Inhibition of (Ca2+ + Mg2+)-ATPase by carbodiimides. A structure-activity study

Inhibition of sarcoplasmic reticulum Ca2+-ATPase by miconazole

The inhibition of sarcoplasmic reticulum Ca2+-ATPase activity by miconazole was dependent on the concentration of ATP and membrane protein. Half-maximal inhibition was observed at 12 microM miconazole when the ATP concentration was 50 microM and the membrane protein was 0.05 mg/ml. When ATP was 1 mM, a low micromolar concentration of miconazole activated the enzyme, whereas higher concentrations inhibited it. A qualitatively similar response was observed when Ca2+ transport was measured. Likewise, the half-maximal inhibition value was higher when the membrane concentration was raised. Phosphorylation studies carried out after sample preequilibration in different experimental settings shed light on key partial reactions such as Ca2+ binding and ATP phosphorylation. The miconazole effect on Ca2+-ATPase activity can be attributed to stabilization of the Ca2+-free enzyme conformation giving rise to a decrease in the rate of the Ca2+ binding transition. The phosphoryl transfer reaction was not affected by miconazole.

ATPase and phosphatase activities are differentially inhibited by photo-oxidation of the sarcoplasmic reticulum Ca(2+)-ATPase

We have already described that photo-oxidation of the sarcoplasmic reticulum Ca(2+)-ATPase with the halogenated dye erythrosin B produces inhibition of the ATPase activity (J.A. Mignaco et al., Biochemistry 35 (1996) 3886-3891). We now show that the Ca(2+)-dependent and Ca(2+)-independent p-nitrophenylphosphatase activities are also inhibited by this treatment. Modification of rapidly (< 10 min) oxidized residue(s) is responsible for the major loss of ATPase activity, whereas photo-inhibition of the phosphatase activities occurs more slowly (t1/2 20-30 min). Here we have focused on photo-inhibition of the Ca(2+)-independent pNPPase activity, and the counteracting effects of ATP and FITC. Following photo-oxidation, the Ca(2+)-independent pNPPase activity decreases monotonically. ATP partially protects against the inactivation of the pNPPase, whereas labeling the enzyme with FITC does not. However, the protective effect of ATP is completely abolished by the attached FITC. These data are interpreted in terms of two different sites that are susceptible to photo-oxidation and are involved in different events related to substrate hydrolysis.

Ca2+ binding and translocation by the sarcoplasmic reticulum ATPase: functional and structural considerations

Three experimental systems are described including sarcoplasmic reticulum (SR) vesicles, reconstituted proteoliposomes, and recombinant protein obtained by gene transfer and expression in foreign cells. It is shown that the Ca(2+) ATPase of sarcoplasmic reticulum (SR) includes an extramembranous globular head which is connected through a stalk to a membrane bound region. Cooperative binding of two calcium ions occurs sequentially, within a channel formed by four clustered helices within the membrane bound region. Destabilization of the helical cluster is produced following enzyme phosphorylation by ATP at the catalytic site in the extramembranous region. The affinity and orientation of the Ca2+ binding site are thereby changed, permitting vectorial dissociation of bound Ca2+ against a concentration gradient. A long range linkage between phosphorylation and Ca2+ binding sites is provided by an intervening peptide segment that retains high homology in cation transport ATPases, and whose function is highly sensitive to mutational perturbations.

Intracellular signaling through long-range linked functions in the Ca2+ transport ATPase

The Ca2+ transport ATPases of intracellular membranes exhibit an intracellular long-range functional linkage which is the basic mechanistic device for Ca2+ transport through ATP utilization. The functional linkage operates between a phosphorylation (catalytic) domain located in the extramembranous region, and a Ca2+ binding domain located in the membrane bound region of the enzyme. The two domains are separated by a distance of approximately 50 A, and are both affected by binding of a single molecule of the highly specific inhibitor, thapsigargin, to the enzyme. Functional and structural features are here described to explain the long-range linkage through the protein structure.

The effect of calmodulin on the interaction of carbodiimides with the purified human erythrocyte (Ca2+ + Mg2+)-ATPase

The activity of the solubilized and purified (Ca2+ + Mg2+)-ATPase from human erythrocyte membranes was inhibited by N,N'-dicyclohexylcarbodiimide in a concentration-dependent manner. The carbodiimide prevented formation of the phosphorylated intermediate during the catalytic cycle of the enzyme. Treatment of the enzyme with N,N'-dicyclohexyl[14C]carbodiimide resulted in the formation of a 14C-labelled polypeptide corresponding to the enzyme monomer (molecular weight 136,000). The tryptic fragmentation of this 14C-labelled enzyme resulted in the formation of three major 14C-labelled fragments with molecular weights of 58,000, 36,500 and 23,000, the latter two probably representing transmembrane and calmodulin-binding domains of the enzyme, respectively. In the absence of calmodulin, 6.7 molecules of N,N'-dicyclohexyl[14C]carbodiimide covalently bound to each molecule of Ca2+-ATPase; in the presence of calmodulin, the number of molecules of carbodiimide bound was 13.1. The binding of N,N'-dicyclohexylcarbodiimide to the (Ca2+ + Mg2+)-ATPase greatly reduced its ability to bind to a calmodulin-agarose gel.

A kinetic model for Ca2+ efflux mediated by the Ca2+ + Mg2+-activated ATPase of sarcoplasmic reticulum

We present a model for Ca2+ efflux from vesicles of sarcoplasmic reticulum (SR). It is proposed that efflux is mediated by the Ca2+ + Mg2+-activated ATPase that is responsible for Ca2+ uptake in this system. In the normal ATPase cycle of the ATPase, phosphorylation of the ATPase is followed by a conformational change in which the Ca2+-binding sites change from being outward-facing and of high affinity to being inward-facing and of low affinity. To mediate Ca2+ efflux, it is proposed that the ATPase can adopt a conformation in which the Ca2+-binding sites are of low affinity but still outward-facing. It is shown that experimental data on the rates of Ca2+ efflux can be simulated in terms of this model, with Ca2+-binding-site affinities previously proposed to explain ATPase activity [Gould, East, Froud, McWhirter, Stefanova & Lee (1986) Biochem. J. 237, 217-227]. Effects of Mg2+ and adenine nucleotides on efflux rates are explained. It is suggested that Ca2+ efflux from SR mediated by the ATPase could be important in excitation-contraction coupling in skeletal muscle.

Calcium-dependent inhibition of the erythrocyte Ca2+ translocating ATPase by carbodiimides

The ATP hydrolytic activity of the solubilized and purified Ca2+-translocating ATPase from human erythrocyte plasma membrane was strongly inhibited by the nonpolar compound, N,N'-dicyclohexylcarbodiimide, both in the presence and in the absence of calmodulin. However, the more water-soluble carbodiimides, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and 1-cyclohexyl-3-(2-morpholinoethyl)carbodiimide had little inhibitory effect on the enzyme. The inhibitory effect of N,N'-dicyclohexylcarbodiimide was most pronounced at acid pH, and declined sharply at alkaline pH values. In addition, the optimum pH for the enzyme activity also shifted to more alkaline values in the presence of the carbodiimide. Calcium ion appears to favor the inhibition induced by the carbodiimide, in contrast to the observed protection by Ca2+ in the sarcoplasmic reticulum Ca2+-translocating ATPase. N,N'-Dicyclohexylcarbodiimide also dramatically decreased the stimulatory effect of calmodulin on the activity of the enzyme.