Electrical conductivity as a driver of biological and geological spatial heterogeneity in the Puquios, Salar de Llamara, Atacama Desert, Chile

Reputed to be the driest desert in the world, the Atacama Desert in the Central Andes of Northern Chile is an extreme environment with high UV radiation, wide temperature variation, and minimum precipitation. Scarce lagoons associated with salt flats (salars) in this desert are the surface expression of shallow groundwater; these ponds serve as refugia for life and often host microbial communities associated with evaporitic mineral deposition. Results based on multidisciplinary field campaigns and associated laboratory examination of samples collected from the Puquios of the Salar de Llamara in the Atacama Desert during austral summer provide unprecedented detail regarding the spatial heterogeneity of physical, chemical, and biological characteristics of these salar environments. Four main lagoons ('Puquios') and more than 400 smaller ponds occur within an area less than 5 km2, and are characterized by high variability in electrical conductivity, benthic and planktonic biota, microbiota, lagoon bottom type, and style of mineral deposition. Results suggest that electrical conductivity is a driving force of system heterogeneity. Such spatial heterogeneity within the Puquios is likely to be expanded with temporal observations incorporating expected seasonal changes in electrical conductivity. The complexity of these Andean ecosystems may be key to their ability to persist in extreme environments at the edge of habitability.

Elastic and anelastic relaxation behaviour of perovskite multiferroics II: PbZr0.53Ti0.47O3 (PZT)-PbFe0.5Ta0.5O3 (PFT)

Elastic and anelastic properties of ceramic samples of multiferroic perovskites with nominal compositions across the binary join PbZr_0.53Ti_0.47O₃-PbFe_0.5Ta_0.5O₃ (PZT-PFT) have been assembled to create a binary phase diagram and to address the role of strain relaxation associated with their phase transitions. Structural relationships are similar to those observed previously for PbZr_0.53Ti_0.47O₃-PbFe_0.5Nb_0.5O₃ (PZT-PFN), but the magnitude of the tetragonal shear strain associated with the ferroelectric order parameter appears to be much smaller. This leads to relaxor character for the development of ferroelectric properties in the end member PbFe_0.5Ta_0.5O₃. As for PZT-PFN, there appear to be two discrete instabilities rather than simply a reorientation of the electric dipole in the transition sequence cubic-tetragonal-monoclinic, and the second transition has characteristics typical of an improper ferroelastic. At intermediate compositions, the ferroelastic microstructure has strain heterogeneities on a mesoscopic length scale and, probably, also on a microscopic scale. This results in a wide anelastic freezing interval for strain-related defects rather than the freezing of discrete twin walls that would occur in a conventional ferroelastic material. In PFT, however, the acoustic loss behaviour more nearly resembles that due to freezing of conventional ferroelastic twin walls. Precursor softening of the shear modulus in both PFT and PFN does not fit with a Vogel-Fulcher description, but in PFT there is a temperature interval where the softening conforms to a power law suggestive of the role of fluctuations of the order parameter with dispersion along one branch of the Brillouin zone. Magnetic ordering appears to be coupled only weakly with a volume strain and not with shear strain but, as with multiferroic PZT-PFN perovskites, takes place within crystals which have significant strain heterogeneities on different length scales.

Elastic and magnetoelastic relaxation behaviour of multiferroic (ferromagnetic + ferroelectric + ferroelastic) Pb(Fe0.5Nb0.5)O3 perovskite

Resonant Ultrasound Spectroscopy has been used to characterize elastic and anelastic anomalies in a polycrystalline sample of multiferroic Pb(Fe(0.5)Nb(0.5))O(3) (PFN). Elastic softening begins at ~550 K, which is close to the Burns temperature marking the development of dynamical polar nanoregions. A small increase in acoustic loss at ~425 K coincides with the value of T(*) reported for polar nanoregions starting to acquire a static or quasi-static component. Softening of the shear modulus by ~30-35% through ~395-320 K, together with a peak in acoustic loss, is due to classical strain/order parameter coupling through the cubic → tetragonal → monoclinic transition sequence of ferroelectric/ferroelastic transitions. A plateau of high acoustic loss below ~320 K is due to the mobility under stress of a ferroelastic microstructure but, instead of the typical effects of freezing of twin wall motion at some low temperature, there is a steady decrease in loss and increase in elastic stiffness below ~85 K. This is attributed to freezing of a succession of strain-coupled defects with a range of relaxation times and is consistent with a report in the literature that PFN develops a tweed microstructure over a wide temperature interval. No overt anomaly was observed near the expected Néel point, ~145 K, consistent with weak/absent spin/lattice coupling but heat capacity measurements showed that the antiferromagnetic transition is actually smeared out or suppressed. Instead, the sample is weakly ferromagnetic up to ~560 K, though it has not been possible to exclude definitively the possibility that this could be due to some magnetic impurity. Overall, evidence from the RUS data is of a permeating influence of static and dynamic strain relaxation effects which are attributed to local strain heterogeneity on a mesoscopic length scale. These, in turn, must have a role in determining the magnetic properties and multiferroic character of PFN.

Quaternary structure of Dictyostelium discoideum nucleoside diphosphate kinase counteracts the tendency of monomers to form a molten globule

Multimeric enzymes that lose their quaternary structure often cease to be catalytically competent. In these cases, conformational stability depends on contacts between subunits, and minor mutations affecting the surface of the monomers may affect overall stability. This effect may be sensitive to pH, temperature, or solvent composition. We investigated the role of oligomeric structure in protein stability by heat and chemical denaturation of hexameric nucleoside diphosphate kinase from Dictyostelium discoideum and its P105G mutant over a wide range of pH. The wild-type enzyme has been reported to unfold without prior dissociation into monomers, whereas monomer unfolding follows dissociation for the P105G mutant (Giartosio et al. (1996) J. Biol. Chem. 271, 17845-51). We show here that these features are also preserved at alkaline pH, with the wild-type enzyme always hexameric at room temperature whereas the mutant dissociates into monomers at pH >or=10. In acidic conditions (pH <or=6), even in the absence of denaturant, the predominant species for both proteins is an intermediate monomeric form with the characteristics of a molten globule: disordered tertiary native structure but preserved secondary structure. Monomers therefore seem to have a low intrinsic stability, which is overcome by the conformational organization in the oligomeric structure.

