Regulation of endogenously catalyzed ADP-ribosylation in adipocyte plasma membranes by Ca and calmodulin

The effects of Ca²⁺ and calmodulin on endogenously catalyzed ADP-ribosylation were investigated in adipocyte plasma membranes. Four specific proteins of 70, 65, 61 and 52 kDa were labeled with [³²P]ADP-ribose and ADP-ribosylation of the proteins was highly dependent upon the conditions employed. ADP-ribosylation of the 70 kDa protein was observed only in membranes supplemented with Ca²⁺. Maximal incorporation of [³²P] into the protein was achieved with free Ca²⁺ concentrations of 90 μM. Calcium-stimulated ADP-ribosylation of the 70 kDa protein was inhibited by calmodulin. Half-maximal inhibition was observed in membranes incubated with 1.2 μM calmodulin. The effect of calmodulin was characterized by an inhibition of the incorporation of [³²P]ADP-ribose as opposed to a stimulation of its removal. ADP-ribosylation of the 61 kDa protein was not altered by added Ca²⁺ and/or calmodulin whereas ADP-ribosylation of the 65 kDa protein was partially (50%) inhibited by free Ca²⁺ concentrations between 10⁻⁶ – 10⁻⁵ M. These results provide evidence that the adipocyte plasma membrane contains ADP-ribosyltransferase activities and demonstrate that ADP-ribosylation of a 70 kDa protein is regulated by Ca²⁺ and calmodulin.     

Relation between Ca2+-ATPase and endogenous calmodulin of human erythrocyte membranes

Short incubation of erythrocyte membranes with oleic acid releases Ca2+-independently bound endogenous calmodulin together with a minor fraction of membrane-associated proteins without destruction of the membranes. The released endogenous calmodulin is similar if not identical to cytosolic calmodulin reversibly bound to ghosts in a Ca2+-dependent manner. The release of endogenous calmodulin proceeds without affecting the activity of Ca2+-ATPase when ghosts are incubated with oleic acid in the presence of Ca2+ plus ATP and thereafter freed from oleic acid by washings with serum albumin. Kinetic parameters of Ca2+-ATPase of ghosts with and without endogenous calmodulin are identical as are amounts of exogenous calmodulin bound to these ghosts. Thus, endogenous calmodulin does not function as an essential part of Ca2+-ATPase.     

The sequestration of Ca2+ by mitochondria in rat heart cells

Rat heart ventricular cells, purified by Percoll density gradient centrifugation, were incubated in the presence of 1.3 mM CaCl2. After 20 min incubation, samples of the cells were lysed in medium containing 0.3 mM digitonin, ruthenium red and EGTA, and a mitochondrial fraction was isolated at intervals thereafter. Extrapolation of the mitochondrial 45Ca2+ contents to zero time enabled the endogenous 45Ca2+ to be estimated at the time of cell lysis. The lysis conditions yielded essentially complete release of lactate dehydrogenase from the cells, but caused negligible damage to the mitochondria as judged by their retention of glutamate dehydrogenase, and their ability to accumulate and retain Ca2+ in the absence of ruthenium red and EGTA. The data indicate that about 13% of total cell Ca2+ only may be mitochondrial in vivo.     

The stoichiometry of the Ca2+ pump in human erythrocyte vesicles: modulation by Ca2+, Mg2+ and calmodulin

