共查询到20条相似文献,搜索用时 15 毫秒
1.
Meryl S. Rubin Norbert I. Swislocki Martin Sonenberg 《Archives of biochemistry and biophysics》1973,157(1):252-259
The response of liver plasma membranes prepared from hypophysectomized, bovine growth hormone-treated hypophysectomized, and normal rats to addition of bovine growth hormone in vitro were studied by circular dichroism and spectrofluorometry. Membranes of hypophysectomized but not normal or treated rats showed increased negative ellipticity in the presence of bovine growth hormone (10?9) without any change in trough position; at higher concentrations (10?7?10?6, m) there was less negative ellipticity. Membranes of hypophysectomized, normal, and growth hormone-treated rats all showed decreased emission of fluorescence and a small shift of the emission peak from 333 to 338 nm in the presence of bovine growth hormone (10?17?10?7, m). The excitation of fluorescence was quenched by bovine growth hormone. Polarization of excitation of fluorescence was unchanged. 相似文献
2.
Marie-Catherine Postel-Vinay Martin Sonenberg Norbert I. Swislocki 《生物化学与生物物理学报:生物膜》1974,332(2):156-165
Conformational changes produced by in vitro bovine growth hormone addition to plasma membranes of hypophysectomized rat liver proteins and lipids have been studied by circular dichroism as well as intrinsic and extrinsic fluorescence. 7,12-Dimethylbenzanthracene has been used as a fluorescent probe of changes in membrane structure. The exposure of membranes to bovine growth hormone produced a change in membrane negative ellipticity. Dimethylbenzanthracene at concentrations similar to those employed in fluorescence studies had no effect on the membrane circular dichroism spectrum. Its presence did, however, prevent a response to growth hormone. There was a decrease in peak fluorescence intensity and a peak shift when bovine growth hormone (0.5 · 10?12 M) was added to liver membranes. The addition of dimethylbenzanthracene (1.6 · 10?6 M) to membranes resulted in a decrease in the intensity of the protein fluorescence peak at 335 nm and the appearance of two peaks at 430 and 407 nm, assignable to the probe. The addition of bovine growth hormone (0.5 · 10?12 M) produced a decrease in fluorescence at 335 nm and also in the peaks at 407 and 430 nm. These data are consistent with the conclusion that bovine growth hormone produces a conformational change in rat liver plasma membrane proteins and lipids. 相似文献
3.
N.Charle Morcos 《生物化学与生物物理学报:生物膜》1982,688(3):747-756
N-Ethylmaleimide was employed as a surface label for sarcolemmal proteins after demonstrating that it does not penetrate to the intracellular space at concentrations below 1·10?4 M. The sarcolemmal markers, ouabain-sensitive (Na+ + K+)-ATPase and Na+/Ca2+-exchange activities, were inhibited in N-ethylmaleimide perfused hearts. Intracellular activities such as creatine phosphokinase, glutamate-oxaloacetate transaminase and the internal phosphatase site of the Na+ pump (K+-p-nitrophosphatase) were not affected. Almost 20% of the (Ca2+ + Mg2+)-ATPase and Ca2+ pump were inhibited indicating the localization of a portion of this activity in the sarcolemma. Sarcolemma purified by a recent method (Morcos, N.C. and Drummond, G.I. (1980) Biochim. Biophys. Acta 598, 27–39) from N-ethylmaleimide-perfused hearts showed loss of approx. 85% of its (Ca2+ + Mg2+-ATPase and Ca2+ pump compared to control hearts. (Ca2+ + Mg2+)-ATPase and Ca2+ pump activities showed two classes of sensitivity to vanadate ion inhibition. The high vanadate affinity class ( for inhibition approx. 1.5 μM) may be localized in the sarcolemma and represented approx. 20% of the total inhibitable activity in agreement with estimates from N-ethylmaleimide studies. Sucrose density fractionation indicated that only a small portion of Mg2+-ATPase and Ca2+-ATPase may be associated with the sarcolemma. The major portion of these activities seems to be associated with high density particles. 相似文献
4.