Binding of nucleotides to nucleoside diphosphate kinase: a calorimetric study

The source of affinity for substrates of human nucleoside diphosphate (NDP) kinases is particularly important in that its knowledge could be used to design more effective antiviral nucleoside drugs (e.g., AZT). We carried out a microcalorimetric study of the binding of enzymes from two organisms to various nucleotides. Isothermal titration calorimetry has been used to characterize the binding in terms of Delta G degrees, Delta H degrees and Delta S degrees. Thermodynamic parameters of the interaction of ADP with the hexameric NDP kinase from Dictyostelium discoideum and with the tetrameric enzyme from Myxococcus xanthus, at 20 degrees C, were similar and, in both cases, binding was enthalpy-driven. The interactions of ADP, 2'deoxyADP, GDP, and IDP with the eukaryotic enzyme differed in enthalpic and entropic terms, whereas the Delta G degrees values obtained were similar due to enthalpy--entropy compensation. The binding of the enzyme to nonphysiological nucleotides, such as AMP--PNP, 3'deoxyADP, and 3'-deoxy-3'-amino-ADP, appears to differ in several respects. Crystallography of the protein bound to 3'-deoxy-3'-amino-ADP showed that the drug was in a distorted position, and was unable to interact correctly with active site side chains. The interaction of pyrimidine nucleoside diphosphates with the hexameric enzyme is characterized by a lower affinity than that with purine nucleotides. Titration showed the stoichiometry of the interaction to be abnormal, with 9--12 binding sites/hexamer. The presence of supplementary binding sites might have physiological implications.

Structural and catalytic properties and homology modelling of the human nucleoside diphosphate kinase C, product of the DRnm23 gene

The human DRnm23 gene was identified by differential screening of a cDNA library obtained from chronic myeloid leukaemia-blast crisis primary cells. The over-expression of this gene inhibits differentiation and induces the apoptosis of myeloid precursor cell lines. We overproduced in bacteria a truncated form of the encoded protein lacking the first 17 N-terminal amino acids. This truncated protein was called nucleoside diphosphate (NDP) kinase CDelta. NDP kinase CDelta had similar kinetic properties to the major human NDP kinases A and B, but was significantly more stable to denaturation by urea and heat. Analysis of denaturation by urea, using size exclusion chromatography, indicated unfolding without the dissociation of subunits, whereas renaturation occurred via a folded monomer. The stability of the protein depended primarily on subunit interactions. Homology modelling of the structure of NDP kinase CDelta, based on the crystal structure of NDP kinase B, indicated that NDP kinase CDelta had several additional stabilizing interactions. The overall structure of the two enzymes appears to be identical because NDP kinase CDelta readily formed mixed hexamers with NDP kinase A. It is possible that mixed hexamers can be observed in vivo.

Nucleoside diphosphate kinase beta (Nm23-R1/NDPKbeta) is associated with intermediate filaments and becomes upregulated upon cAMP-induced differentiation of rat C6 glioma

Nucleoside diphosphate kinases (Nm23/NDPK) are enzymes functional in cell proliferation, differentiation, development, tumor progression, and metastasis. Nevertheless, no consensus exists about the molecular mechanism by which Nm23/NDPK isoforms exert their role in these processes. We investigated the expression of the rat Nm23-R1/NDPKbeta and Nm23-R2/NDPKalpha isoforms, homologues of the human Nm23-H1/NDPK A and Nm23-H2/NDPK B proteins, respectively, upon cAMP-induced differentiation of rat C6 glioma cells and demonstrated a differential interaction with intermediate filaments. Semiquantitative RT-PCR, immunoblotting, and flow cytometry showed a constitutive expression of both Nm23 isoforms. After induction of differentiation in C6 cells with cAMP analogs or isoproterenol, a dose-dependent 2- and 2.5-fold upregulation of the Nm23-R1 mRNA and protein, respectively, was observed. In contrast, the expression of Nm23-R2 remained unchanged. Localization of both isoforms with confocal laser scanning microscopy demonstrated a punctate reticular staining pattern for both Nm23 isoforms in the cytosol and processes of the cells which was particularly intense in the perinuclear region. In addition, while Nm23-R2 was colocalized and coimmunoprecipitated with vimentin in nondifferentiated cells, both isoforms were associated with GFAP in differentiated cells. The significance of these findings in relation to a possible function of Nm23 isoforms in cell proliferation, differentiation, and tumor-associated mechanisms is discussed.

The catalytic mechanism of nucleoside diphosphate kinases

Nucleoside diphosphate kinases catalyze the reversible transfer of the gamma phosphate of nucleoside triphosphates to nucleoside diphosphates. This minireview presents recent advances in understanding the reaction mechanism using steady-state and fast kinetic studies, X-ray crystallography, and site-directed mutagenesis. We also briefly discuss the physiological relevance of in vitro studies.

The human nm23-H4 gene product is a mitochondrial nucleoside diphosphate kinase

We demonstrate here the catalytic activity and subcellular localization of the Nm23-H4 protein, product of nm23-H4, a new member of the human nm23/nucleoside diphosphate (NDP) kinase gene family (Milon, L., Rousseau-Merck, M., Munier, A., Erent, M., Lascu, I., Capeau, J., and Lacombe, M. L. (1997) Hum. Genet. 99, 550-557). Nm3-H4 was synthesized in escherichia coli as the full-length protein and as a truncated form missing the N-terminal extension characteristic of mitochondrial targeting. The truncated form possesses NDP kinase activity, whereas the full-length protein is inactive, suggesting that the extension prevents enzyme folding and/or activity. X-ray crystallographic analysis was performed on active truncated Nm23-H4. Like other eukaryotic NDP kinases, it is a hexamer. Nm23-H4 naturally possesses a serine residue at position 129, equivalent to the K-pn mutation of the Drosophila NDP kinase. The x-ray structure shows that the presence of Ser(129) has local structural effects that weaken subunit interactions. Site-directed mutagenesis shows that the serine is responsible for the lability of Nm23-H4 to heat and urea treatment, because the S129P mutant is greatly stabilized. Examination of human embryonic kidney 293 cells transfected with green fluorescent protein fusions by confocal microscopy shows a specific mitochondrial localization of Nm23-H4 that was also demonstrated by Western blot analysis of subcellular fractions of these cells. Import into mitochondria is accompanied by cleavage of the N-terminal extension that results in NDP kinase activity. Submitochondrial fractionation indicates that Nm23-H4 is associated with mitochondrial membranes, possibly to the contact sites between the outer and inner membranes.

