The distribution of ATPase activities in purified transverse tubular membranes

Abstract

Vesiculated fragments of transverse tubules (TT) and sarcoplasmic reticulum (SR) membranes were purified from heterogeneous microsomal membrane fractions of chicken breast muscle by a modification of an iterative calcium-oxalate loading technique. The distribution of ATPase activities were determined for the TT and SR and were compared to enriched fractions of sarcolemma (SL) membranes. The TT membranes were characterized by high rates of magnesium-stimulated ATPase (Mg-ATPase) and 5′-nucleotidase activities but were virtually devoid of calcium-stimulated, magnesium-dependent ATPase (Ca,Mg-ATPase) activity. Moderate levels of a latent sodium and potassium-stimulated ATPase (Na,K-ATPase) were observed for TT membranes when unmasked with valinomycin and monensin. In contrast to the behavior of TT membranes, highly purified SR membranes displayed an active Ca,Mg-ATPase but negligible Na,K-ATPase, Mg-ATPase, and 5′-nucleotidase activities. High levels of Na,K-ATPase and 5′-nucleotidase activities were observed for SL membranes; however, the SL displayed no appreciable Ca,Mg-ATPase and Mg-ATPase activities. The lack of significant Mg-ATPase activity in the SR and SL fractions suggested that the Mg-ATPase was uniquely associated with the TT membranes. The TT Mg-ATPase was further characterized by its pH and temperature dependences, and its sensitivity to pharmacologic agents. The Mg-ATPase of the TT was insensitive to inhibition by sodium azide and oligomycin in concentrations shown to exert maximum inhibition on the F 1 ATPase of submitochondrial particles. The Mg-ATPase was also resistant to the effects of ouabain and orthovanadate in concentrations which abolished the Na,K-ATPase and Ca,Mg-ATPase activities of the SL and SR, respectively. The Mg-ATPase displayed temperature and pH optima (25 °C, pH 7.3) which were distinguishable from the Ca,Mg-ATPase (45 °, pH 7.0) of highly purified SR fractions but which were very similar to the temperature and pH dependencies of the mixed microsomal fractions (MMF) from which the TT membranes were derived. Similarities in the pH and temperature dependencies of the TT and MMF Mg-ATPases plus the absence of appreciable Mg-ATPase activity in highly purified SR membranes suggests that the “basic” Mg-ATPase often seen in crude SR fractions may originate from TT membrane contamination. The resistance of the TT Mg-ATPase to inhibition by the pharmacologic agents tested plus its unique temperature and pH dependences indicate that this ATPase is distinguishable from other ATPases and may, therefore, be of value as a specific biochemical marker for TT membranes.