Calcium transport and ATPase activities of sarcoplasmic reticulum with adenosine 5'-O-(2-thiotriphosphate) diastereomers as substrates

Phenylarsine oxide increases intracellular calcium mobility and inhibits Ca(2+)-dependent ATPase activity in thymocytes

A rise in intracellular Ca(2+) levels has been implicated as a regulatory signal for the initiation of lymphocyte proliferation. In the present study the mechanism underlying the elevation of [Ca(2+)] induced by phenylarsine oxide [PAO] was investigated in thymocytes. This agent inhibits HIV-1 replication and also NF-kappaB-mediated activation. It has been reported that the PAO-induced Ca(2+) elevation results from an enhanced plasma membrane calcium permeability in T cells. Here, we present biochemical evidence that the PAO-induced Ca(2+) increase was independent of external Ca(2+). Consistent with these facts, when [Ca(2+)](i) was depleted by prolonged incubation of the cells in Ca(2+)-free medium, PAO addition did not lead to [Ca(2+)](i) increase. These data indicate the involvement of intracellular organelles of thymocytes as the source of Ca(2+). Moreover, evidence is presented that PAO inhibited Ca(2+)-dependent ATPase activity from thymocytes and sarcoplasmic reticulum from skeletal muscle. This inhibition was dose-dependent, with a IC(50) of about 30 microM for both preparations of enzyme. The ability of PAO to inhibit Ca(2+)-dependent ATPase represents a novel mechanism of action for this drug. Present data suggest that the PAO-dependent [Ca(2+)](i) increase could be mainly the result of inhibition of Ca(2+)-dependent ATPase. In addition, we describe also a Ca(2+)-dependence for PAO effect on tyrosine phosphorylation.

The reaction mechanism of Ca(2+)-ATPase of sarcoplasmic reticulum. Direct measurement of the Mg.ATP dissociation constant gives similar values in the presence or absence of calcium

Combining rapid filtration and rapid acid quenching, we have directly measured, at pH 7.0 and 5 degrees C, the association and dissociation rate constants of Mg.ATP binding to the sarcoplasmic reticulum (SR) ATPase in the presence of 50 microM calcium and 5 mM MgCl2 (3-4 x 10(6) M-1.s-1 and 9 s-1, respectively). Therefore, we have determined the true affinity for Mg.ATP (Kd = 3 microM) in the presence of calcium, which can not be measured at equilibrium because of spontaneous and fast phosphorylation. At low concentrations, Mg.ATP binding is the rate limiting step in the phosphorylation process, and Mg.ATP dissociation is slower than dephosphorylation. The kinetics of Ca2+ binding measured by rapid filtration are biphasic, reflecting a two-step mechanism, both steps being accelerated by Mg.ATP. Combining rapid filtration and rapid monitoring of the intrinsic fluorescence of SR Ca(2+)-ATPase, we showed that rate constants for calcium binding are always lower than those of Mg.ATP binding to an EGTA-incubated enzyme. We measured dissociation and association rate constants of Mg.ATP binding in the absence of calcium (k-1 = 25 s-1 and k1 = 7.5 10(6) M-1.s-1). This gives a Kd similar to that obtained by equilibrium measurements (3-4 microM). Both non-phosphorylated conformations of the enzyme have similar affinity for Mg.ATP. Therefore, activation of ATPase activity by an excess of ATP cannot be explained by a change in affinity of the non-phosphorylated enzyme for Mg.ATP. In conjunction with previous results, these data are used to discuss the molecular mechanism for the Ca(2+)-ATPase cycle, in which ATP is sequentially substrate and activator on a multiple-function single site.

Calcium release induced by inositol 1,4,5-trisphosphate in thymocyte microsomes. Inhibition by barium and strontium

The properties of calcium transport in microsomes and the effect of inositol 1,4,5-trisphosphate (IP3) on accumulated calcium were studied in rat thymocytes. Active calcium transport shows an apparent affinity constant for calcium of 0.2 +/- 0.01 microM and a maximal velocity of 2.3 +/- 0.6 nmol/mg/30 min (mean +/- SD). IP3 was able to induce release of calcium only in the absence of oxalate. At 6 microM ambient free calcium, half-maximal effect of IP3 was attained at 2 microM and maximal calcium release was produced by IP3 concentrations over 5 microM. Barium and strontium did not modify calcium uptake by microsomes but markedly inhibited the action of IP3.