Catalytic mechanism of nucleoside diphosphate kinase investigated using nucleotide analogues, viscosity effects, and X-ray crystallography

Nucleoside diphosphate (NDP) kinases display low specificity with respect to the base moiety of the nucleotides and to the 2'-position of the ribose, but the 3'-hydroxyl is found to be important for catalysis. We report in this paper the enzymatic analysis of a series of derivatives of thymidine diphosphate (TDP) where the 3'-OH group was removed or replaced by fluorine, azido, and amino groups. With Dictyostelium NDP kinase, kcat decreases 15-200-fold from 1100 s-1 with TDP, and (kcat/Km)NDP decreases from 12 x 10(6) to 10(3) to 5 x 10(4) M-1 s-1, depending on the substrate. The poorest substrates are 3'-deoxyTDP and 3'-azido-3'-deoxyTDP, while the best modified substrates are 2',3'-dehydro-3'-deoxyTDP and 3'-fluoro-3'-deoxyTDP. In a similar way, 3'-fluoro-2',3'-dideoxyUDP was found to be a better substrate than 2',3'-dideoxyUDP, but a much poorer substrate than 2'-deoxyUDP. (kcat/Km)NDP is sensitive to the viscosity of the solution with TDP as the substrate but not with the modified substrates. To understand the poor catalytic efficiency of the modified nucleotides at a structural level, we determined the crystal structure of Dictyostelium NDP kinase complexed to 3'-fluoro-2',3'-dideoxyUDP at 2.7 A resolution. Significant differences are noted as compared to the TDP complex. Substrate-assisted catalysis by the 3'-OH, which is effective in the NDP kinase reaction, cannot occur with the modified substrate. With TDP, the beta-phosphate, which is the leaving group when a gamma-phosphate is transferred to His122, hydrogen bonds to the 3'-hydroxyl group of the sugar; with 3'-fluoro-2',3'-dideoxyUDP, the beta-phosphate hydrogen bonds to Asn119 and moves away from the attacking Ndelta of the catalytic His122. Since all anti-AIDS nucleoside drugs are modified at the 3'-position, these results are relevant to the role of NDP kinase in their cellular metabolism.

Substitution of an alanine residue for glycine 146 in TMP kinase from Escherichia coli is responsible for bacterial hypersensitivity to bromodeoxyuridine

The wild-type TMP kinases from Escherichia coli and from a strain hypersensitive to 5-bromo-2'-deoxyuridine were characterized comparatively. The mutation at codon 146 causes the substitution of an alanine residue for glycine in the enzyme, which is accompanied by changes in the relative affinities for 5-Br-UMP and TMP compared to those of the wild-type TMP kinase. Plasmids carrying the wild-type tmk gene from Escherichia coli or Bacillus subtilis, but not the defective tmk gene, restored the resistance to bromodeoxyuridine of an E. coli mutant strain.

Bovine serum fetuin is unfolded through a molten globule state

The reversible heat- and GuHCl-induced unfolding of bovine serum fetuin (BSF) has been studied by differential scanning calorimetry, circular dicroism, tryptophan fluorescence, and size-exclusion chromatography. We show here that thermal unfolding of BSF occurs in two distinct steps corresponding to transitions from the native (N) to an intermediate (I) and from the intermediate to the unfolded state (U). The Nleft and right arrow I transition is highly cooperative and can well be accounted for by a two-state mechanism. The Ileft and right arrow U transition is also cooperative but to a lesser extent than the Nleft and right arrow I transition. CD spectra show that the protein in the I state retains nearly all of the native secondary structure and has a largely disrupted tertiary structure. However, the hydrophobic environment of the single tryptophan residue is not changed, and some compactness is retained in the I state. The structural properties of this intermediate state are apparently characteristic of a molten globule. The GuHCl-induced unfolding is also a two-step process with an I state around 2 M GuHCl. Although the structural features of the denaturant-induced I state are somewhat different from those of the heat-induced I state, the unfolding free energies DeltaG degreesNleft and right arrow I and DeltaG degreesIleft and right arrow U as well as DeltaG degreesNleft and right arrow U obtained from these two methods are comparable. We argue that the observed two-state Nleft and right arrow I transition is due to the melting of the tertiary packing, while leaving quasi-intact the secondary structure and some long-range interactions in the I state. These long-range interactions, together with the secondary structural elements, would be responsible for the observed cooperativity of the Ileft and right arrow U transition.

A point mutation of human nucleoside diphosphate kinase A found in aggressive neuroblastoma affects protein folding

The point mutation serine 120 to glycine in the human nucleoside diphosphate kinase A has been identified in several aggressive neuroblastomas (Chang, C. L., Zhu, X. X., Thoraval, D. H., Ungar, D., Rawwas, J., Hora, N., Strahler, J. R., Hanash, S. M. & Radany, E. (1994) Nature 370, 335-336). We expressed in bacteria and purified wild-type and S120G mutant nucleoside diphosphate kinase A. The mutant enzyme had enzymatic and structural properties similar to the wild-type enzyme, whereas its stability to denaturation by heat and urea was markedly reduced. More importantly, upon renaturation of the urea-denatured mutant protein, a folding intermediate accumulated, having the characteristics of a molten globule. It had no tertiary structure, as shown by near UV circular dichroism, whereas the secondary structure was substantially recovered. The hydrophobic probe 8-anilino-1-naphthalene sulfonate bound to the intermediate species with an increase in fluorescence intensity and a blue shift. The hydrodynamic size was between that expected for a folded and an unfolded monomer. Finally, electrophoresis in a transverse urea gradient displayed no renaturation curve, and the protein showed the tendency to aggregate at the lowest urea concentrations. The existence of a molten globule folding intermediates resulting from an altered folding in the mutated protein might be related to the aggressiveness of neuroblastomas.

nm23-H4, a new member of the family of human nm23/nucleoside diphosphate kinase genes localised on chromosome 16p13

A novel human nm23/nucleoside diphosphate (NDP) kinase gene, called nm23-H4, was identified by screening a human stomach cDNA library with a probe generated by amplification by reverse transcription-polymerase chain reaction. The primers were designed from publicly available database cDNA sequences selected according to their homology to the human nn23-H1 putative metastasis suppressor gene. The full-length cDNA sequence predicts a 187 amino acid protein possessing the region homologous to NDP kinases with all residues crucial for nucleotide binding and catalysis, strongly suggesting that Nm23-H4 possesses NDP kinase activity. It shares 56, 55 and 60% identity with Nm23-H1, Nm23-H2 and DR-Nm23, respectively, the other human Nm23 proteins isolated so far. Compared with these proteins, Nm23-H4 contains an additional NH2-terminal region that is rich in positively charged residues and could indicate routing to mitochondria. The nm23-H4 gene has been localised to human chromosomal band 16p13.3. The corresponding 1.2 kb mRNA is widely distributed and expressed in a tissue-dependent manner, being found at very high levels in prostate, heart, liver, small intestine and skeletal muscle tissues and in low amounts in the brain and in blood leucocytes. Nm23-H4 naturally possesses the Pro-Ser substitution equivalent to the K-pn mutation (P97S) of Drosophila.

