Optimal Concentrations Of Mg Research Articles

Study on Photocatalytic Degradation of MG by PW<sub>12</sub>-TiO<sub>2</sub>/SiO<sub>2</sub>

To investigate the photocatalytic degradation of MG by PW12-TiO2/SiO2, in this study PW12-TiO2/SiO2 composite oxides were prepared by sol-gel and hydrothermal strategy. The prepared photocatalysits were characterized by XRD, BET,TEM and EDS techniques and the configuration charateristics of PW12-TiO2/SiO2 nanoparticles were systematically investigated. Photocatalytic activity was checked by the photocatalytic oxidation of MG dye in aqueous solution. The results showed that the phosphotungstic acid still has the Keggin structure. The anatase phase of titanium oxid is identified by XRD. The result of the present work implied that compared with pure TiO2, the PW12-TiO2/SiO2 composite oxides have stronger catalytic activity, good particles dispersion and bigger specific surface area. The optimal concentration of MG was 20mg.L-1, the best dosage of PW12-TiO2/SiO2 was 1g.L-1. Moreover, after three times of recycling, the catalyst still has high catalytic activity.

Kinetic properties of the C-terminal Src kinase, p50 csk

Csk is an important regulator of tyrosine kinases of the Src family. In this paper, we have characterised the kinetics and catalytic properties of a highly active and stable enzyme obtained in milligram amounts by expressing the enzyme as a fusion protein with glutathione- S-transferase (GST) in Escherichia coli. Using the synthetic polyamino acid poly(Glu,Tyr) as substrate, phosphotransferase activity was linear for 7–8 min with Mg 2+ and 5 min with Mn 2+. With Mg 2+ and Mn 2+, respectively, K m(ATP) was 56.9±6.2 and 5.4±0.6 μM and V max was 293±52 and 217±38 pmol phosphate transferred ( μg Csk) −1 min −1. Optimal concentrations of Mg 2+ and Mn 2+ were 4–10 mM and 2–3 mM, respectively, and higher concentrations of both cations were inhibitory. The Csk activity was highly sensitive to monovalent (Na +, K +) and divalent (Ca 2+) cations, the sensitivity being 2–5-fold higher with Mg 2+ than Mn 2+. Physiological concentrations of Ca 2+ (less than 10 μM) were without effect. Autophosphorylation of Csk was demonstrated in vitro, but did not influence the catalytic activity. Addition of inorganic phosphate above 100 μM strongly inhibited Csk catalytic activity towards poly(Glu,Tyr) in the presence of Mn 2+, but not in the presence of Mg 2+. Phosphorylation of a physiological substrate (Lck) and autophosphorylation of Csk was not inhibited by phosphate, indicating that the phosphate-dependent inhibition of Csk activity was substrate specific.

Differential effects of spermine on phosphatidylinositol 3-kinase and phosphatidylinositol phosphate 5-kinase

The metabolism of phosphoinositides plays an important role in the signal transduction pathways. We report here that naturally occuring polyamines affect the activities of phosphatidylinositol (PI) 3-kinase and PI 4-phosphate (PIP) 5-kinase differently. While polyamines inhibited the PI 3-kinase activity, they stimulated the activity of PIP 5-kinase in the order of spermine > spermidine > putrescine. Spermine inhibited the PI 3-kinase activity in a concentration-dependent manner with an IC 50 of 100 μM. On the other hand, spermine (5 mH) stimulated the activity of PIP 5-kinase 2–3 fold. Kinetic studies of spermine-mediated inhibition of PI 3-kinase revealed that it was noncompetitive with respect to ATP. The effect of Mg 2+ and PIP, concentration on kinase activity was sigmoidal, with spermine inhibiting PI 3-kinase activity at all PIP 2 concentrations. While 1 mH calcium stimulated PI 3-kinase activity at submaximal concentrations of Mg 2+ (1.25 mH), inhibition was observed at optimal concentration of Mg 2+(2 mM). We propose that spermine may modulate the cellular signal by virtue of its differential effects on phosphoinositide kinases.

