Refolding and reactivation of calf intestinal alkaline phosphatase with excess magnesium ions

It is well known that Mg(2+) is an essential component in many biological processes. This research investigated the courses of both the reactivation and the refolding in the absence and presence of Mg(2+) ions. Calf intestinal alkaline phosphatase (CIP) was extensively denatured in 3 M guanidine hydrochloride (GdnHCl) solution for 2 h. Under suitable renaturation conditions, about 60-70% of the activity was recovered in the absence and presence of different magnesium ion concentrations. The refolding processes followed two-phase courses, whereas the reactivation processes were monophasic after dilution in proper solutions with or without Mg(2+). The magnesium ions affected both the reactivation and the refolding courses of unfolded CIP. A comparison of rate constants for the refolding of unfolded CIP with those for recovery of enzyme activity at different Mg(2+) concentrations showed that they were not synchronized. The activity recovery was speeded up due to the presence of Mg(2+) ions; while the refolding course of unfolded CIP was somewhat inhibited by the excess Mg(2+).     

Effect of metal ions on the secondary structure and activity of calf intestine phosphatase

Cobalt is an essential microelements in many biological processes involving enzymatic activity. We found that Zn2+ and Mg2+, which are in the active site of native calf intestine alkaline phosphatase (CIP), can be replaced by Co2+ directly in solution. The effect of Co2+ concentration on the substitution reaction was examined at ratios of [Co2+]/[CIP] from 0:1 to 8:1. The quantity of Zn2+ in CIP decreased progressively as the ratio was increased, but the amount of Mg2+ changed in irrregular fashion. A series of active site models of the reaction mechanism of CIP are proposed. Low pH was found to promote the replacement of Mg2+ by Co2+. To understand how the substitution affects the enzyme, we also solved the secondary structure of CIP after reaction with Co2+ in different conditions.     

Engineered allosteric ribozymes that respond to specific divalent metal ions

In vitro selection was used to isolate five classes of allosteric hammerhead ribozymes that are triggered by binding to certain divalent metal ion effectors. Each of these ribozyme classes are similarly activated by Mn2+, Fe2+, Co2+, Ni2+, Zn2+ and Cd2+, but their allosteric binding sites reject other divalent metals such as Mg2+, Ca2+ and Sr2+. Through a more comprehensive survey of cations, it was determined that some metal ions (Be2+, Fe3+, Al3+, Ru2+ and Dy2+) are extraordinarily disruptive to the RNA structure and function. Two classes of RNAs examined in greater detail make use of conserved nucleotides within the large internal bulges to form critical structures for allosteric function. One of these classes exhibits a metal-dependent increase in rate constant that indicates a requirement for the binding of two cation effectors. Additional findings suggest that, although complex allosteric functions can be exhibited by small RNAs, larger RNA molecules will probably be required to form binding pockets that are uniquely selective for individual cation effectors.     

Capacity of the outer membrane of a gram-negative marine bacterium in the presence of cations to prevent lysis by Triton X-100.

Cells of marine pseudomonad B-16 (ATCC 19855) washed with a solution containing 0.3 M NaCl, 50 mM MgCl2, and 10 mM KCl (complete salts) could be protected from lysis in a hypotonic environment if the suspending medium contained either 20 mM Mg2+, 40 mM Na+, or 300 mM K+. When the outer double-track layer (the outer membrane) of the cell envelope was removed to yield mureinoplasts, the Mg2+, Na+ or K+, requirements to prevent lysis were raised to 80, 210, and 400 mM, respectively. In the presence of 0.1% Triton X-100, 220, 320, and 360 mM Mg2+, Na+ or K+, respectively, prevented lysis of the normal cells. Mureinoplasts and protoplasts, however, lysed instantly in the presence of the detergent at all concentrations of Mg2+, Na+, or K+ tested up to 1.2 M. Thus, the structure of the outer membrane appears to be maintained by appropriate concentrations of Mg2+ or Na+ in a form preventing the penetration of Triton X-100 and thereby protecting the cytoplasmic membrane from dissolution by the detergent. K+ was effective in this capacity with cells washed with complete salts solution but not with cells washed with a solution of NaCl, suggesting that bound Mg2+ was required in the cell wall membrane for K+ to be effective in preventing lysis by the detergent. At high concentrations (1 M) K+ and Mg2+, but not Na+, appeared to destabilize the structure of the outer membrane in the presence of Triton X-100.     