Phosphorylation of nucleoside diphosphate kinase at the active site studied by steady-state and time-resolved fluorescence

Nucleoside diphosphate (NDP) kinase is the enzyme responsible in the cell for the phosphorylation of nucleoside or deoxynucleoside diphosphates into the corresponding triphosphates at the expense of ATP. Transfer of the gamma-phosphate is very fast (turnover number above 1000 s-1) and involves the phosphorylation of a histidine residue at the active site of the enzyme. We have used intrinsic protein fluorescence of the single tryptophan of Dictyostelium discoideum NDP kinase as a sensitive probe for monitoring the interaction of the enzyme with its substrates. We demonstrate that the 20% quenching of steady-state fluorescence observed upon addition of ATP is due to formation of the phosphorylated intermediate. Time-resolved fluorescence indicates that the Trp-137 side chain is rigidly bound to the protein core with a unique lifetime of 4.5 ns for the free enzyme at 20 degrees C and that it remains tightly immobilized during the time course of the reaction. Phosphorylation of this catalytic residue (His-122) in the presence of ATP induces a similar decrease in mean lifetime, due to the splitting of the signal and the appearance of a shorter decay. This splitting is discussed in terms of a slow conformational equilibrium. We demonstrate that, in the wild-type enzyme, the conserved His-55 quenches the fluorescence of Trp-137 as the H55A mutant protein fluorescence displays an increase in quantum yield. Even though H55A mutant enzyme is active, the absence of the imidazole ring prevents the detection of the phosphorylated state of His-122 by Trp-137. We conclude that His-55 serves as a relay between His-122 and Trp-137.

Thermal stability of hexameric and tetrameric nucleoside diphosphate kinases. Effect of subunit interaction

The eukaryotic nucleoside diphosphate (NDP) kinases are hexamers, while the bacterial NDP kinases are tetramers made of small, single domain subunits. These enzymes represent an ideal model for studying the effect of subunit interaction on protein stability. The thermostability of NDP kinases of each class was studied by differential scanning calorimetry and biochemical methods. The hexameric NDP kinase from Dictyostelium discoideum displays one single, irreversible differential scanning calorimetry peak (Tm 62 degrees C) over a broad protein concentration, indicating a single step denaturation. The thermal stability of the protein was increased by ADP. The P105G substitution, which affects a loop implicated in subunit contacts, yields a protein that reversibly dissociates to folded monomers at 38 degrees C before the irreversible denaturation occurs (Tm 47 degrees C). ADP delays the dissociation, but does not change the Tm. These data indicate a "coupling" of the quaternary structure with the tertiary structure in the wild-type, but not in the mutated protein. We describe the x-ray structure of the P105G mutant at 2.2-A resolution. It is very similar to that of the wild-type protein. Therefore, a minimal change in the structure leads to a dramatic change of protein thermostability. The NDP kinase from Escherichia coli behaves like the P105G mutant of the Dictyostelium NDP kinase. The detailed study of their thermostability is important, since biological effects of thermolabile NDP kinases have been described in several organisms.

Expression of NM23 in human melanoma progression and metastasis

NM23 is a putative metastasis-suppressor gene for some human cancers. Here we have studied NM23 expression during melanoma progression using Northern blotting and immunocytochemistry. There was no significant difference in the average amounts of NM23 mRNA between cell lines derived from metastatic and primary melanomas. The level of NM23 mRNA was also determined for three pairs of poorly metastatic parental (P) and their highly metastatic variant (M) cell lines; the ratios for M/P were 1.2, 0.98 and 0.80. Next we used immunocytochemistry to study NM23 protein in normal skin, benign naevi and primary and metastatic melanomas. Melanocytes in all normal skin and benign samples were positive for NM23; however most primary melanomas (7/11) were not stained by the antibody. All metastatic melanoma samples (5/5) were positively stained. Findings were similar with an antiserum reactive with both forms of NM23 (H1 and H2), and with an antibody specific for NM23-H1. No relationship was apparent between NM23 immunoreactivity in primary tumours and their aggressiveness or prognosis. Hence, in contrast to the situation described for murine melanoma, the amount of NM23 mRNA or protein in human melanoma did not correlate inversely with metastasis.

Cellular phosphorylation of anti-HIV nucleosides. Role of nucleoside diphosphate kinase

Nucleotide analogs are widely used in antiviral therapy and particularly against AIDS. Delivered to the cell as nucleosides, they are phosphorylated into their active triphospho derivative form by cellular kinases from the host. The last step in this series of phosphorylations is performed by nucleoside diphosphate (NDP) kinase, an enzyme that can use both purine or pyrimidine and oxy- or deoxynucleotides as substrates. Using pure recombinant human NDP kinase type B (product of the gene nm23-H2), we have characterized the kinetic parameters of several nucleotide analogs for this enzyme. Contrary to what is generally assumed, diphospho- and triphospho- derivatives of azidothymidine as well as of dideoxyadenosine and dideoxythymidine are very poor substrates for NDP kinase. The rate of phosphorylation of these analogs varies between 0.05% and 0.5%, as compared to the corresponding natural nucleotide, a result that is not due to the inability of the analogs to bind to the enzyme. Using the data from the high resolution crystal structure of NDP kinase, we provide an interpretation of these results based on the crucial role played by the 3'-OH moiety of the nucleotide in catalysis.

Increased levels of nm23 H1/nucleoside diphosphate kinase A mRNA associated with adenocarcinoma of the prostate

Overexpression of the nm23H1 gene has been associated with the suppression of metastasis in several solid tumors. However, in colorectal carcinoma and neuroblastoma, increased levels of nm23 H1 nucleoside diphosphate kinase A (NDPKA) mRNA are associated with tumorigenesis. To determine the role of nm23 H1/NDPKA in the prostate, normal and/or malignant tissue samples from 29 consecutive patients were studied. Levels of nm23 H1/NDPKA mRNA and nm23 H1/NDPKA mRNA protein were determined in tissue from 18 and 27 patients, respectively. In all, 16 of the 18 tumor samples expressed increased levels of nm23 H1/NDPKA mRNA as compared with those measured in normal tissue. The level of nm23 H1/NDPKA mRNA was > 10-fold higher in a metastatic lymph node than in normal prostate tissue. All cancer specimens and areas of prostatic intraepithelial neoplasia showed immunoreactivity with the nm23 H1/NDPKA antibody; however, normal prostatic tissue was unreactive. These findings suggest that overexpression of the nm23 H1/NDPKA gene occurs frequently in adeno-carcinomas of the prostate and may be an early event in prostate cancer tumorigenesis.

