Acid and Alkaline Phosphatases in a Brain Tubulin Preparation

The hydrolysis of p ‐Nitrophenylphosphate has been studied in vitro in a tubulin preparation from bovine brain. The activity at pH 6.8 was 16.4 ± 2.2 nmol/mg protein, h. At least two phosphatases were responsible for this activity. They were found to have pH‐optima at 5.1 and 10.4. respectively, and their apparent K M values were 1.23 ± 0.10 m m and 0.17 ± 0.03 m m . respectively. Mg 2+ was found to stimulate activity at both pHs while Zn 2+ inhibited at pH 5.1 and stimulated activity at pH 10.4. All of the alkaline and part of the acid phosphatase activity were found to be closely associated with microtubules/tubulin. Tubulin purified by phosphocellulose chromatography contained phosphatase activity, and it is suggested that such activity is an intrinsic property of tubulin itself. Phosphatase activity was also found in association with the microtubule‐associated proteins that co‐purify with tubulin. Two proteins of high molecular weight constituted the major part of the associated material. The results indicate an association of phosphatase activity with the larger of these two proteins.