Nucleotides and divalent cations as effectors and modulators of exocytosis in permeabilized rat mast cells.

The idea that the universal trigger to exocytosis (the terminal step in the secretory process) is an elevation of the cytosol concentration of Ca2+, and that it is dependent on ATP, is no longer tenable. Working with streptolysin-O-permeabilized mast cells (and other myeloid cells) we have shown that non-hydrolysable analogues of GTP can stimulate exocytosis after depletion of Ca2+ (i.e. at concentrations below 10(-9) M) and ATP. Such Ca2+- and ATP-independent exocytosis is strongly dependent on the presence of Mg2+, and the requirement for Mg2+ declines as the concentration of Ca2+ is brought up to 10(-7) M. We argue that Ca2+ serves to regulate the binding of guanine nucleotides to GE, a GTP-binding protein that regulates exocytosis through its interaction with CE, a calcium-binding protein which serves as an intracellular pseudo-receptor. The onset of exocytosis, following provision of Ca2+ and guanine nucleotides to the permeabilized cells, is preceded by delays which are sensitive to the order of provision of the two effectors (i.e. Ca2+ and guanine nucleotides), the presence or absence of Mg2+, and the identity of the activating guanine nucleotide. In view of the similarity of these features with the activation kinetics of adenylyl cyclase, we argue that GE behaves as a member of the heterotrimeric class of signal transducing G-proteins such as GS.     

Studies on the hysteretic properties of chloroplast fructose-1,6-bisphosphatase

Chloroplast fructose-1,6-bisphosphatase hysteresis in response to modifiers was uncovered by carrying out the enzyme assays in two consecutive steps. The activity of chloroplast fructose-1,6-bisphosphatase, assayed at low concentrations of both fructose-1,6-bisphosphatase and Mg2+, was enhanced by preincubating the enzyme with dithiothreitol, thioredoxin f, fructose 1,6-bisphosphate, and Ca2+. In the time-dependent activation process, fructose 1,6-bisphosphate and Ca2+ could be replaced by other sugar biphosphates and Mn2+, respectively. Once activated, chloroplast fructose-1,6-bisphosphatase hydrolyzed fructose 1,6-bisphosphate and sedoheptulose 1,7-bisphosphate in the presence of Mg2+, Mn2+, or Fe2+. The A0.5 for fructose 1,6-bisphosphate (activator) was lowered by reduced thioredoxin f and remained unchanged when Mg2+ was varied during the assay of activity. On the contrary, the S0.5 for fructose 1,6-bisphosphate (substrate) was unaffected by reduced thioredoxin f and depended on the concentration of Mg2+. Ca2+ played a dual role on the activity of chloroplast fructose-1,6-bisphosphatase; it was a component of the concerted activation and an inhibitor in the catalytic step. Provided dithiothreitol was present, the activating effectors were not required to maintain the enzyme in the active form. Considered together these results strongly suggest that the regulation of fructose-1,6-bisphosphatase in chloroplast occurs at two different levels, the activation of the enzyme and the catalysis.     

Isolation and characterization of the Mg2(+)-ATPase from rabbit skeletal muscle sarcoplasmic reticulum membrane preparations