Overexpression of nm23-H1 and nm23-H2 genes in colorectal carcinomas and loss of nm23-H1 expression in advanced tumour stages

Although a reduced expression of nm23 has been shown to correlate with a high metastatic potential in some human cancers, in colorectal cancers, conflicting data have been reported. As there are two homologous genes, nm23-H1 and nm23-H2, which encode the A and B subunits of nucleoside diphosphate kinase, efficient and simplified techniques were designed to selectively study nm23-H1 and -H2 expression in 35 colorectal cancers at both the protein and mRNA levels by immunoblotting, immunohistochemistry, and reverse transcription polymerase chain reaction (RT PCR) using specific antibodies and primers. Nm23-H1 and Nm23-H2 proteins were overexpressed in tumours compared with adjacent mucosa. This overexpression was lost, however, in some advanced cases: 89% and 81% of TNM (tumour, node, metastases) stages 0-II showed Nm23-H1 and -H2 overexpression, respectively, which significantly differed from 47% and 38% of stage III-IV tumours. Similar results were seen with nm23-H1 mRNA. Heterogenous labelling of tumoral cells was seen by immunohistological staining. This suggests a dichotomy: an overexpression of nm23-H1 and -H2 linked to early stages of cancer and a loss of nm23-H1 overexpression seen in more advanced stages. Therefore specific nm23-H1 determination should be evaluated as a prognostic factor in human colorectal carcinoma.

Phosphorylation mechanism of nucleoside diphosphate kinase: 31P-nuclear magnetic resonance studies

The phosphorylation mechanism of Dictyostelium discoideum nucleoside diphosphate (NDP) kinase was investigated by NMR. 31P chemical shifts were measured on both native and denatured enzyme. In the enzymatically phosphorylated enzyme denatured by 9 M urea or 7 M guanidine hydrochloride, the NDP kinase phosphohistidine signal appeared between the signals of N delta and N epsilon free monophosphohistidines used as reference compounds and added to the sample. A signal with the same intermediate position was also observed in the pronase digest of the alkaline-denatured phosphorylated enzyme. However, when phosphohistidines of the phosphorylated synthetic peptide pGlu-His-Gly were taken as references, the NDP kinase and the N delta peptide phosphohistidine signals were shown to be identical, providing evidence that phosphorylation occurs on the N delta of the active site histidine residue. Moreover, the rate of hydrolysis of the histidine-bound phosphate is in agreement with a modification at the N delta position. Phosphorylation of the NDP kinase by phosphoramidate provided a result similar to that of the enzymatic phosphorylation. In both cases, phosphorylation could not be detected on any amino acid other than histidine. Particularly, no phosphoserine residue was observed.

Mechanism of phosphate transfer by nucleoside diphosphate kinase: X-ray structures of the phosphohistidine intermediate of the enzymes from Drosophila and Dictyostelium

Nucleoside diphosphate kinase (NDP kinase) has a ping-pong mechanism with a phosphohistidine intermediate. Crystals of the enzymes from Dictyostelium discoideum and from Drosophila melanogaster were treated with phosphoramidate, and their X-ray structures were determined at 2.1 and 2.2 A resolution, respectively. The atomic models, refined to R factors below 20%, show no conformation change relative to the free proteins. In both enzymes, the active site histidine was phosphorylated on N delta, and it was the only site of phosphorylation. The phosphate group interacts with the hydroxyl group of Tyr56 and with protein-bound water molecules. Its environment is compared with that of phosphohistidines in succinyl-CoA synthetase and in phosphocarrier proteins. The X-ray structures of phosphorylated NDP kinase and of previously determined complexes with nucleoside diphosphates provide a basis for modeling the Michaelis complex with a nucleoside triphosphate, that of the phosphorylated protein with a nucleoside diphosphate, and the transition state of the phosphate transfer reaction in which the gamma-phosphate is pentacoordinated.

Refined X-ray structure of Dictyostelium discoideum nucleoside diphosphate kinase at 1.8 A resolution

The X-ray structure of the nucleoside diphosphate kinase (NDP kinase) from Dictyostelium discoideum has been refined at 1.8 A resolution from a hexagonal crystal form with a 17 kDa monomer in its asymmetric unit. The atomic model was derived from the previously determined structure of a point mutant of the protein. It contains 150 amino acid residues out of 155, and 95 solvent molecules. The R-factor is 0.196 and the estimated accuracy of the average atomic position, 0.25 A. The Dictyostelium structure is described in detail and compared to those of Drosophila and Myxococcus xanthus NDP kinases. The protein is a hexamer with D3 symmetry. Residues 8 to 138 of each subunit form a globular alpha/beta domain. The four-stranded beta-sheet is antiparallel; its topology is different from other phosphate transfer enzymes, and also from the HPr protein which, like NDP kinase, carries a phosphorylated histidine. The same topology is nevertheless found in several other proteins that bind mononucleotides, RNA or DNA. Strand connections in NDP kinase involve alpha-helices and a 20-residue segment called the Kpn loop. The beta-sheet is regular except for a beta-bulge in edge strand beta 2 and a gamma-turn at residue Ile120 just preceding strand beta 4. The latter may induce strain in the main chain near the active site His122. The alpha 1 beta 2 motif participates in forming dimers within the hexamer, helices alpha 1 and alpha 3, the Kpn loop and C terminus, in forming trimers. The subunit fold and dimer interactions found in Dictyostelium are conserved in other NDP kinases. Trimer interactions probably occur in all eukaryotic enzymes. They are absent in the bacterial Myxococcus xanthus enzyme which is a tetramer, even though the subunit structure is very similar. In Dictyostelium, contacts between Kpn loops near the 3-fold axis block access to a central cavity lined with polar residues and filled with well-defined solvent molecules. Biochemical data on point mutants highlight the contribution of the Kpn loop to protein stability. In Myxococcus, the Kpn loops are on the tetramer surface and their sequence is poorly conserved. Yet, their conformation is maintained and they make a similar contribution to the substrate binding site.

