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Anion regulation of Ca2+ transport ATPase of the human erythrocyte membrane

University of British Columbia

The mechanism of regulation of the Ca²⁺ pump ATPase of the human erythrocyte membrane by calmodulin, cyclic AMP and the anion channel was studied using membrane fragments, resealed "ghosts", inside-out vesicles and a Triton X-100 solubilized enzyme preparation. The (Ca²⁺ + Mg²⁺ )-ATPase activity in erythrocyte membranes or a Triton X-100 solubilized enzyme preparation showed biphasic (high and low affinity) Ca²⁺ activation kinetics. The anionic calcium binding protein, calmodulin, increased both the calcium sensitivity (Kca²⁺) and the maximum velocity (Vmax ) of the enzyme. Certain polyanionic agents (poly-L-aspartic acid, poly-L-glutamic acid), alicyclic sulfonic acids (HEPES,N-2-hydroxyethylpiperazine-N¹-2-ethanesulfonic acid, MES,2-N- (morpholinoethanesulfonic acid)), and aromatic carboxylic acids (benzoic and salicylic acids) increased the Kca²⁺ but not the Vmax of (Ca²⁺ + Mg²⁺ )-ATPase in erythrocyte membranes and Triton X-100 solubilized enzyme preparations. Trifluoperazine (30 μM) antagonized activation of the enzyme by calmodulin and poly-L-aspartic acid, but not by sodium-HEPES or sodium-MES. Limited trypsin proteolysis of (Ca²⁺ + Mg²⁺ )-ATPase in the erythrocyte membrane abolished activation by calmodulin, poly-L-aspartic acid and sodium-HEPES. These results suggest that the modulation of the Ca²⁺ sensitivity of (Ca²⁺ + Mg²⁺ )-ATPase by calmodulin may be associated with the anionic properties of this protein, and that this property can be mimicked by some other anions, probably by interacting at an anion-regulatory site on the enzyme. Cyclic AMP (5 μM) was found to inhibit the (Ca²⁺ + Mg²⁺)-ATPase activity (approx. 20%) in erythrocyte membranes, probably via endogenous cyclic AMP protein kinase, since this effect could be blocked by cyclic AMP protein kinase inhibitor (PKI) from the rabbit skeletal muscle, By contrast, bovine heart PKI stimulated (Ca²⁺ + Mg²⁺ )-ATPase activity (approx. 100%) by increasing the Kca²⁺ but not the Vmax of the enzyme in membrane or Triton X-100 solubilized preparations. At a low calcium concentration the stimulation by bovine heart PKI and saturating levels of calmodulin was additive, suggesting that the two effectors acted by distinct mechanisms. The stimulation of (Ca²⁺ + Mg²⁺ )-ATPase activity by bovine heart PKI was not solely due to its antagonism of the protein kinase because a) modification of arginine residues of bovine heart PKI abolished its inhibition of cyclic AMP protein kinase, but had no effect on the stimulation of (Ca²⁺ + Mg²⁺ )-ATPase; b) trifluoperazine (20 μM) antagonized the stimulation of (Ca²⁺ + Mg²⁺ )-ATPase by PKI, similarly to its antagonism of calmodulin stimulation, but it did not affect the inhibition of protein kinase by PKI. It is suggested that different mechanisms are involved in the inhibition of cyclic AMP protein kinase and the stimulation of (Ca²⁺ + Mg²⁺ )-ATPase by bovine heart cyclic AMP PKI. Next, the role of anion channel blockers on the (Ca²⁺ + Mg²⁺ )- ATPase was studied. The photolabeling reagent N-(4-azido-2-nitrophenyl)- 2 aminoethylsulfonate (NAP-taurine) was found to inhibit the (Ca²⁺+ Mg²⁺ )-ATPase of fragmented red cell membranes. Half maximal inhibition occurred between 25 μM and 50 μM. At these concentrations Mg²⁺ -ATPase and (Na⁺ + K⁺)-ATPase activities in the membranes were not affected. The reversible inhibition of (Ca²⁺ + Mg²⁺ )-ATPase produced by NAP-taurine in the dark became irreversible after photolysis in the presence of this reagent. Incubation of the membranes with Ca²⁺ , Mg²⁺ , ATP or calmodulin, prior to photolysis in the presence of NAP-taurine, did not protect the enzyme from Inhibition. Limited trypsin proteolysis of (Ca²⁺ + Mg²⁺ )-ATPase in fragmented membranes, which abolished activation by calmodulin, did not affect the inhibition by NAP-taurine. NAP-taurine was found to Inhibit the (Ca²⁺ + Mg²⁺ )-ATPase activity from the cytoplasmic side of the membrane, as determined from the following experiments. Addition of NAP-taurine (50 μM) to resealed erythrocyte ghosts inhibited less than 5% of the (Ca²⁺ + Mg²⁺ )-ATPase activity, compared to 50-60% Inhibition in ghosts resealed in the presence of 50 μM NAP-taurine. Furthermore, NAP-taurine inhibited ATP-dependent Ca²⁺ - transport into inside-out vesicles at a similar concentration (50 μM). The inhibition of the (Ca²⁺ + Mg²⁺ )-ATPase activity of membranes by NAP-taurine appeared to be a direct action on the enzyme, rather than through inhibition of the anion channel, as (Ca²⁺ + Mg²⁺ )-ATPase activity was not inhibited in membranes made from red blood cells reacted irreversibly with 50 μM NAP-taurine or the anion channel blocker 4,4'-diisothiocyano- 2,2' stilbene disulfonate (DIDS) (5 μM) or in membranes assayed in the presence of another anion channel blocker, probenecid (125 μM). This is the first reported selective antagonist of the Ca²⁺ pump, and it is suggested that NAP-taurine could be a useful tool for studying the Ca²⁺- transport ATPase in a variety of cells.

Text

2010-04-20

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http://hdl.handle.net/2429/23912

Pharmaceutical Sciences

University of British Columbia

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