An orthovanadate-inhibited, nitrate-insensitive, phospholipid-requiring Mg2+-ATPase has been partially purified (approx. 40-fold) from microsomal preparations from 24 h germinated radish seedlings. The specific activity obtained was 10–13 μmol Pi · min?1 per mg protein, namely by 4- to 10-fold higher than that reported for the known similar enzyme preparations from corn and oat roots, and by 3- to 10-fold lower than that of the extensively purified plasmalemma enzymes from Neurospora and yeast. The partially purified activity was fairly specific for ATP, other nucleotide triphosphates being hydrolysed at less than 10% the rate with ATP; no activity was present towards ADP, AMP, p-nitrophenyl phosphate and other phosphate esters. The activity was strongly dependent on the presence of phospholipids with a marked preference for lysophosphatidylcholine, and showed an absolute requirement for Mg2+ or some other divalent cations (CO2+, Mn2+, Mg2+, Ni2+, Zn2+ in order of decreasing effectiveness); Ca2+ could not substitute for Mg2+ and was strongly inhibitory in its presence. K+, Rb+ and Na+ and also to a lesser extent NH4+ and Li+ were significantly stimulatory, while the anions NO3?, H2PO4?, Cl? and SO42? were ineffective. Orthovanadate, N,N′-dicyclohexylcarbodiimide, diethylstilbestrol, p-chloromercuribenzensulfonate, tetraiodofluorescein and tetrachlorotetraiodofluorescein were strongly inhibitory. The coincidence of the Km for ATP with that for Mg2+ suggested that ATP-Mg is the true substrate. Accordingly, the enzyme showed a normal Michaelis-Menten kinetics for ATP-Mg with an apparent Km of approx. 0.5 mM. The similarity of the characteristics of this enzyme with those of the plasmalemma enzymes from lower plants suggests its location at the plasma membrane, while some data ‘in vivo’ and in native sealed vesicle systems indicate its involvement in active proton transport. 相似文献
5.
6.
《FEBS letters》1985,182(1):31-33
Three types of proteoliposome containing mitochondrial H+-ATPase have been prepared: Mg2+-‘free’, one-side and two-side Mg2+-containing proteoliposomes. The ATPase activity as well as its sensitivity to oligomycin or N,N'-dicyclohexylcarbodiimide of the three proteoliposome preparations has been compared. They decreased in the order : L ·(H+-ATPase)+Mg2+ > L · (H+-ATPase)+Mg2+ > L · (H+-ATPase)−Mg2+. The fluidity of the proteoliposomes has also been compared by fluorescence polarization probes diphenylhexatriene (DPH) or 7-(9-anthroyloxy)stearic acid (7-AS). The degree of polarization for DPH in these proteoliposomes decreased in the order: L · (H+-ATPase)+Mg2+ > L · (H+-ATPase)+Mg2+ > L · (H+-ATPase)−Mg2+, while that for 7-AS: L · (H+-ATPase)+Mg2+ ≈ L · (H+-ATPase)+Mg2+ > L · (H+-ATPase)−Mg2+.Lipid fluidityMitochondrial H+-ATPaseOne-side Mg2+ effectTwo-side Mg2+ effectLipid-protein interaction 相似文献
7.
(1) Octylglucoside stimulates an Mg2+-specific ATPase activity with CF1 preparations from different higher plants and the alga Chlamydomonas reinhardii. (2) Tentoxin at high concentrations (10?4–10?3 M) in the presence of octylglucoside further stimulates the Mg2+-ATPase activity of CF1 from tentoxin-sensitive species and inhibits the activity of CF1 from tentoxin-resistant species. The extent of tentoxin stimulation and inhibition varies among species. A maximal stimulation of over 2-fold was obtained with spinach CF1 and a maximal inhibition of 50% was obtained with C. reinhardii CF1. In Nicotiana spp., tentoxin had only a marginal effect on the Mg2+-ATPase activity induced by octylglucoside. 相似文献
8.