Chapter 64 Flagellar Adenylyl Cyclases in Chlamydomonas

Gametic flagellar enzyme, but not the vegetative enzyme, is regulated by phosphorylation and dephosphorylation. Both the regulation by cell–cell interactions and the differences between the vegetative and gametic forms of the enzyme are matter of interest. Cyclic adenosine monophosphate (cAMP) is important for the control of flagellar motility; gametes must carefully regulate low, basal levels of this molecule when they are not interacting with gametes of the opposite type. But, on flagellar adhesion with a gamete of the opposite mating type, cAMP levels raise several folds as the cells prepare for fusion. While this adhesion-dependent regulation is critical for gametes, evidence is lacking that vegetative cells, which do not undergo flagellar adhesion, exhibit any dramatic regulation of the enzyme. This chapter summarizes methods for studying Chlamydomonas adenylyl cyclase in both vegetative and gametic flagella. The methods detail the chemicals, reagents, and solutions, the processes of flagellar isolation and assaying adenylyl cyclase. The chapter also describes the evaluating parameters of the assay conditions—divalent cations and detergents. Both gametic and vegetative adenylyl cyclases have requirement for Mg 2+ , the optimal concentration of Mg 2+ for both enzymes being about 2.5 m M. Both the gametic and vegetative enzymes are inhibited by free Ca 2+ in the millimolar range. The gametic adenylyl cyclase is more sensitive to Ca 2+ than the vegetative enzyme and is inhibited by micromolar concentrations of free Ca 2+ . The vegetative enzyme seems to be less sensitive to detergents than the gametic enzyme. There is discussion on the regulation of gametic and vegetative adenylyl cyclase—ATP-dependent inhibition and heat-induced activation of gametic adenylyl cyclase; and adhesion-induced activation of adenylyl cyclase in gametic flagella. During gametogenesis the appearance of the gametic form of the enzyme is coincident with the acquisition of the ability of the cells to undergo sexual signaling and cell fusion.

Characteristics of the translation of thymosin α 1, precursor mRNA by cell-free wheat germ system. evidence for the acetylation of thymosin α 1, precursor

1. 1. At the optimal concentrations of Mg 2+ and K + to translate total thymus PolyA +-RNA, the purified thymosin α 1 precursor mRNA saturate the protein synthetic activity at lower concentration than the total thymus mRNAs. 2. 2. The polyamine spermidine increases the translation rate of the messenger, which is modulated by magnesium, rather than improve the yield in full-length chains of thymosin α 1 precursor. 3. 3. As other eukariotic mRNAs, this messenger presents the “cap” modification at the 5'-end terminalposition. 4. 4. The incorporation of [ 3H]acetate into the translation product of the messenger, shows an evidence for the acethylation of the thymosin α 1 precursor during its biosynthesis in vitro.

The study of spermidine-stimulated polypeptide synthesis in cell-free translation of mRNA from lactating mouse mammary gland

The stimulatory effect of spermidine on the translation of poly(A) + mRNA from lactating mouse mammary glands in a wheat germ system was studied. Spermidine stimulated total polypeptide synthesis about 2.5-fold relative to that occurring in the presence of an optimal concentration of Mg 2+ alone. The size and the number of polysomes were about 1.6-times larger in the presence of spermidine than in its absence. A similar magnitude of increase in peptide chain initiation, 1.4-fold, was found when the extent of peptide chain initiation was measured by determining the residual polypeptide synthesis subsequent to the addition of inhibitor(s) of peptide chain initiation to the in vitro translation system with or without spermidine at various times of the incubation. Time-course study of the release of polypeptide from polysomes showed that spermidine stimulated this process to a much greater extent than peptide chain initiation, indicating that the polyamine also increases the rate of peptide chain elongation. The extent of stimulation of peptide chain elongation by spermidine was estimated to be about 1.5-fold when the disappearance of isotope-labeled nascent peptides from polysomes was measured by pulse-chase experiments. These results indicate that spermidine stimulates the cell-free translation of mammary mRNA by increasing the rates of both initiation and elongation of polypeptide synthesis to almost the same extent. The polyamine also reduced the relative amount of incomplete polypeptides, thereby increasing the yield of full-length translational products.