Preparations of sarcoplasmic reticulum vesicles, obtained according to the method of Eletr and Inesi (Biochim. Biophys. Acta (1972) 282, 174), contained both Mg2(+)-ATPase and Ca2+, Mg2(+)-ATPase activity. The two enzymes were solubilized by a mixture of digitonin and lysophosphatidylcholine and separated on a DEAE-cellulose column eluted with a discontinuous gradient of NaCl. The Mg2(+)-ATPase activity was eluted with 0.43 M NaCl. The Ca2+,Mg2(+)-ATPase was obtained by increasing the NaCl concentration of the elution medium to 0.40 M. The fraction eluted with 0.043 M NaCl was insensitive to micromolar concentrations of calcium, resistant to oligomycin, ouabain, orthovanadate and thiocyanate, and was inhibited by low concentrations of Triton X-100. The enzyme showed a single apparent Km for MgATP in the range of 0.2 mM and a Vm of 2.9 mumol Pi.min-1.mg-1 protein. Activity was maximal over a broad peak between pH 6.0-8.0. Hydrolysis of ATP was unaffected by dimethylsulfoxide concentrations up to 20% (v/v) and was inhibited at higher concentrations. The enzyme was not phosphorylated by either 32Pi or [gamma-32P]ATP at significant levels when compared with the Ca2+,Mg2(+)-ATPase in an EGTA-containing medium. The kinetic pattern of the Mg2(+)-ATPase was distinctly different from that of the Ca2+,Mg2(+)-ATPase under the same conditions. The fraction eluted from the DEAE-cellulose column was subjected to electrophoresis under non-denaturing conditions. Only one band with Mg2(+)-ATPase activity was detected. The Mg2(+)-ATPase migrated much slower than the Ca2+,Mg2(+)-ATPase under non-denaturing conditions, whereas both enzymes had a molecular mass of 105 kDa on SDS gel electrophoresis.     

Effects of zinc chloride on the hydrolysis of cyclic GMP and cyclic AMP by the activator-dependent cyclic nucleotide phosphodiesterase from bovine heart.

In the presence of 10 micrometer Ca2+ and 5 mM Mg2+ (or 0.25 mM Mg2+), the addition of 100 micrometer Zn2+, Ni2+, Co2+, Fe2+, Cu2+ or 1 mM Mn2+ resulted in varying degrees of stimulation or inhibition of 10(-6) M cyclic GMP and cyclic AMP hydrolysis by the activator-dependent cyclic nucleotide phosphodiesterase from bovine heart in the absence or presence of phosphodiesterase activator. The substrate specificity of the enzyme was altered under several conditions. The addition of Zn2+ in the presence of 5 mM Mg2+ and the absence of activator resulted in the stimulation of cyclic GMP hydrolysis over a narrow substrate range while reducing the V 65% due to a shift in the kinetics from non-linear with Mg2+ alone to linear in the presence of Zn2+ and Mg2+. Zn2+ inhibited the hydrolysis of cyclic GMP and cyclic AMP in the presence of activator with Ki values of 70 and 100 micrometer, respectively. Zn2+ inhibition was non-competitive with substrate, activator and Ca2+ but was competitive with Mg2+. In the presence of 10 micrometer Ca2+ and activator, a Ki of 15 micrometer for Zn2+ vs. Mg2+ was noted in the hydrolysis of 10(-6) M cyclic GMP. Several effects of Zn2+ are discussed which have been noted in other studies and might be due in part to changes in cyclic nucleotide levels following phosphodiesterase inhibition.     

The effects of storage of sarcoplasmic reticulum fragments on the Ca2+, Mg2+-ATPase.

The effects of K+ and Na+ on the Ca2+,Mg2+-ATPase of sarcoplasmic reticulum fragments (SRF) were investigated at 1 mM ATP. There was an alteration of the sensitivity of the ATPase to the monovalent cations during storage of the SRF preparation. The Ca2+, Mg2+-ATPase of freshly prepared SRF was slightly activated by 5-10 mM K+ and Na+. Mg2+-ATPase was inhibited by both the monovalent cations to the same extent, and this response to the ions was independent of the freshness of the preparations. After storage of SRF, however, the Ca2+,Mg2+-ATPase was markedly activated by higher concentrations of K+ and Na+ (0.2-0.3 M). K+ and Na+ reduced the Ca uptake at the steady state in freshly prepared SRF, but did not affect pre-steady state uptake. In the presence of oxalate, the rate of Ca accumulation both in fresh and stored preparations was activated by 0.1-0.2 M K+ and Na+. The Ca2+, mg2+-ATPase with oxalate, so-called "extra ATPase," showed the same response to the ions as did the activity without oxalate during storage.     