Manuel J. Datiles 《BBA》2008,1777(4):362-368
Melittin, a cationic, amphiphilic polypeptide, has been reported to inhibit the ATPase activity of the catalytic portions of the mitochondrial (MF1) and chloroplast (CF1) ATP synthases. Gledhill and Walker [J.R. Gledhill, J.E. Walker. Inhibition sites in F1-ATPase from bovine heart mitochondria, Biochem. J. 386 (2005) 591-598.] suggested that melittin bound to the same site on MF1 as IF1, the endogenous inhibitor polypeptide. We have studied the inhibition of the ATPase activity of CF1 and of F1 from Escherichia coli (ECF1) by melittin and the cationic detergent, cetyltrimethylammonium bromide (CTAB). The Ca2+- and Mg2+-ATPase activities of CF1 deficient in its inhibitory ε subunit (CF1-ε) are sensitive to inhibition by melittin and by CTAB. The inhibition of Ca2+-ATPase activity by CTAB is irreversible. The Ca2+-ATPase activity of F1 from E. coli (ECF1) is inhibited by melittin and the detergent, but Mg2+-ATPase activity is much less sensitive to both reagents. The addition of CTAB or melittin to a solution of CF1-ε or ECF1 caused a large increase in the fluorescence of the hydrophobic probe, N-phenyl-1-naphthylamine, indicating that the detergent and melittin cause at least partial dissociation of the enzymes. ATP partially protects CF1-ε from inhibition by CTAB. We also show that ATP can cause the aggregation of melittin. This result complicates the interpretation of experiments in which ATP is shown to protect enzyme activity from inhibition by melittin. It is concluded that melittin and CTAB cause at least partial dissociation of the α/β heterohexamer. 相似文献
9.
T E Donnelly 《Archives of biochemistry and biophysics》1976,173(1):375-385
The effects of imidazole on the hydrolysis of cyclic AMP and cyclic GMP by crude and partially purified phosphodiesterases obtained from bovine heart and rat liver were studied in order to determine if imidazole has an activity on cyclic nucleotide hydrolysis under conditions which might explain its ability to antagonize the effects of several hormones. Imidazole-Cl (40 mm, pH 7.4) had no effect on the hydrolysis of cyclic AMP or cyclic GMP at substrate levels below 10 μm by the crude enzymes but increasing stimulation was observed with increasing substrate concentrations reaching a twofold stimulation at 1 mm cyclic nucleotide. Three phosphodiesterases with varying substrate specificities were partially purified from bovine heart by ammonium sulfate precipitation and diethyl aminoethyl cellulose chromatography. With these enzymes imidazole had less stimulatory activity and some inhibitory effect on the hydrolysis of 10?4m cyclic AMP and cyclic GMP but was without significant effect on the hydrolysis of 10?6m cyclic AMP or cyclic GMP. The stimulatory activity of imidazole on the hydrolysis of high levels of cyclic nucleotide was dependent on the presence of phosphodiesterase activator. The stimulatory effect of the activator and imidazole plus activator on the hydrolysis of 10?4m cyclic GMP by the rather cyclic GMP-specific enzyme could be eliminated by the addition of ethylene glycol-bis-(β-aminoethyl ether)N,N′-tetraacetate (EGTA) and restored by Ca2+. Imidazole was without effect on the binding of cyclic AMP to a cyclic AMP-dependent protein kinase from bovine heart. The lack of effect of imidazole on the hydrolysis of physiological levels of cyclic AMP or cyclic GMP suggests that the activity of imidazole to antagonize the effects of various hormones is probably not due to a direct action of imidazole on the hydrolysis of cyclic AMP or cyclic GMP. 相似文献
10.
Noriyuki Koyama Kazuo Koshiya Yoshiaki Nosoh 《Archives of biochemistry and biophysics》1980,199(1):103-109
ATPase was purified from an alkalophilic Bacillus. The enzyme has a molecular weight of 410,000 and consists of five types of subunits of molecular weights of 60,000 (α), 58,000 (β), 34,000 (γ), 14,000 (δ), and 11,000 (?). The subunit structure is suggested to be α3β3γδ?. The enzyme is activated by Mg2+ and Ca2+. The pH optima of the enzyme with 0.1 and 2.0 mm Mg2+ are 9 and 6, and those with 1 and 10 mm Ca2+ are 8–9 and 7, respectively. Ca2+-ATPase hydrolyzes only ATP, whereas Mg2+-ATPase hydrolyzes GTP and, to a lesser extent, ATP. The values of V and Km of the enzyme with ATP in the presence of 10 mm Ca2+ or 0.6 mm Mg2+ at pH 7.2 are 17 or 0.5 units/mg protein and 1.2 or 0.3 mm, respectively. The enzyme with Mg2+ is appreciably activated by HCO?3. Relationship of the ATPase to the active transport system in the bacterium is suggested. 相似文献
11.
