaction and the associated enthalpy for the formation of ATP at 35 degrees C, pH 8.2, and excess [Mg2+] were 3.5 x 10(9) M-1 and -2.4 +/- 0.5 kcal/mol, respectively. The rates at equilibrium at 35 degrees C catalyzed by 1 unit/ml CK were 12.4 and 10.7 microM/s at pH 7 and 8, respectively. The rate constants per 1 unit CK/1 ml at 35 degrees C, pH 7, were 1.3 x 10(8) s-1 M-2 and 9.9 x 10(-3) s-1 M-1 in the direction of ATP and PCr formation, respectively. The activation energies in both directions were similar and corresponded to 15 +/- 2 kcal/mol at pH 7 and 17.5 +/- 1.5 kcal/mol at pH 8. Comparison of in vivo results with the above in vitro data may provide information regarding the activity and kinetics of the CK reaction.

Histidyls in lobster arginine kinase. 1H-NMR of native and ethoxyformylated enzyme, 1H- and 31P-NMR of its complexes with substrates and analogues

The role of histidyls in lobster arginine kinase (EC 2.7.3.3) has been studied by 1H-NMR spectroscopy of the enzyme and its complexes with substrates or their analogues and 31P-NMR spectroscopy of complexes with ADP. Five histidyls were detected by 1H-NMR in native enzyme (His 1 to His 5). Three of them appeared possibly to be implicated in catalysis: His 3, whose pH/titration was affected by arginine binding, and His 1 and 4, shown from paramagnetic relaxation by Mn2+ to be close (less than or equal to 1.2 and less than or equal to 1.27 nm respectively) to the metal cofactor. His 4 was broadened beyond detection in the presence of any adenine nucleotide. In the enzyme reversibly inactivated by histidine ethoxyformylation, the modified histidyl was His 1. In the transition state analogue complex (in which NO3- mimics the transferred phosphoryl), Hill plots of histidyl pH/titration curves showed that His 1 and His 3 were both interacting with the same set of three titratable groups and hence spatially close. 31P-NMR demonstrated that ADP binding in this complex was unaffected by the chemical modification of His 1. It is concluded that His-ethoxyformyl-enzyme is inactive because ethoxyformyl-His 1 is unable to titrate. This is consistent with His 1 acting as the acid-base catalyst. However our results, which do not indicate any catalytic role of His 3, exclude any H-bonding of His 1 on either substrate. Involvement is needed of at least one other titratable residue for the proton evolved in the catalysis to exchange directly with the guanidino substrate.

Kinetics of creatine kinase in heart: a 31P NMR saturation- and inversion-transfer study

The kinetics of the phosphate exchange by creatine kinase (CK) was studied in solution and in the Langendorff-perfused rat heart at 37 degrees C. 31P inversion-transfer (IT) and saturation-transfer (ST) methods were applied. The kinetic parameters obtained by the two magnetization transfer methods were the same, whether in solution or in the perfused heart. Inversion transfer is the more efficient method, yielding the kinetic constants for the exchange and the relaxation rates of the transferred phosphate in both substrates, in one experiment. In solution the forward (kF) and reverse (kR) pseudo-first-order rate constants for the CK reaction (kF = k1[MgADP][H+]; kR = k-1[creatine]) as well as the concentrations of phosphocreatine (PCr), MgATP, and creatine (Cr) remained constant between pH 6.9 and pH 7.8. Equilibrium at this pH region is therefore maintained by compensating changes in the concentration of MgADP. The forward and reverse fluxes in the perfused heart were equal with an average flux ratio (fluxF/fluxR) of 0.975 +/- 0.065 obtained by both methods. Average values of kF and kR were 0.725 +/- 0.077 and 1.12 +/- 0.14 s-1, respectively. These results clearly indicate that the CK reaction in the Langendorff-perfused heart is in equilibrium and its rate is not limited by the diffusion of substrates between different locations of the enzyme. There is therefore no indication of compartmentation of substrates of the CK reaction.

Metal-ion-promoted dephosphorylation of the 5'-triphosphates of uridine and thymidine, and a comparison with the reactivity in the corresponding cytidine and adenosine nucleotide systems

First-order rate constants (50 degrees C; I = 0.1 M, NaClO4) for the dephosphorylation of UTP and TTP (1 mM) in the pH range 2-10 are compared with those of ATP and CTP; they all show the same properties indicating that the nucleic base has no influence on the rate. In the presence of Cu2+ or Zn2+ (NTP:M2+ = 1:1) this changes drastically: ATP-M2+ much greater than UTP-M2+ approximately equal to TTP-M2+ approximately equal to CTP-M2+ greater than NTP, the Cu2+ systems being always more reactive than the Zn2+ systems, and these more than the Ni2+ systems. An interaction between the nucleic base and metal ion is important for the Cu2+-ATP and Zn2+-ATP systems, but not for the pyrimidine-nucleotide systems (these behave like methyltriphosphate). Accordingly, prevention of the Cu2+-purine interaction by the addition of one equivalent of 2,2'-bipyridyl, leading to Cu(Bpy) (NTP)2-, strongly reduces the activity and all four ternary Cu2+ systems now show the same dephosphorylation rate. Addition of a second equivalent of Cu2+ to the Cu2+-UTP 1:1 system enhances the dephosphorylation rate significantly and Job's method provides evidence that a 2:1 complex is the most reactive intermediate. The relation between the initial rate, vo = d[PO3-4]/dt, and the concentration of Cu2+-UTP in 1:1 and 2:1 systems was determined. The results suggest that the reactive complex with pyrimidine nucleotides is a monomeric, dinuclear species of the type M2(NTP) (OH)- (its formation is inhibited by ligands like tryptophanate), while with M2+-ATP the reactive complex is a dimer. The connection between the indicated dephosphorylations in vitro, i.e. trans-phosphorylations to H2O, and related reactions in vivo are discussed.