X-ray structure of nucleoside diphosphate kinase complexed with thymidine diphosphate and Mg2+ at 2-A resolution

We report the crystal structure of nucleoside diphosphate kinase (NDP kinase) from Dictyostelium discoideum with thymidine diphosphate (dTDP) and Mg2+ bound at the active site. The structure has been refined to an R-factor of 18.3% at 2-A resolution. The base stacks on the aromatic ring of Phe 64 near the protein surface and is wedged between the side chains of Phe 64 and Val 116. The sugar and the pyrophosphate are deeper inside the protein and make numerous H-bonds with protein side chains. There is no backbone interaction with the nucleotide. A Mg2+ ion bridges the alpha- and beta-phosphates and interacts with the protein via water molecules. NDP kinase shows little specificity toward ribonucleotides and deoxyribonucleotides. This property, required by the enzyme biological function, can now be analyzed by comparing the crystal structures of free, ADP-ligated, and dTDP-ligated enzymes. The most significant differences are located in residues 60-64, which adapt their conformation to allow Phe 64 to stack on both types of bases. Nonspecific binding is achieved by the absence of polar interaction between the base and protein atoms. The ribose of ADP and the deoxyribose of dTDP occupy similar positions, their hydroxyl groups interacting with Lys 16 and Asn 119. The H-bond between Lys 16 and the O2' hydroxyl of ADP is replaced by a similar interaction with a water molecule in the dTDP complex. The beta-phosphate position is the same for ADP and dTDP, suggesting that the mechanism of phosphate transfer is the same for all substrates ofNDP kinase.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorylation of nm23/nucleoside diphosphate kinase by casein kinase 2 in vitro

We have investigated phosphorylation of human nucleoside diphosphate kinase (NDPK) and of homologous NDPK from different species by human casein kinase 2 (CK-2). The human NDPK isotypes A and B were phosphorylated by CK-2 in vitro both when the purified proteins and total lysate of HL-60 leukemia cells were used. The homologous NDPK's from Yeast and E. coli were also substrates for CK-2 in vitro, but not Drosophila NDPK. Phosphorylation of all NDPK types by the CK-2 holoenzyme was entirely polyamine-dependent. The CK-2 phosphorylation site in human NDPK A, that was about 2.5 times stronger phosphorylated than was the B isotype, was tentatively assigned to Ser-122. The location of the corresponding residue in the 3D-structure of the 80% homologous Drosophila NDPK suggests that its phosphorylation may directly influence substrate binding and/or catalysis.

Adenosine 5'-diphosphate binding and the active site of nucleoside diphosphate kinase

The X-ray structure of nucleoside diphosphate kinase (NDP kinase) from the slime mold Dictyostelium discoideum has been determined to 2.2-A resolution and refined to an R-factor of 0.19 with and without bound ADP-Mg2+. The nucleotide binds near His 122, a residue which becomes phosphorylated during the catalytic cycle. The mode of binding is different from that observed in other phosphokinases, and it involves no glycine-rich sequence. The adenine base makes only nonpolar contacts with the protein. It points outside, explaining the lack of specificity of NDP kinase toward the base. The ribose 2'- and 3'-hydroxyls and the pyrophosphate moiety are H-bonded to polar side chains. A Mg2+ ion bridges the alpha- to the beta-phosphate which approaches the imidazole group of His 122 from the N delta side. The geometry at the active site in the ADP-Mg2+ complex suggests a mechanism for catalysis whereby the gamma-phosphate of a nucleoside triphosphate can be transferred onto His 122 with a minimum of atomic motion.

NDP-K/nm23 expression in human breast cancer in relation to relapse, survival, and other prognostic factors: an immunohistochemical study

The nm23 gene was originally identified by Steeg et al. by screening of cDNA libraries from murine melanoma cell lines of varying metastatic potential. An inverse relationship between metastatic potential and nm23 RNA and/or protein was found in four different metastatic model systems. It was proposed that nm23 may function as a suppressor gene for tumour metastasis. It has recently been found that the sequence of nm23 and NDP-kinase (NDP-K) is identical. Using an immunohistochemical technique and employing a polyclonal antibody to purified NDP-K A, we have determined NDP-K expression in a series of 197 breast carcinomas. One hundred and sixty (81.2 per cent) of these tumours were scored positive for NDP-K and 37 (18.8 per cent) scored negative. No relationship was found between NDP-K/nm23 expression and patient relapse or survival. Furthermore, no relationship was found between NDP-K/nm23 expression and a number of other prognostic factors including tumour grade, oestrogen receptor, progesterone receptor, and p53 expression. Our results contradict the hypothesis concerning the possible role of NDP-K/nm23 as a metastatic suppressor gene in human breast cancer, but further studies using antibodies specific for NDP-K/nm23 subtypes are clearly indicated.

Crystal structure of the Awd nucleotide diphosphate kinase from Drosophila

BACKGROUND

Nucleotide diphosphate kinase (NDP kinase) is a phosphate transfer enzyme involved in cell regulation and in animal development. Drosophila NDP kinase is the product of the abnormal wing disc (awd) developmental gene, a point mutation in which can produce the killer of prune (K-pn) conditional lethal phenotype. The highly homologous mammalian genes control metastasis and a human NDP kinase acts as a transcription factor.

RESULTS

The X-ray structure of the Awd protein prepared from Drosophila was solved at 2.4 A resolution by molecular replacement from the homologous Dictyostelium protein. Both are hexamers, and both have the same fold and the same active site. Subunit contacts differ as a result of sequence changes in the carboxy-terminal segment and in the loop that is the site of the K-pn mutation.

CONCLUSIONS

Regulatory properties of animal NDP kinases depend on interactions with other macromolecules, such as DNA and the product of the Drosophila prune gene. The Awd structure suggests an allosteric mechanism of action of NDP kinase where DNA is the effector and the protein undergoes a major conformational change, possibly dissociating to dimers.