Active Ca2+ uptake and the associated (Ca2+ + Mg2+)-ATPase activity were studied under the same conditions in an inside-out vesicle preparation of human red blood cells made essentially by the procedure of Quist and Roufogalis (Journal of Supramolecular Structure 6, 375-381, 1977). Some preparations were treated with 1 mM EDTA at 30 degrees to further deplete them of endogenous levels of calmodulin. As the Ca2+ taken up by the EDTA-treated inside-out vesicles, as well as the non-EDTA treated vesicles, was maintained after addition of 4.1 mM EGTA, the vesicles were shown to be impermeable to the passive leak of Ca2+ over the time course of the experiments. In the absence of added calmodulin, both active Ca2+ uptake and (Ca2+ + Mg2+)-ATPase were sensitive to free Ca2+ over a four log unit concentration range (0.7 microM to 300 microM Ca2+) at 6.4 mM MgCl2. Below 24 microM Ca2+ the stoichiometry of calcium transported per phosphate liberated was close to 2:1, both in EDTA and non-EDTA treated vesicles. Above 50 microM Ca2+ the stoichiometry approached 1:1. When MgCl2 was reduced from 6.4 mM to 1.0 mM, the stoichiometry remained close to 2:1 over the whole range of Ca2+ concentrations examined. In contrast to the results at 6.4 mM MgCl2, the Ca2+ pump was maximally activated at about 2 microM free Ca2+ and significantly inhibited above this concentration at 1 mM MgCl2. Calmodulin (0.5-2.0 microgram/ml) had little effect on the stoichiometry in any of the conditions examined. The possible significance of a variable stoichiometry of the Ca2+ pump in the red blood cell is discussed.     

Plant and fungal calmodulin: Ca2+-dependent regulation of plant NAD kinase

Although little is known about the role(s) of second messengers, including free Ca2+, in plant cells there has been increasing evidence for a role for Ca2+ in metabolic regulation in plants. The recent demonstration that the Ca2+-binding protein, calmodulin exists in extracts of higher plants and basidiomycete fungi provides a basis for understanding Ca2+-dependent metabolic regulation in plant cells. In this review we summarize the similarities and differences of plant, fungal and mammalian calmodulin. We also discuss the known in vitro functions of calmodulin in higher plants. A Ca2+-calmodulin-dependent NAD kinase has been purified to homogeneity from extracts of pea seedlings and shown to be absolutely dependent upon calmodulin and microM levels of free Ca2+ for activity. The available evidence suggest that this Ca2+-calmodulin-dependent NAD kinase is the major form of plant NAD kinase and that this regulatory enzyme is localized in the chloroplast. A model is presented which predicts that the rate of photosynthesis is regulated by a receptor-mediated change in the level of chloroplastic free Ca2+ upon illumination. Free Ca2+, acting as a second messenger, forms a Ca2+-calmodulin complex thus converting calmodulin to its active conformation. This Ca2+-calmodulin complex then activates chloroplastic NAD kinase resulting in an increased NADP/NAD ratio.     

Characterization of a (Ca2+ + Mg2+)-ATPase activator bound to human erythrocyte membranes

Incubation of human erythrocyte ghosts with an equal volume of 0.2 mM EDTA in isotonic KCl decreased both the activity and Ca2+ sensitivity of the (Ca2+ + Mg2+)-ATPase remaining associated with the membrane. Readdition of the EDTA-extract activated the (Ca2+ + Mg2+)-ATPase activity. The activator activity was trypsin sensitive, heat stable and retained by a phenothiazine affinity column, consistent with properties expected of calmodulin. However, unlike calmodulin, the activity was not retained by DEAE Sephadex A-50 and it eluted from Sephacryl S-200 as heterogeneous peaks of activator activity of apparent molecular weight between 107,000 and 178,000. Nevertheless, the activator in the EDTA extract both before and after gel filtration contained calmodulin, as determined by radioimmunoassay and by its activation of calmodulin - deficient phosphodiesterase. SDS-gel electrophoresis of the activator isolated by gel filtration showed a protein of Mr 56,000 in addition to a low molecular weight protein corresponding to calmodulin. It is suggested that the red cell membrane contains a calmodulin binding protein which tightly binds calmodulin as a polymeric complex in a Ca2+-independent manner.     

Stimulation by calmodulin of Ca2+ uptake and (Ca2+-Mg2+) ATPase activity in membrane fractions from ox neurohypophyses

Calmodulin stimulated ⁴⁵Ca²⁺ uptake into a plasma membrane enriched fraction from ox neurohypophysial nerve endings and into a microsome fraction. The ⁴⁵Ca²⁺ uptake and the (Ca²⁺-Mg²⁺) ATPase activity in the plasma membrane fraction exhibited similar pCa and calmodulin sensitivities, suggesting that the enzyme activity is the biochemical expression of a high affinity Ca²⁺ pump. Calmodulin thus seems to play a role in regulation of the intracellular free Ca²⁺ concentration in the neurohypophysis.     

