The lower limit for the first order rate constant for dissociation of GDP from the tubulin E-site has been determined to be 0.14 s-1; this corresponds to a reaction with a half-life of 5 s. Using this rate constant and the previously determined equilibrium constant for GDP dissociation, equal to 6.1 X 10(-8) M (Zeeberg, B., and Caplow, M. (1979) Biochemistry 18, 3880-3886), the calculated association rate constant is 2.2 X 10(6) M-1 s-1. The tubulin E-site is highly reactive and it is calculated that: the half-life is 5 s for quantitative displacement of E-site bound radioactive GDP, by added excess nonradioactive GDP; the half-life is about 260 ms for isotopic equilibration when a trace amount of radioactive GDP is added to 20 microM tubulin-GDP; the half-life is about 850 ms for re-establishing the equilibrium for GDP binding, when 20 microM tubulin is diluted 20-fold. Thus, tubulin-GDP nucleotide exchange is rapid, so that added radioactive guanine nucleotides can be used in studies of relatively rapid reactions involving the tubulin subunit
