Cross-linking of the (Ca2+ + Mg2+)-ATPase protein

Effects of peroxynitrite on sarcoplasmic reticulum Ca2+ pump in pig coronary artery smooth muscle

Peroxynitrite generated in arteries from superoxide and NO may damage Ca(2+) pumps. Here, we report the effects of peroxynitrite on ATP-dependent azide-insensitive uptake of Ca(2+) into pig coronary artery vesicular membrane fractions F2 [enriched in plasma membrane (PM)] and F3 [enriched in sarcoplasmic reticulum (SR)]. Membranes were pretreated with peroxynitrite and then with DTT to quench this agent. This pretreatment inhibited Ca(2+) uptake in a peroxynitrite concentration-dependent manner, but the effect was more severe in F3 than in F2. The inhibition was thus not overcome by excess DTT used to quench peroxynitrite and was not affected if catalase, SOD, or mannitol was added along with peroxynitrite. Such damage to the pump protein would be difficult to repair if produced during ischemia-reperfusion. The acylphosphates formed with ATP in F3 corresponded mainly to the SR Ca(2+) pump (110 kDa), but in F2 both PM (140 kDa) and 110-kDa bands were observed. Peroxynitrite treatment of F2 inhibited only the 110-kDa band. Inhibition of Ca(2+) uptake and acylphosphate formation from ATP correlated well in peroxynitrite-treated F3 samples. However, inhibition of acylphosphates from orthophosphate (reverse reaction of the pump) was slightly poorer. Peroxynitrite treatment also covalently cross-linked the pump protein, yielding no dimers but only larger oligomers. In contrast, cross-linking of the SR Ca(2+) pump in skeletal and cardiac muscles gives dimers as the first oligomers. Therefore, we speculate that SERCA2 has a different quaternary structure in the coronary artery smooth muscle.

Microdomain arrangement of the SERCA-type Ca2+ pump (Ca2+-ATPase) in subplasmalemmal calcium stores of paramecium cells

We localized SERCA pumps to the inner region of alveolar sac membranes, facing the cell interior, by combining ultrastructural and biochemical methods. Immunogold labeling largely predominated in the inner alveolar sac region which displayed aggregates of intramembrane particles (IMPs). On image analysis, these represented oligomeric arrangements of approximately 8-nm large IMP subunits, suggesting formation of SERCA aggregates (as known from sarcoplasmic reticulum). We found not only monomers of typical molecular size ( approximately 106 kD) but also oligomeric forms on Western blots (using anti-SERCA antibodies, also against endogenous SERCA from alveolar sacs) and on electrophoresis gelautoradiographs of 32P-labeled phosphoenzyme intermediates. Selective enrichment of SERCA-pump molecules in the inner alveolar sac membrane region may eliminate Ca2+ after centripetal spread observed during exocytosis activation, while the plasmalemmal Ca2+ pump may maintain or reestablish [Ca2+] in the narrow subplasmalemmal space between the outer alveolar sac membrane region and the cell membrane. We show for the first time the microzonal arrangement of SERCA molecules in a Ca2+ store of a secretory system, an intensely discussed issue in stimulus-secretion coupling research.

Oligomerization of sarcoplasmic reticulum Ca2+-ATPase from rabbit skeletal muscle

Although the primary structure and catalytic cycle of the sarcoplasmic reticulum Ca2+-ATPase has been revealed, it is not well understood whether functional Ca2+ pump proteins exist in a monomeric or an oligomeric state in native skeletal muscle membranes. Here, we show that the Ca2+-ATPase tends to form high molecular weight complexes, estimated to be dimers and tetramers using immunoblotting of two-dimensionally separated microsomal membranes following crosslinking. This agrees with both electron microscopical and biochemical findings which demonstrate that Ca2+-ATPase clusters are the predominant molecular species in native membranes and that oligomerization may play a role in cooperative kinetics and enzyme stabilization.

