Characteristics of Ca2+-stimulated ATPase in rat heart sarcolemma in the presence of dithiothreitol and alamethicin

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Abstract
We have studied the activities of Ca2+-stimulated ATPase in rat heart sarcolemma upon modulating the redox state of membrane thiol groups with dithiothreitol (DTT). The suitability of alamethicin to unmask the latent activity of this enzyme was also investigated. The Ca2+-stimulated ATPase in sarcolemma exhibited two activation sites — one with low affinity (Km = 0.70 ± 0.2 mM; Vmax = 10.0 ± 2.2 μmol Pi/mg/h) and the other with high affinity (Km = 0.16 ± 0.7 mM; Vmax = 4.6 ± 0.8 μmol Pi/mg/h) for Mg2+ATP. Alamethicin at a ratio of 1:1 with the sarcolemmal protein caused a 3-fold activation of Ca2+-stimulated ATPase without affecting its sensitivity to Ca2+ or Mg2+ATP. Treatment of sarcolemma with deoxycholate or sodium dodecyl sulfate resulted in a total loss of the enzyme activity; high concentrations of alamethicin also showed a detergent-like action on the sarcolemmal vesicles. DTT at 5–10 mM concentrations caused a 4–5 fold activation of Ca2+-stimulated ATPase in sarcolemma and this effect was observed to be dependent on the concentration of Mg2+ATP. DTT increased the affinity of the enzyme to Mg2+ATP at the high affinity site and enhanced the Vmax at the low affinity site in addition to increasing the sensitivity of Ca2+-stimulated ATPase to Ca2+. DTT protected the Ca2+-stimulated ATPase against deterioration by detergents and restored the enzyme activity after treatment with N-ethylmaleimide. The mechanism of action of DTT on Ca2+-stimulated ATPase may involve the reduction of essential thiols at the active site of the enzyme or its interaction with specific DTT-dependent inhibitor protein. No changes in the sensitivity of sarcolemmal Ca2+-stimulated ATPase to orthovanadate was evident in the absence or presence of DTT and alamethicin. The results suggest the use of both DTT and alamethicin for the determination of Ca2+-stimulated ATPase activity in sarcolemmal preparations.

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