Mechanism of aminoacylation of tRNA: Influence of spermine on the kinetics of aminoacyl-tRNA synthesis by isoleucyl- and valyl-tRNA synthetases from Mycobacterium smegmatis

Isoleucyl-tRNA synthetase has been purified to homogeneity from Mycobacterium smegmatis. The influence of spermine on the kinetics of valyl-tRNA and isoleucyl-tRNA formation has been investigated by Cleland's method (Cleland, W.W. (1963) Biochim. Biophys. Acta 67, 104–137, 173–187, 188–196). The results suggest that in the presence of spermine and suboptimal concentration of Mg 2+, the formation of valyl-tRNA and isoleucyl-tRNA follows a sequential ∗ ∗ In Cleland's nomenclature ‘sequential’ mechanism is equivalent to the ‘concerted’ mechanism used by others for aminoacylation. The ‘ping-pong’ mechanism of Cleland is equivalent to ‘step-wise’ or ‘sequential’ mechanism as used for aminoacylation. In the summary and results, Cleland's nomenclature is used; in discussion, a general one is used. mechanism. In the presence of an optimal concentration of Mg 2+, both valyl-tRNA and isoleucyl-tRNA formation proceeds by a ping-pong mechanism. However, in the presence of spermine and optimal concentrations of Mg 2+, valyl-tRNA formation follows the ping-pong mechanism while isoleucyl-tRNA formation follows the sequential mechanism.

Polyamines stimulate natural RNA-directed DNA synthesis by Rauscher murine leukemia virus DNA polymerase

In the presence of optimal concentrations of Mg 2+, spermine and spermidine were found to stimulate rabbit globin mRNA-directed cDNA synthesis by Rauscher murine leukemia virus (R-MuLV) DNA polymerase. Stimulation of DNA synthesis did not occur with the polyamines putrescine or cadaverine, nor could exogenously provided salt or ammonium ions duplicate the stimulation. Analysis of the mechanism of stimulation showed that inclusion of spermine in reaction mixtures a) increased Vmax and decreased apparent Km with respect to the globin mRNA-oligo(dT) temṕlate-primer complex, and b) decreased the quantity of oligo (dT) required for optimal rates of cDNA synthesis on a fixed quantity of mRNA template. Genomic 70S RNA-directed cDNA synthesis was also stimulated by spermine addition to reaction mixtures, but only at supra-optimal RNA concentrations. Our results suggest that stimulation of R-MuLV DNA polymerase activity by polyamines is primarily due to stabilization of the enzyme-templateprimer initiation complex resulting in increased efficiency of initiation of cDNA synthesis.

Factors affecting the inhibition of yeast plasma membrane ATPase by vanadate

Inhibition of yeast plasma membrane ATPase by vanadate occurs only if either Mg 2+ or MgATP 2− is bound to the enzyme. The dissociation constant of the complex of vanadate and inhibitory sites is 0.14–0.20 μM in the presence of optimal concentrations of Mg 2+ and of the order of 1 μM if the enzyme is saturated with MgATP 2−. The dissociation constants of Mg 2+ and MgATP 2− for the sites involved are 0.4 and 0.62–0.73 mM, respectively, at pH 7. KCl does not increase the affinity of vanadate to the inhibitory sites as was found with ( Na + + K +)- ATPase . On the other hand, the effect of Mg 2+ upon vanadate binding is similar to that upon ( Na + + K +)- ATPase , and the corresponding affinity constants of Mg 2+ and vanadate for the two enzymes are of the same order of magnitude.

Stimulation by polyamines of enzymatic methylation of two adjacent adenines near the 3′ end of 16S ribosomal RNA of [formula omitted

Effect of polyamines on the methylation of adenine in 16S rRNA was examined using the purified methylating enzyme. When 23S core particles were used as substrate, the activity was stimulated by Mg 2+, Ca 2+ and monovalent cations. Even in the presence of optimal concentrations of Mg 2+ and NH 4 +, the addition of 1 mM spermidine stimulated the methylation approximately 1.7-fold. When 30S ribosomal subunits were used as substrate, the rate of methylation was 20% of that of the methylation of 23S core particles. The activity was not influenced significantly by Mg 2+, Ca 2+ or monovalent cations. The addition of spermidine inhibited the methylation.

Properties of mitochondrial Mg 2+-HCO −3-stimulated and SCN −-inhibited ATPase in rat liver, kidney and small intestinal mucosa and some relationships between ATPase and supernatant carbonic anhydrase

1. 1. The optimal concentration of Mg 2+ for mitochondrial Mg 2+-ATPase was 1.5 mM in all organs tested. 2. 2. The optimal concentration of HCO − 3 for MG 2+-HCO − 3-ATPase was 30 mM of liver, kidney, ileum and 60 mM of jejunum. 3. 3. The optimal pH value for maximum Mg 2+-and Mg 2+-HCO − 3-ATPase activities was about 9.2 at 37°C. 4. 4. SCN − inhibited both Mg 2+- and Mg 2+-HCO − 3-ATPase at concentrations above 10 −3 M. 5. 5. Acetazolamide inhibited intestinal Mg 2+-HCO − 3-ATPase activity at concentrations above 10 −3 M with no effect on hepatic and renal ATPase activities. 6. 6. Mg 2+- and Mg 2+ -HCO − 3-ATPase activities in duodenal mucosa were highest in all organs tested and may have some correlation with carbonic anhydrase.