《Animal reproduction science》1996,44(3):139-148
Different ATPases may control the various functional changes that spermatozoa undergo just prior to fertilization, with the enzyme's specific location within the cell reflecting its function. The activities of Mg2+-ATPase, Ca2+Mg2+-ATPase, Na+K+-ATPase and Ca2+-ATPase were determined for head plasma membranes (HPM) and sperm body membrane (SBM) from both fresh (n = 4) and cryopreserved bovine spermatozoa (n = 4) and fresh homogenized whole spermatozoa (HWS) (n = 6). No activity of Ca2+Mg2+-ATPase was found in any preparation from spermatozoa. Ca2+-ATPase was detected in fresh SBM and HWS but not in HPM. Activity of Mg2+-ATPase and Na+K+-ATPase was higher in HPM than HWS or SBM (P < 0.01). Cryopeserving the whole sperm reduced the activities of all three enzymes, but Na+K+-ATPase was more sensitive to cryopreservation than Mg2+-ATPase (P ≤ 0.05). Enzyme location suggests that Ca2+-ATPase may be associated with events in the flagellum, while Mg2+-ATPase and Na+K+-ATPase may affect functions in the sperm head. Cryopreservation-induced damage to ATPases might be involved in reducing the fertilizing ability of cryopreserved spermatozoa. 相似文献
12.
An ATPase activity stimulated by divalent ions (Mg2+, Ca2+, Mn2+, Zn2+) has been observed in intact hamster fibroblasts cultured in vitro (BHK line). Such activity has been determined by the incubation (30 min at 37°C) of washed cell suspensions (about 1 mg of proteins) in a medium containing 100 mM NaCl, 20 mM KCl, 15 mM Tris—HCl (pH 7.4), 10 mM NaHCO3, 5 mM glucose and equimolar concentrations of ATP and divalent cation. Mg2+-ATPase activity is insensitive to ouabain and lacks specificity towards nucleoside triphosphate substrates. AMP and ADP are not hydrolyzed under these conditions. Apparent Km of 0.76 mM and Vmax of 1.46 μmol Pi · mg proteins?1 · h?1 have been calculated for Mg-ATP complex. This ATPase is an ectoenzyme, therefore its activity could be used as a suitable index of the action of chemicals like chromium compounds known for their cytotoxic effects on membrane functions.Salts of trivalent (CrCl3) and hexavalent (K2Cr2O7) chromium at concentrations ranging from 1 mM to 5 mM inhibit Mg2+-ATPase. The inhibition by K2Cr2O7 is observed after pretreatment of the cells with this compound followed by its absence from the assay medium “per se” for Mg2+-ATPase, and it is referred to the alterations of membrane bound enzyme structures by the oxidizing hexavalent chromium. The inhibition by CrCl3 is mainly evident when this compound is present in the incubation medium, and is referred to the interaction of trivalent chromium with Mg2+-ATP as it is partially reversed by increasing Mg2+-ATP concentration. 相似文献
13.
Fred L. Larsen Beat U. Raess Thomas R. Hinds Frank F. Vincenzi 《Journal of cellular biochemistry》1978,9(2):269-274
Red blood cells contain a protein that activates membrane-bound (Ca2+ + Mg2+)-ATPase and Ca2+ transport. The red blood cell activator protein is similar to a modulator protein that stimulates cyclic AMP phosphodiesterase. Wang and Desai [Journal of Biological Chemistry 252:4175–4184, 1977] described a modulator-binding protein that antagonizes the activation of cyclic AMP phosphodiesterase by modulator protein. In the present work, modulator-binding protein was shown to antagonize the activation of (Ca2+ + Mg2+)-ATPase and Ca2+ transport by red blood cell activator protein. The results further demonstrate the similarity between the activator protein from human red blood cells and the modulator protein from bovine brain. 相似文献
14.