Mg2,Ca2+-ATPase activity of sarcolemmas of intestinal smooth muscle cells in the rabbit

Preparations of rabbit small intestine smooth muscle cell sarcolemma are capable of hydrolyzing ATP in the presence of millimolar concentrations of Mg2+ and Ca2+ and possess the activity of Mg2+,Ca2+-ATPase having a high affinity for Ca2+ (Km = 5.8 X 10(-6) M). The optimal conditions for the Mg2+,Ca2+-ATPase reaction were established. It was demonstrated that sarcolemmal preparations hydrolyze ATP, GTP, ITP and UTP almost at the same rates. The enzyme contains SH-groups that are unequally exposed to the water phase and are inhibited by 50% by p-chloromercurybenzoate and by 90% by dithionitrobenzoate. The Mg2+,Ca2+-ATPase activity is highly sensitive to oxytocin: at the concentration of 10(-7) MU/ml, the hormone completely inhibits the enzyme without affecting its Mg2+-, Ca2+- and Na+,K+-ATPase activities.     

The three conformations of the anticodon loop of yeast tRNA(Phe)

The complex conformational states of the anticodon loop of yeast tRNA(Phe) which we had previously studied with relaxation experiments by monitoring fluorescence of the naturally occurring Wye base, are analyzed using time and polarization resolved fluorescence measurements at varying counterion concentrations. Synchrotron radiation served as excitation for these experiments, which were analyzed using modulating functions and global methods. Three conformations of the anticodon loop are detected, all three occurring in a wide range of counterion concentrations with and without Mg2+, each being identified by its typical lifetime. The fluorescence changes brought about by varying the ion concentrations, previously monitored by steady state fluorimetry and relaxation methods, are changes in the population of these three conformational states, in the sense of an allosteric model, where the effectors are the three ions Mg2+, Na+ and H+. The population of the highly fluorescent M conformer (8ns), most affine to magnesium, is thus enhanced by that ligand, while the total fluorescence decreases as lower pH favors the H+-affine H conformer (0.6ns). Na+-binding of the N conformer (4ns) is responsible for complex fluorescence changes. By iterative simulation of this allosteric model the equilibrium and binding constants are determined. In turn, using these constants to simulate equilibrium fluorescence titrations reproduces the published results.     

Characterization of enzymes of the branched-chain amino acid biosynthetic pathway in Methanococcus spp.

Methanococcus aeolicus, Methanococcus maripaludis, and Methanococcus voltae contain similar levels of four enzymes of branched-chain amino acid biosynthesis: acetohydroxy acid synthase, acetohydroxy acid isomeroreductase, dihydroxy acid dehydratase, and transaminase B. Following growth at low partial pressures of H2-CO2, the levels of these enzymes in extracts of M. voltae are reduced three- to fivefold, which suggests that their synthesis is regulated. The enzymes from M. aeolicus were found to be similar to the eubacterial and eucaryotic enzymes with respect to molecular weights, pH optima, kinetic properties, and sensitivities to O2. The acetohydroxy acid isomeroreductase has a specific requirement for Mg2+, and other divalent cations were inhibitory. It was stimulated threefold by K+ and NH4+ ions and was able to utilize NADH as well as NADPH. The partially purified enzyme was not sensitive to O2. The dihydroxy acid dehydratase is extremely sensitive to O2, and it has a half-life under 5% O2 of 6 min at 25 degrees C. Divalent cations were required for activity, and Mg2+, Mn2+, Ni2+, Co2+, and Fe2+ were nearly equally effective. In conclusion, the archaebacterial enzymes are functionally homologous to the eubacterial and eucaryotic enzymes, which implies that this pathway is very ancient.     

Conformational changes in Mycobacterium smegmatis glutamine synthetase induced by certain divalent cations