Partial fractionation and characterization of nuclear protein kinases in HeLa S 3 cells

Five fractions of phosphoprotein kinase activity were separated by phosphocellulose chromatography of nuclear non-histone phosphoproteins isolated from exponentially-growing HeLa S 3 cells. Each fraction contained a heterogeneous group of proteins and possibly more than one protein kinase. These fractions were characterized with respect to specific activity and optimal concentrations of Mg 2+ and Mn 2+. The ability of each kinase fraction to catalyze the incorporation of 32P from [γ- 32P]ATP into various molecular weight classes of heat-inactivated non-histone nuclear phosphoproteins (as substrate) was examined by electrophoretically resolving the proteins in SDS-polyacrylamide gels. Each kinase fraction induced a distinctive pattern of protein phosphorylation. Substrate preferences (non-histone phosphoprotein or histone) were determined for each fraction by examining the incorporation of 32P from [γ- 32P]ATP into both TCA-precipitable material and into specific molecular weight classes of proteins. The activities of two of the fractions to phosphorylate non-histone nuclear phosphoprotein were stimulated by the presence of histones. Studies were undertaken to assess the effects of cAMP or cGMP on the activity of each kinase fraction. No significant sensitivity to the cyclic nucleotides was observed for any of the kinase fractions.

The presence and properties of a Mg 2+-dependent HCO 3− stimulated ATPase in the Malpighian tubules of Locusta migratoria

A 50% stimulation of ATPase activity by added NaHCO 3 was observed in homogenates of Malpighian tubules. The activities of the Mg 2+-dependent and Mg 2+-dependent HCO 3 −-stimulated ATPase were highest in the 20,000 g fraction although substantial activity was present in the 100,000 g fraction. The properties of Mg 2+-dependent HCO 3 −-stimulated ATPase in the 100,000 g fraction have been determined. The pH optimum was ca. 7.5 although maximal stimulation due to the presence of HCO 3 − was nearer pH 8.0. Optimal concentrations of Mg 2+ and HCO 3 − for Mg 2+-dependent HCO 3 −-stimulated ATPase were 2 mM and 20 mM respectively. In the presence of HCO 3 − the K m for ATP was increased from 0.16 mM to 0.26 mM. SCN − inhibited both the Mg 2+-dependent and the Mg 2+-dependent HCO 3 −-stimulated ATPase whilst 1 mM ouabain and sodium acetazolamide had no effect on enzyme activity.

Effects of cyclic adenosine 3′: 5′-monophosphate on phosphoprotein kinase and phosphatase fractions prepared from rat liver nuclei

A soluble rat liver nuclear extract containing total RNA polymerase activities also exhibits appreciable amounts of protein kinase activity. This unfractionated protein kinase catalyzes the phosphorylation of both endogenous proteins and exogenous lysine-rich histone in the presence of [γ- 32P]ATP and Mg 2+. The optimal concentration of Mg 2+ is 5 m m for histone phosphorylation and 25 m m for the phosphorylation of endogenous proteins. Cyclic AMP has no effect on the phosphorylation of lysine-rich histone by this unfractionated nuclear protein kinase. However, addition of cyclic AMP causes a reduction in the 32P-labeling of an endogenous protein (CAI) which can be characterized by its mobility during SDS-acrylamide gel electrophoresis and elution in the unbound fraction of a DEAESephadex column. If CAI is first labeled with 32P and then incubated with 10 −6 m cyclic AMP under conditions where protein kinase activity is inhibited, the presence of the cyclic nucleotide causes a loss of the 32P-labeling of this protein, implying the activation of a substrate-specific protein phosphatase. When rat liver RNA polymerases are purified by DEAE-Sephadex chromatography, protein kinase activity is found in the unbound fraction and in those column fractions containing RNA polymerase I and II. The fractionated protein kinases exhibit different responses to cyclic AMP, the unbound protein kinase being stimulated and the RNA polymerase-associated protein kinases being dramatically inhibited. A second protein (CAII) whose phosphorylated state is modified by cyclic AMP is found within the DEAE-Sephadex column fractions containing RNA polymerase II. The cyclic nucleotide in this case appears to reduce labeling of CAII by inhibition of the protein kinase activity which co-chromatographs with both CAII and RNA polymerase II. Based on molecular weight estimates, neither CAI nor CAII appears to be an RNA polymerase subunit. The identity of CAI as a protein factor whose phosphorylated state influences nuclear RNA synthesis is suggested by the fact that addition of fractions containing CAI to purified RNA polymerase II inhibits the activity of this enzyme, but only if CAI has been previously incubated in the presence of cyclic AMP.

