The interactions of tropolone with magnesium ions and tubulin

Tropolone, a single analog of colchicine, interacts with Mg2 with the formation of a 1 : 1 complex and an apparent equilibrium binding constant Kb of 1.4 x 10(4) M(-1) in neutral aqueous solution at 25 degrees C. The tropolone-Mg2 complex, but not tropolone, is fluorescent. Since tubulin binds Mg2 (Frigon, R.P. and Timasheff, S.N. (1975) Biochemistry 14, 4567-4573), previous reports of tropolone interaction with tubulin in Mg2 -containing buffers must be critically re-examined. Fluorescence and difference absorption spectroscopy experiments performed at essentially constant Mg2 activity indicate that tubulin does bind tropolone, but the optical effects are too weak to use in quantitative studies.