Hydroxyl radical-mediated reduction of Ca(2+)-ATPase activity of masseter muscle sarcoplasmic reticulum

To understand the effect of oxygen free radicals on Ca(2+)-ATPase, we used sarcoplasmic reticulum (SR) microsomes of canine masseter muscle as a model system in which to explore the effects of oxidation on a biological membrane, and we investigated the effect of hydroxyl radicals (.OH) generated from Fenton's reagent (H2O2/FeSO4). H2O2 (10 mM) alone had no effect on Ca(2+)-ATPase activity; in the presence of FeSO4 (0.2 mM), H2O2 inhibited the enzyme activity. Oxygen free radical species generated from H2O2/FeSO4 under the conditions employed in the Ca(2+)-ATPase assay were verified by highly sensitive electron spin resonance spectroscopy and the spin-trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) in the absence of SR vesicles; the 1:2:2:1 quartet (AN = A beta H = 1.49 mT), characteristic of the DMPO-OH spin adduct, was observed. The Ca(2+)-ATPase activity was inversely correlated with the calculated signal intensity of DMPO-OH, which is indicative of the amount of .OH radical generated. The effect of Fenton's reagent was effectively inhibited by catalase, dimethylsulfoxide, and dimethylthiourea; the effect was also inhibited by sulfhydryl (SH) group reducing agents, cysteine and dithiothreitol. The SH group modifying agents, p-chloromercuric benzoate and 5,5'-dithiobis(2-nitrobenzoic acid) depressed Ca(2+)-ATPase activity; the effects of the SH group modifying agents used were potentiated in the presence of Fenton's reagent. It is suggested that .OH radical-induced oxidant injury may be caused primarily by modification of the key SH group(s) on the ATPase molecule of masseter muscle SR vesicles.

Uncoupling of Ca2+ transport from ATP hydrolysis activity of sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase

In reconstituted rabbit skeletal muscle (Ca2+ + Mg2+)-ATPase proteoliposomes, Ca(2+)-uptake is decreased by more than 90% with T2 cleavage (Arg-198). However, no difference in the ATP dependence of hydrolysis activity is seen between SR and trypsin-treated SR. A large decrease in E-P formation and hydrolysis activity of the enzyme appear only at T3 cleavage, which represents the cleavage of A1 fragment to A1a + A1b forms. The disappearance of hydrolysis activity due to digestion is prior to the disappearance of E-P formation. No significant difference is found in the passive Ca2+ efflux between control SR and tryptically digested SR in the absence of Mg2+ + ruthenium red or in the presence of ATP. However, the passive Ca2+ efflux rate for tryptically digested SR is much larger than control SR in the presence of Mg2+ + ruthenium red. These results show that the Ca2+ channel cannot be closed after trypsin digestion of SR membranes by the presence of the Ca2+ channel inhibitors, Mg2+ and ruthenium red. In the reconstituted proteoliposomes, the Ca2+ efflux rates are the same regardless of digestion (T2); also, efflux is not affected by the presence or absence of Mg2+ + ruthenium red. These results indicate that T2 cleavage causes 'uncoupling' of the 'Ca(2+)-pump' from ATP hydrolytic activity. A theoretical model is developed in order to fit the extent of tryptic digestion of the A fragment of the (Ca2+ + Mg2+)-ATPase polypeptide with the loss of Ca(2+)-transport. Fits of the theoretical equations to the data are consistent with that Ca(2+)-transport system appears to require a dimer of the polypeptide (Ca2+ + Mg2+)-ATPase.