Manganese ion, like Mg2+, has been found to produce high biosynthetic activity of the unadenylylated form of glutamine synthetase obtained from Mycobacterium smegmatis, and the activity with each of these cations was decreased by the adenylylation of the enzyme. Further, the gamma-glutamyltransferase reaction was catalyzed in the presence of either Mn2+, Mg2+, or Co2+ with both unadenylylated and adenylylated enzyme; however, each of these divalent cation-dependent activities was also decreased by one order of magnitude by adenylylation of the enzyme. From studies of UV-difference spectra, it was found that the ability of M. smegmatis glutamine synthetase to assume a number of distinctly different configurations was the result of the varied response of the enzyme to different cations. When either Mn2+, Mg2+, Ca2+, or Co2+ was added to the relaxed (divalent cation-free) enzyme at saturated concentration, each produced a similar UV-difference spectrum of the enzyme, indicating that the conformational states induced by these cations are similar with respect to the polarity of the microenvironment surrounding the tyrosyl and tryptophanyl groups of the enzyme. The binding of Cd2+, Ni2+, or Zn2+ to the relaxed enzyme each produced a different shift in the UV-absorption spectrum of the enzyme, indicating different conformational states. The kinetics of the spectral change that occurred upon addition of Mn2+, Mg2+, or Co2+ to a relaxed enzyme preparation were determined. The first-order rate constants for the decrease in relaxed enzyme with Mn2+ and Mg2+ were 0.604 min-1 and 0.399 min-1, respectively, at 25 degrees C, pH 7.4. The spectral change with Co2+ was completed within the time of mixing (less than 4 s). For these three metal ions, the total spectral change as well as the time course of the change were the same for both the unadenylylated enzyme and the partially adenylylated enzyme. However, Hill coefficients obtained from spectrophotometric titration data for both Mn2+ and Mg2+ were decreased with adenylylated enzyme to compared with unadenylylated enzyme. These results suggest that covalently bound AMP on each subunit may be involved in subunit interactions within the dodecamer. Circular dichroism measurements also indicated that the various structural changes of the M. smegmatis glutamine synthetase were produced by the binding of the divalent cations.     

The effect of divalent cations on bovine spermatozoal adenylate cyclase activity.

The effect of divalent cations on bovine sperm adenylate cyclase activity was studied. Mn2+, Co2+, Cd2+, Zn2+, Mg2+ and Ca2+ were found to satisfy the divalent cation requirement for catalysis of the bovine sperm adenylate cyclase. These divalent cations in excess of the amount necessary for the formation of the metal-ATP substrate complex were found to stimulate the enzyme activity to various degrees. The magnitude of stimulation at saturating concentrations of the divalent cations was strikingly greater with M2+ than with either Ca2+, Mg2+, Zn2+, Cd2+ or Co2+. The apparent Km was lowest for Zm2+ (0.1 - 0.2 mM) than for any of the other divalent cations tested (1.2 - 2.3 mM). The enzyme stimulation by Mn2+ was decreased by the simultaneous addition of Co2+, Cd2+, Ni2+ and particularly Zn2+ and Cu2+. The antagonism between Mn2+ and Cu2+ or Zn2+ appeared to have both competitive and non-competitive features. The inhibitory effect of Cu2+ on Mn2+-stimulated adenylate cyclase activity was prevented by 2,3-dimercaptopropanol, but not by dithiothreitol, L-ergothioneine, EDTA, EGTA or D-penicillamine. Ca2+ at concentrations of 1-5 mM was found to act synergistically with Mg2+, Zn2+, Co2+ and Mn2+ in stimulating sperm adenylate cyclase activity. The Ca2+ augmentation of the stimulatory effect of Zn2+, Co2+, Mg2+ and Mn2+ appeared to be specific.     

Interaction of divalent cations with beta-galactosidase (Escherichia coli).

Although the addition of various divalent metals to beta-galactosidase resulted in apparent activation, only Mg2+ and Mn2+ actually did activate. The apparent activation by the other divalent metals was shown to be due to Mg2+ impurities. Calcium did not activate, but experiments suggested that it did bind. Other divalent metals which were studied failed to bind. The dissociation constants for Mg2+ and Mn2+ were 2.8 X 10(-7) and 1.1 X 10(-8) M, respectively, and in each case one ion bound per monomer. These constants corresponded very closely to apparent values which were obtained from activation studies. The apparent binding constant for Ca2+, obtained from competition studies, was 1.5 X 10(-5) M. Data were obtained which showed that Mg2+, Mn2+, and Ca2+ all compete for binding at a single site. Of interest and of possible molecular biological importance was the observation that, while Mg2+ bound noncooperatively (n = 1.0), Mn2+ did so in a highly cooperative manner (n = 3.4). The binding of Mn2+ (as compared to Mg2+) resulted in a twofold drop in the Vmax for the hydrolysis and transgalactosylis reactions of lactose but had little effect on the Vmax of hydrolysis of allolactose, p-nitrophenyl beta-D-galactopyranoside (PNPG), or o-nitrophenyl beta-D-galactopyranoside (ONPG); Km values were not effected differently for any of the substrates by Mn2+ as compared to Mg2+. When very low levels of divalent metal ions were present (0.01 M EDTA added) or when Ca2+ was bound with lactose as the substrate, a greater decrease was observed in the rate of the transgalactosylic reaction than in the rate of the hydrolytic reaction, and the Km values for lactose and ONPG were increased. Of the three divalent metal ions which bound to beta-galactosidase, only Mn2+ had significant stabilizing effects toward denaturing urea and heat conditions.     