Purified red blood cell Ca2+-pump ATPase: evidence for direct inhibition by presumed anti-calmodulin drugs in the absence of calmodulin

A variety of presumed anti-calmodulin (anti-CaM) drugs was tested for their potential inhibitory effects on the isolated, purified and reconstituted Ca2+-pump ATPase of human red blood cell membranes. Anti-CaM drugs inhibited the Ca2+-pump ATPase both in the absence and presence of added CaM. Qualitatively similar inhibition was observed in two different ATPase assay systems. In asolectin vesicles in the absence of added CaM trifluoperazine (TFP), N-(6-aminohexyl)-5-chloro-1-naphthalene- sulfonamide (W-7), vinblastine, dibucaine, imipramine, propranolol and dimethylpropranolol (UM-272) were all inhibitory. Potency of anti-CaM drugs was generally greater on the enzyme reconstituted in asolectin vesicles than on the enzyme reconstituted in phosphatidylcholine vesicles, either in the presence or absence of CaM. The results emphasize that anti-CaM drugs have actions other than to bind to CaM. Possible direct interaction of amphipathic cationic anti-CaM drugs with the Ca2+-pump ATPase and/or its lipid environment is suggested.     

Inhibition of Ca2+-dependent protein kinase and Ca2+/Mg2+-ATPase in cardiac sarcolemma by the anti-calmodulin drug calmidazolium

Sarcolemma (SL) vesicles, isolated from pig heart, contain both a Ca2+-calmodulin-dependent protein kinase (CaM-PK) and a Ca2+-dependent Mg2+-ATPase (Ca2+/Mg2+)-ATPase). Some of their properties have been compared: their affinity for Ca2+ ions, dependence on exogenous calmodulin (CaM) and sensitivity to the anti-CaM drug calmidazolium (R24571). The properties of Ca2+-CaM-dependent brain phosphodiesterase (PDE) have also been examined. R24571 appeared to be the most potent inhibitor from brain PDE. For the three enzymes studied, exogenously added CaM was able to antagonize the R24571 inhibition, although the efficiency to counteract was rather low in the case of the SL Ca2+/Mg2+-ATPase. R24571 decreased the affinity of the Ca2+/Mg2+-ATPase for Ca2+ ions (KCa 0.35 versus 0.75 microM) and exerted an inhibition non-competitive with Ca2+ ions on the other CaM-dependent enzymes. Membrane-bound CaM, which is involved in controlling the Ca2+/Mg2+-ATPase, appeared to be present in a stoichiometry varying from 1:1 to 1:4 compared to the 32P-intermediate of the ATPase. R24571 treatment of SL vesicles selectively solubilized a number of proteins in the molecular range of 13-20 kD, which may include CaM. The results suggest that different mechanisms are involved in the CaM control of the two SL enzymes studied.     

Ca2+-independent stimulation of cyclic GMP-dependent protein kinase by calmodulin

Calmodulin purified from bovine brain markedly stimulated cyclic GMP-dependent protein kinase from pig lung in the presence of cyclic GMP. This stimulation by calmodulin did not require Ca²⁺ and was dose-dependent up to optimal amounts, but the extent of stimulation decreased at concentrations over the optimal condition. The concentrations of cyclic GMP and cyclic AMP producing half-maximal stimulation were 4.5 × 10^?8 M and 5.0 × 10^?6 M respectively, under optimal conditions. Calmodulin increased maximum velocity without altering the Km for ATP. These effects of calmodulin on cyclic GMP-dependent protein kinase were similar to those of the stimulatory modulator described by Kuo and Kuo (J. Biol. Chem. $\text{251}$, 4283–4286, 1976). Ouf findings indicate that calmodulin regulates enzyme activity both Ca²⁺-dependently and independently.     