Effect of calcium on the interactions between Ca2+-ATPase molecules in sarcoplasmic reticulum

The interaction between Ca2+-ATPase molecules in the native sarcoplasmic reticulum membrane and in detergent solutions was analyzed by chemical crosslinking, high performance liquid chromatography (HPLC), and by the polarization of fluorescence of fluorescein 5'-isothiocyanate (FITC) covalently attached to the Ca2+-ATPase. Reaction of sarcoplasmic reticulum vesicles with glutaraldehyde causes the crosslinking of Ca2+-ATPase molecules with the formation of dimers, tetramers and higher oligomers. At moderate concentrations of glutaraldehyde solubilization of sarcoplasmic reticulum by C12 E8 or Brij 36T (approximately equal to 4 mg/mg protein) decreased the formation of higher oligomers without significant interference with the appearance of crosslinked ATPase dimers. These observations are consistent with the existence of Ca2+-ATPase dimers in detergent-solubilized sarcoplasmic reticulum. Ca2+ (2-20 mM) and glycerol (10-20%) increased the degree of crosslinking at pH 6.0 both in vesicular and in solubilized sarcoplasmic reticulum, presumably by promoting interactions between ATPase molecules; at pH 7.5 the effect of Ca2+ was less pronounced. In agreement with these observations, high performance liquid chromatography of sarcoplasmic reticulum proteins solubilized by Brij 36T or C12 E10 revealed the presence of components with the expected elution characteristics of Ca2+-ATPase oligomers. The polarization of fluorescence of FITC covalently attached to the Ca2+-ATPase is low in the native sarcoplasmic reticulum due to energy transfer, consistent with the existence of ATPase oligomers (Highsmith, S. and Cohen, J.A. (1987) Biochemistry 26, 154-161); upon solubilization of the sarcoplasmic reticulum by detergents, the polarization of fluorescence increased due to dissociation of ATPase oligomers. Based on its effects on the fluorescence of FITC-ATPase, Ca2+ promoted the interaction between ATPase molecules, both in the native membrane and in detergent solutions.

Stimulated human neutrophils damage cardiac sarcoplasmic reticulum function by generation of oxidants

An important aspect of myocardial injury is the role of neutrophils in post-ischemic damage to the heart. Stimulated neutrophils initiate a series of reactions that produce toxic oxidizing agents. Superoxide rapidly dismutases to H2O2 and neutrophils contain myeloperoxidase which catalyzes the oxidation of Cl- by H2O2 to yield hypochlorous acid (HOCl). The highly reactive HOCl combines non-enzymatically with nitrogenous compounds to generate long-lived, non-radical oxidants, monochloramine and taurine N-monochloramine. We investigated the role of oxygen radicals and long-lived oxidants on cardiac sarcoplasmic reticulum function, which plays a major role in the regulation of intracellular Ca2+ and thereby in the generation of force. Incubation of sarcoplasmic reticulum with phorbol myristate acetate (PMA)-stimulated neutrophils (4 x 10(6) cells/ml) significantly decreased calcium uptake rate (0.85 +/- 0.11 to 0.11 +/- 0.06 mumol/min per mg) and Ca2+-ATPase activity (1.67 +/- 0.08 to 0.46 +/- 0.10 mumol/min per mg). Inclusion of myeloperoxidase inhibitors (cyanide, sodium azide and 3-amino-1,2,4-triazole), catalase, superoxide dismutase plus catalase, and alpha-tocopherol significantly protected (P less than 0.01) calcium uptake rates and Ca2+-ATPase activity of sarcoplasmic reticulum. Superoxide dismutase (10 microgram/ml) alone or deferoxamine (1 mM) had no protective effect in this system. The maximum inhibition of sarcoplasmic reticulum function was observed with (3-4) x 10(6) cells/ml in 4-6 min. HOCl and NH2Cl inhibited calcium uptake rate and Ca2+-ATPase activity of sarcoplasmic reticulum in a dose-dependent manner (2-20 microM), whereas H2O2 damaged sarcoplasmic reticulum at concentrations ranging from 5 to 25 mM. HOCl (20 microM) inhibited 80-90% of Ca2+-uptake rate and Ca2+-ATPase activity and L-methionine (0.1-1 mM) provided complete protection. We conclude that stimulated neutrophils damage cardiac sarcoplasmic function by generation of myeloperoxidase-catalyzed oxidants.