Divalent cation-induced changes in conformation of protein kinase C

Using physical techniques, circular dichroism and intrinsic and extrinsic fluorescence, the binding of divalent cations to soluble protein kinase C and their effects on protein conformation were analyzed. The enzyme copurifies with a significant concentration of endogenous Ca2+ as measured by atomic absorption spectrophotometry, however, this Ca2+ was insufficient to support enzyme activity. Intrinsic tryptophan fluorescence quenching occurred upon addition to the soluble enzyme of the divalent cations, Zn2+, Mg2+, Ca2+ or Mn2+, which was irreversible and unaffected by monovalent cations (0.5 M NaCl). Far ultraviolet (200-250 nm) circular dichroism spectra provided estimations of secondary structure and demonstrated that the purified enzyme is rich in alpha-helices (42%) suggesting a rather rigid structure. At Ca2+ or Mg2+ concentrations similar to those used for fluorescence quenching, the enzyme undergoes a conformational transition (42-24% alpha-helix, 31-54% random structures) with no significant change in beta-sheet structures (22-26%). Maximal effects on 1 microM enzyme were obtained at 200 microM Ca2+ or 100 microM Mg2+, the divalent cation binding having a higher affinity for Mg2+ than for Ca2+. The Ca2(+)-induced transition was time-dependent, while Mg2+ effects were immediate. In addition, there was no observed energy transfer for protein kinase C with the fluorescent Ca2(+)-binding site probe, terbium(III). This study suggests that divalent cation-induced changes in soluble protein kinase C structure may be an important step in in vitro analyses that has not yet been detected by standard biochemical enzymatic assays.     

Axonal surface charges: evidence for phosphate structure.

The effect of different extracellular alkaline-earth cations (Ca2+, Mg2+, Sr2+, Ba2+) upon the threshold membrane potential for spike initiation in crayfish axon has been studied by means of intracellular microelectrodes. This was done at the following extracellular concentrations of the divalent uranyl ion (UO2/2+): 1.0 X 10(-6) M, 3.0 X 10(-6) M, and 9.0 X 10(-6) M. At each concentration employed, extensive neutralization of axonal surface charges by UO2/2+ was evidenced by the fact that equal concentrations (50 mM) of alkaline-earth cations did not have the same effect on the threshold potential. The selectivity sequences observed at the different uranyl-ion concentrations were: 1.0 X 10(-6) M UO2/2+, Ca2+ greater than Mg2+ greater than Sr2+ greater than Ba2+; 3.0 X 10(-6) M UO2/2+, Ca2+ greater than Mg2+ greater than Ba2+ larger than or equal to Sr2+; 9.0 X 10(-6) M UO2/2+, Ca2+ approximately Ba2+ greater than Sr2+ greater than Mg2+. These selectivity sequences are in accord with the equilibrium selectivity theory for alkaline-earth cations. At each of the concentrations used, uranyl ion did not have any detectable effect on the actual shape of the action potential itself. It is concluded that many (if not most) of the surface acidic groups in the region of the sodium gates represent phosphate groups of membrane phospholipids, but that the m gates themselves are probably protein-aceous in structure.     