Effect of calmodulin,Ca2+ and Mg2+ on the (Ca2+ + Mg2+)-ATPase of erythrocyte membranes

Calmodulin-depleted isotonic erythrocyte ghosts contain 200 ng residual calmodulin/mg protein which is not removed by extensive washings at pCa²⁺ > 7. Specific activity and Ca²⁺-affinity of the (Ca²⁺ + Mg²⁺)ATPase increase at increasing calmodulin, with K_{0.5 Ca} of 0.38 μM at calmodulin concentrations corresponding to that in erythrocytes. High Ca²⁺ concentrations inhibit the enzyme. Specific activity and Ca²⁺-affinity of the enzyme decrease at increasing Mg²⁺ concentrations. The Ca²⁺ ? Mg²⁺ antagonism is likewise observed at inhibitory Ca²⁺ concentrations.     

The role of calmodulin in the regulation of (Mg2+ + Ca2+)-ATPase activity in erythrocyte membranes of cystic fibrosis patients and controls

(Mg²⁺ + Ca²⁺)-ATPase activity has been found to be significantly reduced in EDTA-washed erythrocyte membrane preparations from cystic fibrosis patients compared to aged-matched controls. Calmodulin was found to be present in erythrocytes from cystic fibrosis patients and characterized similarly to calmodulin isolated from control preparations. Calmodulin from control erythrocyte preparations stimulated the (Mg²⁺ + Ca²⁺)-ATPase activity of EDTA-washed erythrocyte membranes derived from cystic fibrosis patients to the same extent as those membranes derived from controls. Similarly, calmodulin obtained from erythrocytes of cystic fibrosis patients stimulated the (Mg²⁺ + Ca²⁺)-ATPase activity of control and cystic fibrosis erythrocyte membrane preparations to a similar extent. These results indicate that this decrease in (Mg²⁺ + Ca²⁺)-ATPase activity in erythrocytes from cystic fibrosis patients is not due to an alteration in the regulatory function of calmodulin.     

Reconstitution of the purified calmodulin-dependent (Ca2+ + Mg2+)-ATPase from smooth muscle

The purified calmodulin dependent (Ca2+ + Mg2+)-ATPase (CaMg ATPase) from porcine antral smooth muscle transports Ca2+ after reconstitution in lipid vesicles indicating that this enzyme is indeed a Ca2+-transport ATPase. For CaMg ATPase reconstituted in asolectin vesicles a good correlation was found between the time course of Ca2+ accumulation and the corresponding changes in CaMg ATPase activity. The ATPase activity was stimulated 8-fold by A23187, which further indicates a tight coupling between ATP hydrolysis and Ca2+ transport. Asolectin vesicles with incorporated enzyme accumulated Ca2+ with a ratio approaching one Ca2+ ion transported for each ATP hydrolyzed. For CaMg ATPase reconstituted in phosphatidylcholine vesicles on the other hand, Ca2+ transport and CaMg ATPase were poorly coupled as is shown by the approximately 3.5 fold stimulation by A23187. The activity of the CaMg ATPase when reconstituted in asolectin vesicles was stimulated 1.25 fold by calmodulin while in phosphatidylcholine a value of 4.25 was obtained. The CaMg ATPase activity of the enzyme reconstituted either in asolectin or phosphatidylcholine was, after its stimulation by A23187, still further stimulated by detergent by a factor of 5.     

Inhibition of brain mitochondrial Ca2+ transport by amiloride analogues

Rat brain mitochondrial Ca2+ uptake and release were examined in the presence of amiloride (3,5-diamino-6-chloro-N-(diaminomethylene)-pyrazinecarboxamide) and nineteen amiloride analogues. Amiloride, an inhibitor of Na+-Ca2+ exchange in plasmalemma membranes, did not affect energy-dependent Ca2+ uptake, whereas several other analogues were inhibitors. Similarly, amiloride did not alter Ca2+ release in the presence or absence of Na+. However, some analogues were found that stimulated and others that inhibited Ca2+ release. While many of these analogues reduced mitochondrial respiratory control ratios, two analogues were identified which inhibited Ca2+ uptake but did not alter mitochondrial respiratory control. Similarly two analogues were identified which inhibited Ca2+ efflux without affecting respiratory control.     