The presence and properties of Mg 2+-HCO 3−-stimulated and SCN −-inhibited ATPase in mouse kidney and some relationships between ATPase and carbonic anhydrase

1. 1. Mg 2+- and Mg 2+-HCO 3 −-ATPase activities from mouse kidney were highest in mitochondria. 2. 2. Optimal concentrations of Mg 2+ and HCO 3 − for Mg 2+-HCO 3 −-ATPase activity were 1.5 and 30 mM, respectively. 3. 3. Optimal pH value for maximum Mg 2+- and Mg 2+-HCO 3 −-ATPase activities was about 9.0 at 37°C. 4. 4. SCN − inhibited both Mg 2+- and Mg 2+-HCO 3 −-ATPase at concentrations above 10 −2 M. 5. 5. Mg 2+-HCO 3 −-ATPase had no direct relationship with adrenalectomy and aldosterone administration in contrast to carbonic anhydrase.

Changes in liver composition in phenobarbital-induced hepatomegaly

In a study of molecular events occurring during phenobarbital-induced hepatomegaly, the levels of poly(A)-RNA (heterogeneous nuclear RNA + cytoplasmic mRNA), total RNA, DNA, and protein were compared in untreated rats and those injected with phenobarbital (80 mg/kg/day) for 3 days. Total poly(A)-RNA isolated by phenol extraction and purified by affinity chromatography on oligo(dT)-cellulose, was assayed for its template (messenger RNA) activity in a cell-free protein synthetic system derived from wheat germ. The cell-free system was sufficiently sensitive to detect differences of less than a microgram of mRNA when optimal concentrations of Mg 2+ and K + were used. Using this system, no significant differences in the hepatic content of poly(A)-RNA were observed between the control and treated groups. The DNA level was also unchanged, but the liver weight was increased by 26% in the treated group. Total RNA and protein levels also increased significantly (25 and 38%, respectively, per 100 g body weight, and 21 and 30%, respectively per unit of DNA) in the treated group. Protein levels, when expresed in terms of poly(A)-RNA, showed a significant (22%) increase in the treated group. These results suggest that phenobarbital-induced hepatomegaly: (i) is characterized by hypertropy (cell enlargement), (ii) is not mediated by a generalized increase in the level of poly(A)-RNA, but involves a generalized increase in the synthesis and/or stabilization of RNA other than functional message, and (iii) involves an increased translation of mRNA in vivo.

Antibiotics and compounds affecting tanslation by eukaryotic ribosomes. Specific enhancement of aminoacyl-tRNA binding by methylaxnthines.

The mode and site of action of inhibitors of translation (initiation, elongation and termination of protein synthesis) in eukaryotic systems is reviewed. The isolation and characterization of a factor is described that binds Ac-Phe-tRNA to form a complex made up of binding factors, Ac-Phe-tRNA, and ribosome. The binding of Ac-Phe-tRNA probably occurs at the ribosomal site involved in the binding of the initiator substrate Met-tRNAF. The effect of inhibitors of the intitiation phase of protein synthesis on the nonenzymic Ac-Phe-tRNA binding to ribosomes is investigated. The two sites translocation model for translation in eukaryotic cells is presented and the effects of inhibitors on the various steps of protein synthesis are determined empirically. The site of action of inhibitors of peptide bond formation at the ribosomal peptidyl transferase center is elucidated. The action of inhibitors of translocation is sutdied in model cell-free systems from human cells. In addition, a number of methylxanthines are shown to enhance the elongation phase in polypeptide synthesis by stimulating the enzymic binding of aminoacyl-tRNA. The effect of caffeine, theophylline and its derivatives are shown to be fairly specific and dependent on the ribosome concentration. Aminophylline is shown to have a similar effect but also enhances aminoacyl-tRNA synthetase activity at low Mg++ concentrations, probably displacing the optimal concentration of Mg++ in the reaction. This second effect of aminophylline appears to be due to the ethylenediamine moiety of aminophylline since it is also observed in the presence of different polyamines but not in the presence of caffeine or theophylline.