State of aggregation of the (Ca2+ + Mg2+)-ATPase studied using chemical cross-linking

We have studied cross-linking of the (Ca2+ + Mg2+)-ATPase in sarcoplasmic reticulum and in reconstituted systems, using glutaraldehyde, cupric-1,10-phenanthroline and 3,3'-dithiobis (sulphosuccinimidylpropionate). All reagents produce extensive cross-linking, forming aggregates too large to enter polyacrylamide gels. Only traces of cross-linked dimeric ATPase species are formed. Saturation transfer electron spin resonance spectra of spin-labelled sarcoplasmic reticulum cross-linked with glutaraldehyde are also consistent with the formation of extensively cross-linked aggregates in the membrane. The results are interpreted in terms of dynamic clusters of ATPase molecules in the membrane, probably in the form of rows of ATPase molecules.

Structural and functional degradation of Ca2+:Mg2+-ATPase rich sarcoplasmic reticulum vesicles photosensitized by erythrosin B

Erythrosin B (Red Dye No. 3) and Rose Bengal photosensitize the destruction of the Ca2+:Mg2+-ATPase pump protein in sarcoplasmic reticulum (SR) vesicles with respective quantum efficiencies of (1.53 +/- 0.19) X 10(-3) and (1.25 +/- 0.18) X 10(-3). Damage to vesicle function was assayed by measurements of increases in passive Ca2+ permeability. Rates of passive Ca2+ movement into the SR lumen were increased by dye photosensitization in proportion to radiation absorbed. Active Ca2+ transport into SR vesicles was blocked independent of radiation absorbed by Erythrosin B and Rose Bengal at free concentrations of 0.69 microM and 1.16 microM, respectively. The photochemical lability of the Ca2+ pump protein and alterations in passive and active Ca2+ transport may be dependent on the concentration of the dye in the membrane. The photosensitization results may have implications with respect to the suitability of Erythrosin B usage in vivo, since the brightness of our irradiation source is comparable to that of sunlight at 480 nm.

Crosslinking of sarcoplasmic reticulum ATPase protein with 1,5-difluoro 2,4-dinitrobenzene

The reacton of 1,5-difluoro-2,4-dinitrobenzene with the ATPase protein of rabbit skeletal sarcoplasmic reticulum caused a marked loss of the Ca2+-dependent ATPase (ATP phosphohydrolase, EC 3.6.1.3) activity during an interval when 2 mol of the crosslinking reagent were incorporated/10(5) g of protein. The modified ATPase protein formed non-serial high molecular weight aggregates or oligomers during short (1--5 min) or long exposure (60 min) to the reagent at 25 degrees C or 4 degrees C. The same pattern was found when sarvoplasmic reticulum was treated similarly; only the ATPase protein formed oligomers (homopolymers). In all cases the ATPase protein monomer remained the predominant species present. During the appearance of the high molecular weight oligomers the Ca2+-ATPase activity was unaffected but Ca2+ uptake was inhibited. Major changes in the ATPase activity occurred when the monomeric ATPase protein was modified. Disubstituted dinitrophenylene derivatives of cysteine and tyrosine were found in modified ATPase protein and only a small amount of monosubstituted dinitrophenyl groups were identified. Thiolysis of the modified ATPase protein with 2-mercaptoethanol removed approx. 35% of the incorporated groups, but there was no restoration of the Ca2+-ATPase activity. Substrate MgATP2- protected the Ca2+-ATPase activity of the ATPase protein and sarcoplasmic reticulum but Ca2+ had no effect on the modificaton. Different conformational states of the ATPase protein could be ascertained from a comparison of the effects of Ca2+ and MgATP2- on the bifunctional reagent dinitrophenylation of the ATPOase protein with that of the monofunctional reagent 1-fluoro-2,4-dinitrobenzene (Bailin, G. (1980) Biochim. Biophys. Acta 623, 213-224). Intramolecular crosslinking of the ATPase protein predominated and oligomers which formed during the reaction were not essential for the maintenance of the ATPase activity.