Occluded calcium sites in soluble sarcoplasmic reticulum Ca2+-ATPase

Occlusion of K+ in the Na+/K+/2Cl- cotransporter of Ehrlich ascites tumor cells

Proteins of n-octyl glucoside solubilized membrane vesicles derived from Ehrlich ascites tumor cells can occlude 86Rb+.K+ displaces 86Rb+ and it is assumed that 86Rb+ can be used as a tracer to measure K+ occlusion. The following observations indicate that the Na+/K+/2Cl- cotransporter is responsible for this occlusion: (1) Na+ does not compete for the K+ binding site, but rather stimulates 86Rb+ occlusion. (2) K+ occlusion saturates with increasing [Na+] and [K+], the respective K0.5 values being 50 +/- 7 microM for Na+ and 371 +/- 63 microM for K+. (3) Preincubation with 1 mM ouabain does not inhibit 86Rb+ occlusion, arguing against the Na+/K+-ATPase as being responsible for the occlusion. This notion is supported by the K0.5 value for K+ being higher than reported for Na+/K+-ATPase and by the stimulatory effect of Na+. (4) The K+ occlusion is sensitive to [Cl-], and the occluded ion is protected by the presence of bumetanide during cation exchange chromatography. Our results suggest that occlusion measurements of substrate ions could be a profitable way to study the ion binding mechanism(s) of the Na+/K+/2Cl- cotransporter.

Activation and inhibition of protein kinase C isozymes alpha and beta by Gd3+

Gd3+ was evaluated as a probe for Ca2+ sites on protein kinase C (PKC) by studying its ability to replace Ca2+ in activation of PKC isozymes II (beta) and III (alpha) in the lipid systems phosphatidylserine/1,2-dioleoyl-sn-glycerol (PS/DO) and diheptanoylphosphatidylcholine (PC7)/DO. PKC beta was stimulated by Ca2+ or Gd3+ in PS/DO whereas activity in PC7/DO was independent of these metals. Thus, it is suggested that Gd3+ replaces Ca2+ at a site involving metal-lipid interactions. High concentrations of Ca2+ or Gd3+ inhibited activity in both lipid systems. Analysis of the Gd3+ inhibition in the PC7/DO system suggests that it is due to formation of GdATP, which competes at the MgATP site. Activity of PKC alpha was dependent on low concentrations of Ca2+ in both lipid systems. The ability of Gd3+ to substitute for Ca2+ could not be evaluated in the PS system due to the inability to completely remove contaminating Ca2+ without chelating buffers. Successful reduction of contaminating Ca2+ was achieved in the PC7 system but Gd3+ failed to substitute for Ca2+ in activating PKC alpha and only caused inhibition. This is consistent with binding of Gd3+ to a Ca2+ site at or near the active site of the enzyme rather than to a site on the lipid. These results indicate that interactions between PKC and Gd3+ are complex, involving occupation of more than one class of sites. Conditions for separately evaluating the individual sites can be manipulated by selection of isozyme and lipid system.

High-affinity Ca(2+)- and substrate-binding sites on protein kinase C alpha as determined by nuclear magnetic resonance spectroscopy

Water proton nuclear magnetic resonance (NMR) relaxation rates were used to identify metal sites on protein kinase C (PKC) isozymes alpha and beta using paramagnetic Gd3+ as a probe. The paramagnetic effect of Gd3+ on water proton relaxation was enhanced with PKC isozymes alpha and beta in the presence of diheptanoylphosphatidylcholine/1,2-dioleoyl-sn-glycerol (PC7/DO). The data are consistent with a single class of metal-binding sites on PKC beta and two classes of sites on PKC alpha: a single high-affinity site with a KD for Gd3+ of 0.2 microM and a larger class of sites with a lower affinity for Gd3+. Titration with Ca2+ abolished the observed enhancement of water proton relaxation by the PKC alpha.Gd3+ complex, consistent with displacement of Gd3+ by Ca2+. Titrations of the PKC alpha.Gd3+ complex with Co(NH3)4ATP, a substitution-inert analogue of ATP, caused a substantial decrease in the observed water proton relaxation enhancement, consistent with formation of a ternary enzyme.metal.substrate complex with a KPKC alpha.Gd.[CoATP] of 30-100 nM. Titration of the metal enzyme complex with a model peptide substrate derived from the pseudosubstrate sequence of PKC alpha caused a similar decrease in enhancement at stoichiometric concentrations consistent with the formation of a PKC alpha.Gd3+.peptide complex with a KPKC alpha.Gd.[peptide] of less than or equal to 13 nM. Titrations of the fully formed PKC alpha.Gd3+.peptide complex with Co(NH3)4ATP caused a further decrease in enhancement consistent with formation of a quaternary complex.(ABSTRACT TRUNCATED AT 250 WORDS)

Sphingomyelin and derivatives as cellular signals

This comprehensive review was necessitated by recent observations suggesting that sphingomyelin and derivatives may serve second messenger functions. It has attempted to remain true to the theme of cellular signalling. Hence, it has focussed on the lipids involved primarily with respect to their metabolism and properties in mammalian systems. The enzymology involved has been emphasized. An attempt was made to define directions in which signals may be flowing. However, the evidence presented to date is insufficient to conclusively designate the mechanisms of stimulated lipid metabolism. Hence, the proposed pathways must be viewed as preliminary. Further, the biologic functions of these lipids is for the most part uncertain. Thus, it is difficult to presently integrate this sphingomyelin pathway into the greater realm of cell biology. Nevertheless, the present evidence appears to suggest that a sphingomyelin pathway is likely to possess important bioregulatory functions. Hopefully, interest in this novel pathway will grow and allow a more complete understanding of the roles of these sphingolipids in physiology and pathology.