Bound Forms of Ca Taken Up by the Synaptic Plasma Membrane

Temperature dependent Ca-binding by the synaptic plasma membrane was increased in the presence of ATP and Mg++. Apparent Km for ATP was about 2.8×10−5 M and optimal concentration of Mg++ was 2 mM in the presence of 2 mM ATP. After preincubation with nonradioactive Ca++, ATP and Mg++ to attain a steady state, addition of 45Ca resulted in remarkable labelling of the membrane, indicating rapid turnover of most of the membrane bound Ca. The presence of oxalate (60 mM) greatly increased Ca up-take on prolonged incubation. The Ca uptake in presence and absence of oxalate had similar substrate specificity and was similarly influenced by various monovalent cations. Furthermore, activities for Ca-uptake in the presence and absence of oxalate could not be separated by sucrose density gradient centrifugation of the synaptic plasma membrane fraction. Accordingly, it was considered that Ca++ in the medium was taken up by surface of the membrane, ATP- and temperature-dependently and then transferred into a cavity where the Ca-oxalate complex is formed.

3′:5′-cyclic-nucleotide phosphodiesterases of mammalian sera

Cyclic AMP and cyclic GMP phosphodiesterases (3′:5′-cyclic-nucleotide 5′-nucleotidohydrolase EC 3.1.4.17) were found in the sera of human, dog, rabbit and rat. The formed elements of blood were not present in sera and thus not the source of the phosphodiesterase. More rapid hydrolysis of cyclic GMP than cyclic AMP is observed in the sera of these four species when 0.4 μM of cyclic AMP or cyclic GMP is used as the substrate. Protein activator of the phosphodiesterase is not detectable in the sera of these four species. Serum cyclic AMP and cyclic GMP phosphodiesterase activities are not stimulated by protein activator prepared from bovine brain. The serum phosphodiesterases of these four species are purified through Sepharose 6B column chromatography. Cyclic AMP phosphodiesterase are found in a broad area corresponding to molecular weights ranging from approximately 150 000 to 340 000 with 2 to 3 peaks in all animals tested. Cyclic GMP phosphodiesterase is found in a single area corresponding to molecular weights of 230 000 (rabbit and rat) and 270 000 (human and dog). Serum cyclic AMP and cyclic GMP phosphodiesterase activities of these four species have pH optimum of 7.5–8.5. Optimal concentration of Mg 2+ is about 5 mM for cyclic GMP phosphodiesterase activities of these four species as well as for cyclic AMP phosphodiesterase activities except rabbit. Rabbit serum cyclic AMP phosphodiesterase requires higher concentration of Mg 2+ (50 mM) for its optimal activity. The double reciprocal plots are non-linear for cyclic AMP phosphodiesterases of all animals and cyclic GMP phosphodiesterases of human, dog and rat. Rabbit cyclic GMP phosphodiesterase exhibits a linear reciprocal plot. Cyclic GMP is inhibitor of serum cyclic AMP phosphodiesterase. Rabbit enzyme was most effectively inhibited by cyclic GMP.

Ca 2+Mg 2+-activated ATPase in the shell gland of Japanese quail ( Coturnix coturnix Japonica)

1. 1. An ATPase has been found in the microsomal fraction of the shell gland of the Japanese quail, Coturnix coturnix japonica, which is stimulated by either Ca 2+, Mg 2+ or Mn 2+, but is highest with Mg 2+. Sr 2+ activities the enzyme only slightly. Ca 2+Mg 2+ interaction was studied. 2. 2. The addition of Ca 2+ to a reaction mixture containing an optimal concentration of Mg 2+ reduced hydrolysis of ATP. GTP, ITP and UTP are hydrolyzed about as well as ATP; p- nitrophenyl phosphate is not hydrolyzed. 3. 3. The mean Ca 2+ or Mg 2+ ATPase activity of the shell gland from ovipositing birds was higher than from “precalcifying” adults or immature birds. 4. 4. The data are discussed in relation to the enzymatic basis for active calcium translocation.