Fructose 1,6-bisphosphatase (EC 3.1.3.11) from Saccharomyces cerevisiae has been purified to homogeneity. A molecular weight of 115,000 has been obtained by gel filtration. The enzyme appears to be a dimer with identical subunits. The apparent Km for fructose bisphosphatase varies with the Mg2+ concentration of the enzyme, being 1 × 10?6m at 10 mm Mg2+ and 1 × 10?5m at 2 mm Mg2+. Other phosphorylated compounds are not significantly hydrolyzed by the enzyme. An optimum pH of 8.0 is exhibited by the enzyme. This optimum is not changed by addition of EDTA. AMP inhibits the enzyme with a Ki of 8.0 × 10?5m at 25 °C. The inhibition is temperature dependent, the value of Ki increasing with raising temperature. 2-Deoxy-AMP is also inhibitory with a Ki value at 25 °C of 1.6 × 10?4m. An ordered uni-bi mechanism has been deduced for the reaction with phosphate leaving the enzyme as the first product and the fructose 6-phosphate as the second one. 相似文献
15.
The rate of Ca2+ accumulation in plasmalemma vesicles isolated from quiescent and sprouting potato (Solanum tuberosum L.) tubers and the effect of 10?5–10?10 M jasmonic acid on the accumulation of Ca+2 in plasmalemma vesicles and its efflux were studied. It was found that potato tuber plasmalemma contains a Ca+2,Mg+2-ATPase whose activity decreases upon the transition from forced quiescence to growth. The direction of the effect of jasmonic acid on Ca+2,Mg+2-ATPase (stimulation or suppression) depends on the physiological state of tubers and the phytohormone concentration. 相似文献
16.
We measured the fluorescence static anisotropy and the time-resolved fluorescence anisotropy decay of F-actin labelled with N-iodoacetyl-N'-(5-sulfo-1-naphthyl)ethylenediamine at 20°C in solutions containing 100 mM KCl and free Ca2+ at various concentrations. The average fluorescence anisotropy and the fluorescence rotational correlation time of actin decreased in the presence of micromolar concentrations of free Ca2+. The change of the rotational correlation time of labelled actin could not be explained by a variation of the actin critical concentration. We concluded therefore that F-actin undergoes a conformational change induced by Ca2+ binding. The binding constant was 6 × 106 M?1. 相似文献
17.
M Terai C Furihata T Matsushima T Sugimura 《Archives of biochemistry and biophysics》1976,176(2):621-629
(i) Three forms of cyclic AMP phosphodiesterases (3′,5′-cyclic AMP 5′-nucleotidohydrolase, EC 3.1.4.17), F1, F2-I and F2-II, were partially purified from the soluble fraction of rat pancreas in the presence of excess protease inhibitors by DEAE-cellulose column chromatography and gel filtration and were characterized. (ii) F2-II, which was purified 31-fold, exhibited a single peak of activity on both polyacrylamide-gel electrophoresis and isoelectric focusing. The enzyme had a molecular weight of about 70,000, an isoelectric point of 3.9, and an optimal pH around 8.5 and required Mg2+ or Mn2+ but not Ca2+ for activity. The Km values of this enzyme for cyclic AMP and cyclic GMP were 1 and 50 μm, respectively, while V values of this enzyme for cyclic AMP and cyclic GMP were 36.1 and 12.6 nmol min?1 (mg of protein)?1, respectively. Cyclic GMP competitively inhibited hydrolysis of cyclic AMP by this enzyme. Ro20-1724 [4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone] also inhibited hydrolysis of cyclic AMP competitively, with a Ki value of 1 μm. (iii) Fraction F1, which was purified 10-fold, had a molecular weight of more than 500,000 and required Mg2+ for activity. Its Km values for cyclic AMP were 1 and 5 μm. Its Km value for cyclic GMP was 45 μm. Fraction F2-I, which was purified 26-fold, had a molecular weight of about 70,000. The ratio of the initial velocity of hydrolysis of cyclic GMP to that of cyclic AMP was 0.5 at a substrate concentration of 1 μm. 相似文献
18.
《Biochimica et Biophysica Acta (BBA) - Enzymology》1978,522(1):151-160
In the presence of 10 μM Ca2+ and 5 mM Mg2+ (or 0.25 mM Mg2+), the addition of 100 μM 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 μM, respectively. Zn2+ inhibition was non-competitive with substrate, activator and Ca2+ but was competitive with Mg2+. In the presence of 10 μM Ca2+ and activator, a Ki of 15 μM 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. 相似文献
19.