Separate nuclear genes encode cytosolic and mitochondrial nucleoside diphosphate kinase in Dictyostelium discoideum

We have previously isolated cDNA clones for the gip17 gene encoding the cytosolic nucleoside diphosphate (NDP) kinase from Dictyostelium discoideum, and partial cDNAs for guk, a second member of the NDP kinase gene family (Wallet, V., Mutzel, R., Troll, H., Barzu, O., Wurster, B., Véron, M., and Lacombe, M. L. (1990) J. Natl. Cancer Inst. 80, 1199-1202). We now characterize genomic DNA clones for both NDP kinase genes, and we show that guk defines a nuclear-encoded mitochondrial NDP kinase. Isolated D. discoideum mitochondria contain 3% of the total cellular NDP kinase activity. Antibodies which specifically recognize and inhibit the activity of either cytosolic or mitochondrial NDP kinase unambiguously distinguish between these activities. The nascent mitochondrial NDP kinase contains a presequence of 57 amino acids that is removed during import into the organelle as shown by determination of the NH2 terminus of the mature protein from mitochondria. The genes for mitochondrial and cytosolic NDP kinases contain four and two introns, respectively. The positions of the of the introns in the gene for the cytosolic enzyme match exactly the positions of the second and fourth introns in the coding region of its mitochondrial homologue. From these results we conclude that the isozymes diverged from a common ancestor, and we discuss possible phylogenetic pathways for the evolution of cytosolic and organelle NDP kinases.

Equilibrium dissociation and unfolding of nucleoside diphosphate kinase from Dictyostelium discoideum. Role of proline 100 in the stability of the hexameric enzyme

The Killer-of-prune (K-pn) mutation in Drosophila corresponds to a Pro-Ser substitution in nucleoside diphosphate kinase (Lascu, I. Chaffotte, A., Limbourg-Bouchon, B., and Véron, M. (1992) J. Biol. Chem. 267, 12775-12781). We investigated the role of the equivalent proline (Pro100) in the formation and stability of the Dictyostelium nucleoside diphosphate kinase hexamers. Mutations to serine or glycine had only little effect on the properties of the native enzyme. However, the mutant drastically affected the subunit interaction in the hexamer and the ability of the isolated subunits to associate in vitro. While the wild-type hexamer inactivated and unfolded concomitantly at 5-6 M urea, the mutant proteins dissociated to monomers at 0.5-2 M urea and unfolded at 2.5-4 M urea. At intermediate urea concentrations, the unique species present in solution was a folded, partially active monomer as shown by size-exclusion chromatography, UV, fluorescence, and CD spectroscopy. Proline 100 is located in a loop involved in subunits contact. Altered conformation of the loop in P100S and P100S mutants demonstrates its crucial role in subunit assembly. We propose to explain the conditional dominance of the K-pn mutation by the presence of a monomeric form of the enzyme that would have deleterious effects in vivo.

Assay of nucleoside diphosphate kinase in microtiter plates using a peroxidase-coupled method

A sensitive assay for nucleoside diphosphate kinase which utilizes microtiter plates is described. ATP, formed in the reaction between dGTP and ADP, is trapped by the glycerol kinase reaction. A colored compound is generated by glycerol-3-phosphate oxidase and peroxidase. This assay is useful for testing a large number of samples generated by chromatographic techniques or for screening purposes.

X-ray structure of nucleoside diphosphate kinase

The X-ray structure of a point mutant of nucleoside diphosphate kinase (NDP kinase) from Dictyostelium discoideum has been determined to 2.2 A resolution. The enzyme is a hexamer made of identical subunits with a novel mononucleotide binding fold. Each subunit contains an alpha/beta domain with a four stranded, antiparallel beta-sheet. The topology is different from adenylate kinase, but identical to the allosteric domain of Escherichia coli ATCase regulatory subunits, which bind mononucleotides at an equivalent position. Dimer contacts between NDP kinase subunits within the hexamer are similar to those in ATCase. Trimer contacts involve a large loop of polypeptide chain that bears the site of the Pro----Ser substitution in Killer of prune (K-pn) mutants of the highly homologous Drosophila enzyme. Properties of Drosophila NDP kinase, the product of the awd developmental gene, and of the human enzyme, the product of the nm23 genes in tumorigenesis, are discussed in view of the three-dimensional structure and of possible interactions of NDP kinase with other nucleotide binding proteins.

Nucleoside diphosphate kinase does not directly interact with tubulin nor microtubules

Nucleoside diphosphate kinase has been shown to play a role in proliferation and development. Microtubules have been evoked as a possible target of NDP kinase action; in particular it was proposed that NDP kinase could regulate the cellular pool of polymerizable GTP-tubulin by direct phosphorylation of tubulin bound GDP. We show that this reaction does not occur in vitro and also that NDP kinase does not bind to microtubules both in the presence and absence of MAPs. Thus, any possible physiological effect of NDP kinase on microtubule dynamics is exerted only by modulating the concentrations of free guanine nucleotides in the vicinity of microtubules.

A Pro/Ser substitution in nucleoside diphosphate kinase of Drosophila melanogaster (mutation killer of prune) affects stability but not catalytic efficiency of the enzyme

Nucleoside diphosphate kinase of Drosophila, recently identified as the product of the awd gene, is essential for larval development. The conditional lethal mutation Killer of prune maps to the same gene. We purified the nucleoside diphosphate kinases from wild-type and mutant larvae by a simple procedure involving affinity chromatography on blue Sepharose. Both proteins are purified as hexamers in their native state. The mutant protein, which carries a serine instead of proline at position 97, has structural properties and catalytic efficiency that are very similar to the wild-type protein. However, the mutant protein has a much lower stability to denaturation by heat and urea. Following dilution of urea with buffer the urea-denaturated mutant nucleoside diphosphate kinase accumulates as folded monomers and cannot recover its quaternary structure and enzymatic activity. In contrast, the wild-type enzyme recovers hexameric structure and activity. This suggests that the mutation affects the folding/assembly pathway without affecting the function of the mature protein once folded and assembled into the mature hexameric structure.

Proliferation-related expression of p19/nm23 nucleoside diphosphate kinase

High level expression of the nm23-H1 gene, which encodes for a nucleoside diphosphate kinase, has been found to correlate with diminished metastasis in some tumors but not in others. We have previously identified the protein product of the nm23-H1 gene in two-dimensional electrophoretic gels and have designated it p19/nm23. In neuroblastoma, higher levels of p19/nm23, which are associated with amplification of the N-myc oncogene, large tumor mass, and metastasis, were observed in advanced stage tumors compared with limited stage disease. Because of the variable expression of nm23-H1 in different tumors, we have investigated the relationship between amounts of the protein and cell proliferation. The levels of p19/nm23 were compared between resting and mitotically stimulated normal human PBLs and in leukemia cells. The amount of p19/nm23 increased in normal lymphocytes in response to mitotic stimulation and paralleled the increase in DNA synthesis. In leukemia cells obtained from patients with different subtypes of acute leukemia, p19/nm23 levels were also increased relative to resting normal lymphocytes. Treatment of mitotically stimulated lymphocytes with cyclosporin, which inhibits proliferation, blocked the increase in p19/nm23; treatment of the leukemia cell line HL-60 with dimethylsulfoxide, which induces terminal differentiation, resulted in diminished levels of p19/nm23. Our data therefore provide evidence that nm23-H1 expression is related to cell proliferative activity.

