Adenosine triphosphatase activity of bovine brain microtubule protein.

An ATPase activity is persistently associated with bovine brain microtubules even after multiple cycles of warm-induced microtubule assembly, centrifugation through a 50% sucrose cushion, cold-induced disassembly, and subsequent centrifugation. Likewise, a sucrose extraction method for preparing microtubule proteins free of mitochondrial contamination does not eliminate ATPase activity in the purified microtubule protein. Upon phosphocellulose chromatography, the ATPase activity fractionates with the microtubule-associated proteins. Before such fractionation, ATP and GTP are equally good substrates (Km = 0.8 f 0.2 mm; specific activity, 10 to 40 milliunits/mg); afterwards, ATP is preferentially hydrolyzed (K,,, = 1 to 5 mM; specific activity, 100 to 200 milliunits/mg). The response of the phosphocellulose-purified ATPase to temperature, calcium ion, chlorpromazine, propranolol, ouabain, magnesium ion, and sodium orthovanadate was examined to learn more about the identity of the ATPase. These effectors of well known ATPases of soluble and membrane origin failed to identify the microtubule-associated ATPase as one of these enzymes. Examination of published sodium dodecyl sulfate gel electrophoretic patterns of recycled microtubule protein also excludes the presence of significant amounts of sarcoplasmic reticulum ATPase, mitochondrial F1 ATPase, (Na’,K+)ATPase, myosin ATPase, and dynein ATPase. The ATPase activity, but not the intrinsic protein kinase activity, is rather resistant to heat inactivation, and this suggests that the ATPase does not result from a futile cycle of protein kinase and phosphoprotein phosphatase action.