Calcium transport by sarcoplasmic reticulum of skeletal muscle is inhibited by antibodies against the 53-kilodalton glycoprotein of the sarcoplasmic reticulum membrane

The effects of an antiserum against the 53-kDa glycoprotein (GP-53) of the sarcoplasmic reticulum (SR) and of monoclonal antibodies against GP-53 on Ca2+ transport and ATP hydrolysis by SR of rabbit skeletal muscle have been investigated. Preincubation of SR with an antiserum against GP-53 resulted in decreased ATP-driven Ca2+ transport by the SR but had no effect on Ca2+-stimulated ATP hydrolysis. Preincubation of SR with preimmune serum had no significant effect on either Ca2+ transport or Ca2+-ATPase activity. The effect of anti-GP-53 serum was time and concentration dependent. Preincubation of SR with two monoclonal antibodies against GP-53 had no effect on Ca2+ transport or on Ca2+-stimulated ATP hydrolysis. However, preincubation of SR with either monoclonal antibody against GP-53 together with a monoclonal antibody against the Ca2+-ATPase (at levels which had little effect alone) resulted in markedly decreased rates of Ca2+ uptake and ATP hydrolysis. Preincubation of SR with anti-GP-53-serum or with monoclonal antibodies, under the same conditions that inhibited Ca2+ uptake, did not increase the passive permeability of the SR membrane to Ca2+, did not decrease the permeability of the SR to oxalate, and did not cause significant proteolysis of the Ca2+-ATPase. Our results are consistent with the interpretation that GP-53 may modulate the function of the Ca2+-ATPase of the SR membrane.

NMR studies identify four intermediate states of ATPase and the ion transport cycle of sarcoplasmic reticulum Ca2+-ATPase

Water proton nuclear relaxation measurements are used to detect and characterize four distinct intermediate states for Gd3+ bound to Ca2+ sites of sarcoplasmic reticulum Ca2+-ATPase in complexes with ATP analogues. In the absence of nucleotides, Gd3+ binds to two occluded Ca2+ transport sites on Ca2+-ATPase which have a low accessibility to solvent water. In the presence of the nonhydrolyzable ATP analogue, Co(NH3)4AMPPCP, a new state for bound Gd3+ (still occluded and with fewer waters of hydration) is observed. In the presence of Co(NH3)4ATP or ATP, two additional states for bound Gd3+ are detected in the NMR studies. The first of these probably represents an intermediate state for bound Gd3+ during ATP hydrolysis. The latter is the most occluded Gd3+ site yet observed in these studies and is probably analogous to the highly occluded E1-P state observed with CrATP [(1987) Biochim. Biophys. Acta 898, 313-322].

NMR relaxation measurements detect four intermediate states of ATPase and transport cycle of sarcoplasmic reticulum Ca2+-ATPase

At least four of the intermediate states of Ca2+-ATPase (and presumably ion transport) can be trapped and characterized using water proton relaxation measurements. Gd3+ binds to two occluded Ca2+ transport sites on Ca2+-ATPase which have a low accessibility to solvent water. In the presence of the MgATP analogue Co(NH3)4AMPPCP, a new state for bound Gd3+ with one less water of hydration) is observed. In the presence of Co(NH3)4ATP or ATP, two additional states for bound Gd3+ are detected by NMR, the first of which probably represents an intermediate state of ATP hydrolysis. The latter is the most occluded Gd3+ site yet observed in these studies and corresponds to the highly occluded E1-P state observed with CrATP (Vilsen and Andersen, Biochim. Biophys. Acta 898, 313 (1987).

Inhibition of the sarcoplasmic reticulum Ca2+-ATPase by Reactive Red 120

The triazine dye, Reactive Red 120, was found to bind tightly (Kd = 30) nM) and with low stoichiometry to sarcoplasmic reticulum membranes. Our finding that this high-affinity binding caused noncompetitive inhibition of the Ca2+-ATPase indicates that the dye-binding site is distinct from both the active site and putative regulatory site. Detergent solubilization (monomerization) of the Ca2+-ATPase caused a 25-fold decrease in affinity for Reactive Red 120, while causing no decrease in affinity toward another dye, Reactive Blue 2. For the Reactive-Red-120-inhibited enzyme, the level of steady-state enzyme phosphorylation by ATP was not significantly different from that exhibited by the control Ca2+-ATPase. The rate of dephosphorylation in the presence and absence of ADP, however, was markedly decreased by the presence of the inhibitor. Distance measurements by fluorescence energy transfer from the active (FITC-reactive) site to the Reactive Red 120 site gave a value of 59 A. Similar experiments yielded an average distance of 35 A between the latter site and the tryptophan residues, most of which are postulated by the 'sequence model' (MacLennan et al. (1985) Nature 316, 696-700) to be located in a transmembrane domain.