Investigation of the substrate structure and metal cofactor requirements of the rat liver mitochondrial ATP synthase/ATPase complex

The F1 moiety of the rat liver mitochondrial ATP synthase/ATPase complex contains as isolated 2 mol Mg2+/mol F1, 1 mol of which is nonexchangeable and the other which is exchangeable (N. Williams, J. Hullihen, and P.L. Pedersen, (1987) Biochemistry 26, 162-169). In addition, the enzyme binds 1 mol ADP/mol F1 and 3 mol AMP.PNP, the latter of which can bind in complex formation with divalent cation and displace the Mg2+ at the exchangeable site. Thus, in terms of ligand binding sites the fully loaded rat liver F1 complex contains 3 mol MgAMP.PNP, 1 mol ADP, and 1 mol Mg2+. In this study we have used several metal ATP complexes or analogs thereof to gain further insight into the ligand binding domains of rat liver F1 and the mechanism by which it catalyzes ATP hydrolysis in soluble and membrane bound form. Studies with LaATP confirmed that MgATP is the most likely substrate for rat liver F1, and provided evidence that the enzyme may contain additional Mg2+ binding sites, undetected in previous studies of F1-ATPases, that are required for catalytic activity. Thus, F1 containing the thermodynamically stable LaATP complex in place of MgATP requires added Mg2+ to induce ATP hydrolysis. As Mg2+ cannot readily displace La2+ under these conditions there appears to be a catalytically important class of Mg2+ binding sites on rat liver F1, distinct from the nonexchangeable Mg2+ site and the sites involved in binding MgATP. Additional studies carried out with exchange inert metal-nucleotide complexes involving rhodium and the Mg2+ and Cd2+ complexes of ATP beta S and ATP alpha S imply that the rate-limiting step in the ATPase reaction pathway occurs subsequent to the P gamma-O-P beta bond cleavage steps, perhaps at the level of Mg(ADP)(Pi) hydrolysis or MgADP release. Evidence is presented that Mg2+ remains coordinated to the leaving group of the reaction, i.e., the beta phosphoryl group. Finally, in contrast to soluble F1, F1 bound to F0 in the inner mitochondrial membrane failed to discriminate between the Mg2+ complexes of the ATP beta S isomers. This indicates that a fundamental difference may exist between the catalytic or kinetic mechanism of F1 and the more physiologically intact F0F1 complex.

Cytosolic ionized calcium concentration in isolated chondrocytes from each zone of the growth plate

In isolated chondrocytes from the growth plate, there is an increase in both the cytosolic ionized calcium concentration and in total cell calcium content as the cells approach the mineralization front. The reserve zone chondrocytes have a cytosolic ionized calcium concentration, [Ca2+]i, of 124 +/- 23 nM and a total cell calcium content, CaT, of 12.8 +/- 6.8 nmol/mg cell protein. Proliferative zone chondrocytes have a [Ca2+]i of 172 +/- 29 nM and a CaT of 16.1 +/- 11.8 nmol/mg cell protein. Hypertrophic zone chondrocytes have a [Ca2+]i of 273 +/- 49 nM and a CaT of 35.8 +/- 16 nmol/mg protein. Chondrocytes isolated from the entire growth plate have a [Ca2+]i of 132 +/- 29 nM and a CaT of 13.2 +/- 2.5 nmol/mg cell protein. Chondrocytes isolated from noncalcifying hyaline cartilage have a [Ca2+]i of 85 +/- 10 nM and a CaT of 11.5 +/- 1.4 nmol/mg cell protein. There appears to be a correlation between intracellular calcium accumulation, an increase in the intracellular ionized calcium concentration, and the process of cartilage matrix mineralization.

Mode of action of diethyl ether on ATP-dependent Ca2+ transport by sarcoplasmic reticulum vesicles

The mode of action of diethyl ether on the sarcoplasmic reticulum Ca2+ pump and ATPase activity was investigated in sarcoplasmic reticulum vesicles (SR). The addition of ether (5%, v/v) at 23 degrees nearly doubled the rates of ATP-dependent Ca2+ uptake and ATP hydrolysis by SR for a wide range of ATP concentrations (20 microM to 10 mM). By contrast, the Ca2+-independent ATPase activity of SR decreased with increasing ether concentrations and practically ceased at higher ether solutions (5-7%, v/v). Ether not only enhanced the forward transport of Ca2+ into vesicles but also the reversal of the Ca2+ pump, and higher rates of Ca2+ efflux coupled to ATP synthesis were observed from Ca2+-loaded vesicles. Electron micrographs of SR pellets showed that the average radius of the vesicles increased by about 20% upon exposure to ether. In the range of 5-40 degree the rate of Ca2+ transport increased with temperature; at about 4 degrees active Ca2+ transport by SR normally ceased, but with the addition of ether (5%, v/v) significant Ca2+ transport (1-2 nmoles Ca2+ per mg per sec) occurred at 0 degree. The further stimulation of SR Ca2+ transport induced by ether was particularly effective at low temperatures; ether increased the rate of Ca2+ uptake by a factor of 10 at 5 degrees but only by a factor of 1.5 at 40 degrees. The effect of ether on the (Ca2+-Mg2+)ATPase of SR could be reversed by resuspending the vesicles in ether-free media. The centrifugation and resuspension of SR in ether-free solutions did not make the vesicles leaky nor did it lyse them irreversibly unless they were also mechanically disrupted. The results indicate that, in ether, there was an increase of intravesicular volume and an increase in membrane fluidity of SR, which could account for the dramatic increase in Ca2+ capacity and rate of Ca2+ transport of ether-treated SR.