Characterization of Mg2+- and (Ca2+ + Mg2+)-ATPase activity in adipocyte endoplasmic reticulum

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 K_m 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 K_m^s for magnesium and calcium were 125 and 0.6–1.2 μm, respectively. (Ca²⁺ + Mg²⁺)-ATPase activity was stimulated by potassium with an apparent K_m 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.     

Characteristics of Ca2+ uptake in mitochondria of brown adipose tissue

The rate of calcium uptake in brown adipose tissue mitochondria is here shown to be a sensitive parameter of energisation in this tissue, as demonstrated by high susceptibility to purine nucleotides and albumin. Complete uptake of low amounts of calcium generally requires added phosphate. Bicarbonate can at least partially substitute for phosphate, whereas acetate cannot. Calcium transport in brown fat mitochondria is of interest due to recent indications of an important role of this organelle in regulation of cytosolic calcium levels.     

Transport and control of Ca2+ by pigeon erythrocytes. I. Survey of some cell responses to a range of A23187 doses in the presence of Ca2+

Cells were subjected to a range of 45Ca2+ influx loads with A23187. We measured cell 45Ca2+ with time and A23187 dose, and the apparent 45Ca2+ influxes (identical to "J(in,app)") at Ca2+ steady state. We also measured endogeneous exchangeable and total cell Ca2+, which were 50 and 17-220 microM respectively. The effects of A23187 and Ca2+ on cell ATP, swelling, net Cl- permeability, and cell morphology were measured. These were modest and do not affect our conclusions. J(in,app) congruent to 3 X 10(-4) [A23187]2.9 X [Ca2+(o)]mumoles/l X min with 92-552 microM [Ca2+(o)] (identical to external Ca2+ concentration) and 0-7 microM A23187. J(in,app) was increased an order of magnitude by vanadate and is probably much less than the true influx. The least unlikely explanation found for the high [A23187] exponent, 2.9, was that most of the Ca2+ crossing the membrane is expelled by the pump before it can move deeper into the cell. Calcium pumping increased rapidly in response to increased influx, but the steady state cell 45Ca2+ was approximately proportional to J(in,app) rather than approximately constant between 10 and 120 mumoles/l X min with 184 microM [Ca2+(o)]. This is not the result expected from a simple feedback mechanism. At high A23187 doses the pump appears fully activated resulting in a linear relation between cell/medium 45Ca2+ and [A23187]-2. From the plot we calculated alpha identical to free/total exchangeable Ca2+ = 0.38 +/- 0.08 (n = 3) and a maximum pump rate, "Pmax" = 78 mumole/l X min. Pmax is underestimated insofar as J(in,app) is less than the true influx.     

Antagonism of calmodulin and phosphodiesterase by nifedipine and related calcium entry blockers

Nifedipine, a 1,4-dihydropyridine Ca2+ entry blocker, partially inhibits calmodulin-activated and, to a lesser extent, basal (non-activated) cyclic AMP phosphodiesterase activity at 10-440 microM. The inhibition of calmodulin-activated phosphodieserase does not parallel Ca2+ entry blockade, since analogs of nifedipine, which are 500-fold less potent than nifedipine as Ca2+ entry blockers (Bolger et al. (1982) Biochemical and Biophysical Research Communications 104, 1604-1609), are equal in potency to nifedipine as calmodulin-activated phosphodiesterase inhibitors. Furthermore, the inhibition of calmodulin-activated phosphodiesterase by nifedipine is about 500-fold less potent than its inhibition of Ca2+ entry blockade. It is suggested that the low affinity interaction of nifedipine and related 1,4-dihydropyridines with calmodulin and phosphodiesterase is also of low specificity and therefore is unlikely to contribute to the cardiac and vascular muscle relaxant actions of these drugs at normal pharmacological concentrations.