Homogenates of 7-day-old oat (Avena sativa L. cv. Brighton) roots were highly fluorescent (excitation and emission maxima around 360 and 440 nm, respectively). Less than 1/10 as much fluorescence per g fresh weight was found in oat shoots or in wheat (Triticum aestivum L. cv. Drabant) roots or shoots. Most of the fluorescence of oat roots was found in the soluble fraction (150 000g supernatant). However, some could be detected in the plasma membrane fraction (excitation and emission maxima 365 and 417 nm, respectively), which contained a 3-fold higher fluorescence per mg protein than the homogenate. Growth of oat or wheat in a medium containing, 10-?5M scopoletin (6-methoxy-7-hy-droxy coumarin), a fluorescent compound previously reported to be present in both wheat and oat roots, caused the disappearance of scopoletin from the medium (proportional to the amount of roots) and the appearance of increased fluorescence in the root homogenates but not in the shoot homogenates. In both oat and wheat roots ail of the extra fluorescence was recovered in the soluble fraction and at least in wheat it consisted of unconverted scopoletin. The concentration of scopoletin in wheat roots grown in 10-?5M scopoletin was around 50 nmol (g fresh weight)?1, or about five times the concentration in the growth medium. Scopoletin in the growth medium (10-?5M) or in the assays (up to 10-?4M) did not affect Mg2+-, Mg2++K+- or Ca2+-ATPase activities in wheat or oat roots. The fluorescence properties of the oat plasma membrane were different from those of authentic scopoletin. Either the surroundings modify the fluorescence of membrane-associated scopoletin or the endogenous fluorescent compound is not scopoletin but a glycoside-derivative of scopoletin or some completely unrelated compound. 相似文献
20.
Betty L. Black Jay M. McDonald Leonard Jarett 《Archives of biochemistry and biophysics》1980,199(1):92-102
The presence of an energy-dependent calcium uptake system in adipocyte endoplasmic reticulum (D. E. Bruns, J. M. McDonald, and L. Jarett, 1976, J. Biol. Chem.251, 7191–7197) suggested that this organelle might possess a calcium-stimulated transport ATPase. This report describes two types of ATPase activity in isolated microsomal vesicles: a nonspecific, divalent cation-stimulated ATPase (Mg2+-ATPase) of high specific activity, and a specific, calcium-dependent ATPase (Ca2+ + Mg2+-ATPase) of relatively low activity. Mg2+-ATPase activity was present in preparations of mitochondria and plasma membranes as well as microsomes, whereas the (Ca2+ + Mg2+)-ATPase activity appeared to be localized in the endoplasmic reticulum component of the microsomal fraction. Characterization of microsomal Mg2+-ATPase activity revealed apparent Km values of 115 μm for ATP, 333 μm for magnesium, and 200 μm for calcium. Maximum Mg2+-ATPase activity was obtained with no added calcium and 1 mm magnesium. Potassium was found to inhibit Mg2+-ATPase activity at concentrations greater than 100 mm. The energy of activation was calculated from Arrhenius plots to be 8.6 kcal/mol. Maximum activity of microsomal (Ca2+ + Mg2+)-ATPase was 13.7 nmol 32P/mg/min, which represented only 7% of the total ATPase activity. The enzyme was partially purified by treatment of the microsomes with 0.09% deoxycholic acid in 0.15 m KCl which increased the specific activity to 37.7 nmol 32P/mg/min. Characterization of (Ca2+ + Mg2+)-ATPase activity in this preparation revealed a biphasic dependence on ATP with a Hill coefficient of 0.80. The apparent Kms for magnesium and calcium were 125 and 0.6–1.2 μm, respectively. (Ca2+ + Mg2+)-ATPase activity was stimulated by potassium with an apparent Km of 10 mm and maximum activity reached at 100 mm potassium. The energy of activation was 21.5 kcal/mol. The kinetics and ionic requirements of (Ca2+ + Mg2+)-ATPase are similar to those of the (Ca2+ + Mg2+)-ATPase in sarcoplasmic reticulum. These results suggest that the (Ca2+ + Mg2+)-ATPase of adipocyte endoplasmic reticulum functions as a calcium transport enzyme. 相似文献