Overexpression of nucleoside diphosphate kinase (Nm23) in solid tumours

The product of the nm23-H1 gene, reported to be related to the metastatic potential of tumour cells, was recently identified as the nucleoside diphosphate (NDP) kinase A (Gilles et al., 1991, J Biol Chem, 266, 8784-8789). An analysis of the enzyme by activity measurement and immunological techniques using polyclonal antibodies raised against the NDP kinase A purified from human erythrocytes, was performed on 39 human tissue specimens. Markedly increased activity and higher level of the protein were observed in extracts of solid tumours as compared to the corresponding normal tissues (P less than 0.01). An intense immunolabelling of tumoral cells was observed in sections of the malignant tumours and of some but not all benign neoplasia. The staining is observed in noninvasive and invasive ductal breast carcinomas with or without lymph node involvement as well as in colon and cervix carcinomas and in a case of metastatic melanoma. Therefore, NDP kinase A level is increased in neoplastic tissues but no correlation with metastatic potential could be demonstrated.

Nucleoside diphosphate kinase from human erythrocytes. Structural characterization of the two polypeptide chains responsible for heterogeneity of the hexameric enzyme

Human erythrocyte nucleoside-diphosphate kinase (NDP kinase) is a hexameric enzyme consisting of two kinds of polypeptide chains, A and B. By random association (A6, A5B...AB5, B6) these polypeptides form isoenzymes differing in their isoelectric point. Chains A and B of NDP kinase were purified by ion-exchange chromatography under denaturing conditions. Upon mixing and renaturation, the isozymic pattern of NDP kinase obtained by conventional methods was restored. Antibodies raised against purified chains showed significant cross-reactivity, both in immunoblot experiments and activity inhibition studies. Sequence determination showed that both chains consisted of 152 amino acid residues corresponding to Mr or 17,143 (chain A) and 17,294 (chain B), respectively. There was high homology between the two sequences (88% identity). The phosphorylation site on the enzyme is located at His-118. Chain A was identical with human Nm23 protein, which has been reported as a potential suppressor protein in tumor metastasis and chain B was identical with Nm23-H2 protein.

Nucleoside diphosphate kinase from human erythrocytes: purification, molecular mass and subunit structure

A new procedure for the purification of nucleoside diphosphate kinase from human erythrocytes is described. The enzyme (105 kDa by gel filtration) is made-up of two different kinds of subunits (19.0 and 20.5 kDa), both displaying enzymatic activity. The probable subunit structure of the enzyme is hexameric. The discrepancies related to earlier work are discussed.

Platelet aggregation is inhibited by long chain acyl-CoA

Oleoyl coenzyme A and other acyl-CoA derivatives inhibited ADP or thrombin-induced aggregation of platelets. Arachidonic acid-induced aggregation was also inhibited, but not the slower aggregation caused by 1-oleoyl-2-acetylglycerol or tetradecanoyl-phorbol-13-acetate. Coenzyme A and free fatty acids had little or no effect, and transfer of labeled oleate from oleoyl Co-A to other lipid classes was not detected. Because acyl Co-A compounds have recently been shown to modulate protein kinase C activity, acyl Co-A may provide a useful tool for investigating activation sequences in platelets and other membranes.

Clinical studies on plasma antithrombin III using an antiserum prepared in our laboratories

Using an antiserum obtained in rabbits injected with antithrombin III purified in our laboratories, the plasma level of this inhibitor of coagulation was found to be significantly (p less than 0.001) decreased in 8 patients with decompensated cirrhosis of the liver (15.66 mg/dl +/- 2.22) and in 18 nephrotic patients (21.74 mg/dl +/- 1.75). When compared to values noted in 22 normal-weight normolipidemic control subjects (31.54 mg/dl +/- 0.81), the plasma level of antithrombin III was slightly (p less than 0.05) higher in the 18 hyperlipidemic subjects (36.11 mg/dl +/- 1.9). Owing to an extremely wide dispersion of individual values, the mean level of antithrombin III noted in 16 critically ill postoperative patients (28.16 mg/dl +/- 2.98) did not significantly differ from control values, although this protease inhibitor was obviously decreased in those postoperative patients who developed multiple organ failure or disseminated intravascular-coagulation. Among the 11 patients with recurrent venous thrombosis, two subjects with familial deficiency in antithrombin III were detected. The results provide evidence that the antiserum prepared in our laboratories is a useful tool for detecting acquired and inherited antithrombin III deficiencies.

Structural and catalytic properties of L-alanine dehydrogenase from Bacillus cereus

Alanine dehydrogenase from Bacillus cereus, a non-allosteric enzyme composed of six identical subunits, was purified to homogeneity by chromatography on blue-Sepharose and Sepharose 6B-CL. Like other pyridine-linked dehydrogenases, alanine dehydrogenase is inhibited by Cibacron blue, competitively with respect to NADH and noncompetitively with respect to pyruvate. The enzyme was inactivated by 0.1 M glycine/HCl (pH 2) and reactivated by 0.1 M phosphate (pH 8) supplemented with NAD+ or NADH. The reactivation was characterized by sigmoidal kinetics indicating a complex mechanism involving rate-limiting folding and association steps. Cibacron blue interfered with renaturation, presumably by competition with NADH. Chromatography on Sepharose 6B-CL of the partially renatured alanine dehydrogenase led to the separation of several intermediates, but only the hexamer was characterized by enzymatic activity. By immobilization on Sepharose 4B, alanine dehydrogenase from B. cereus retained 66% of the specific activity of the soluble enzyme. After denaturation of immobilized alanine dehydrogenase with 7 M urea, 37% of the initial protein was still bound to Sepharose, indicating that on the average the hexamer was attached to the matrix via, at most, two subunits. The ability of the denatured, immobilized subunits to pick up subunits from solution shows their capacity to fold back to the native conformation after urea treatment. The formation of "hybrids" between subunits of enzyme from B. cereus and Bacillus subtilis demonstrates the close resemblance of the tertiary and quaternary structures of alanine dehydrogenases from these species.