Evaluation of the metal-ion-coordinating differences between the 2'-, 3'- and 5'-monophosphates of adenosine

The stability constants of the 1:1 complexes formed between Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+ or Cd2+ and 2'AMP2-, 3'AMP2- or 5'AMP2- were determined by potentiometric pH titration in aqueous solution (I = 0.1 M, NaNO3; 25 degrees C). The experimental conditions were carefully selected such that self-association of the nucleotides and their complexes is negligibly small; i.e. it was made certain that the properties of the monomeric divalent-metal-ion--AMP [M(AMP)] complexes were studied. Based on recent measurements with simple phosphate monoesters, R-MP2- where R is a non-coordinating residue [Massoud, S. S. & Sigel, H. (1988) Inorg. Chem. 27, 1447-1453], it is shown that all the M(AMP) complexes of the alkaline earth ions, with the possible exception of Mg(5'AMP), have exactly the stability expected for a sole-phosphate coordination of the metal ion. The same property is revealed for the complexes with Mn2+, Co2+, Zn2+ or Cd2+ and 3'AMP2-; in case of Ni(3'AMP) and Cu(3'AMP) a slight stability increase just at the edge of the experimental-error limits is indicated. This slight stability increase is attributed to the formation of a macrochelate (possibly with N-3); in fact, additional information confirms macrochelation for Cu(3'AMP). About 45% of Cu(2'AMP) exists in aqueous solution as a macrochelate (probably involving N-3); the other M(2'AMP) complexes (M2+ = Mn2+, Co2+, Ni2+, Zn2+, Cd2+) form (if at all) only traces of a base-backbound species. Most pronounced is macrochelate formation with 5'AMP2-: all mentioned 3d ions and Zn2+ or Cd2+ form to some extent macrochelates via N-7 (the structures of these closed species are indicated). In case of M(5'AMP) the base-binding site is certain: replacement of N-7 by a CH unit (tubercidin 5'-monophosphate) eliminates any increased complex stability, whereas formation of the 1,N6-etheno bridge to form 1,N6-ethenoadenosine 5'-monophosphate results in the phenanthroline-like N-6,N-7 site which facilitates macrochelation significantly.     

Activity and properties of fructose bisphosphatase of turbellarian Phagocata sibirica

Activity and properties of fructose bisphosphatase (FBPase) was studied in the free-living turbellarian Phagocata sibirica. All subcellular fractions of P. sibirica (12 000 g cytosol, 105 000 g cytosol, mitochondria, and microsomes) have the FBPase activity. There was studied dependence of the FBPase reaction rate on the substrate concentration. For realization of the enzyme activity, the high affinity to substrate and presence of bivalent cations (Mg2+ or Mn2+) are necessary. The was studied the effect of various effectors as well as of monovalent (Na+, K+, Li+, and NH4+) and bivalent (Zn2+ and Cu2+) cations.     

Enhancement of adenosine triphosphatase activity of purified chloroplast coupling factor 1 in aqueous organic solvent

The ATPase activity of purified coupling factor 1 (CF1) of spinach chloroplasts [EC 3.6.1.3] was reversibly enhanced in some aqueous organic solvents, notably methanol, ethanol, and acetone. Pretreatment of CF1 with 20% (v/v) methanol did not affect the subsequent activity. The activity depended entirely on the final concentration of methanol in the reaction mixture. In the presence of 20% methanol, the Km of Ca2+-ATPase from ATP was lowered from 0.4 mM to 0.2 mM. Not only Ca2+, but also Cd2+, Mg2+, Mn2+, and Zn2+ supported the ATPase activity at rates of higher than 7 mumol.mg protein-1 . min-1. Co2+, Ni2+, and Pb2+ supported the activity at rates of 0.5-1.0 mumol.mg protein-1 . min-1. The activities supported by the following cations, if any, were less than 0.2 mumol.mg protein-1 . min-1; Ba2+, Cu2+, Fe2+, Hg2+, Sn2+, and Sr2+. The optimum concentration of methanol for Ca2+-ATPase and Mg2+-ATPase activities was about 30% (v/v). The optimum pH values for Ca2+-ATPase and Mg2+-ATPase activities were about 8.0 and 8.8, respectively. The enhancing effect of organic solvents appears to be associated with their relative lipophilic character as defined by the octanol-water partition coefficient. The Ca2+-ATPase activities of th trypsin-activated and the heat-activated CF1 were inhibited and their Mg2+-ATPase activities were enhanced by the presence of methanol in the reaction mixture.