guanosine-diphosphate and allocolchicine

Colchicine induces a weak assembly-independent GTPase activity in calf brain tubulin [David-Pfeuty, T., Erickson, H. P., & Pantaloni, D. (1977) Proc. Natl. Acad. Sci. U.S.A. 74, 5372-5376; Andreu, J. M., & Timasheff, S. N. (1981) Arch. Biochem. Biophys. 211, 151-157]. Kinetic analysis shows a turnover number of 2 x 10(-4) s-1 in 0.01 M sodium phosphate and 4 mM MgCl2, pH 7.0, with an apparent Km for GTP of 10 microM. This activity, which requires Mg2+ ions and attains a plateau at 4 mM MgCl2, is independent of pH over the pH range of 6.6-7.4. This GTPase activity was induced by all colchicine analogues that contain rings A and C (or C'), the strength varying in a manner parallel to the free energy of binding of the ligand. The specific GTPase activity was found to be independent of the tubulin-colchicine complex concentration over the range of 2-20 microM. Sedimentation velocity examination of the product of the reaction showed that GDP-tubulin-colchicine generated by hydrolysis of the E-site GTP was indistinguishable from that produced by nucleotide exchange at the site, the protein assuming the "curved" conformation in both cases. Steady-state kinetic analysis in the presence of high concentrations of microtubule-inducing cosolvent additives revealed an increase in kcat/Km of up to 1 order of magnitude that followed the order poly(ethylene glycol) 6000 (PEG-6000 > PEG-1000 = 2-methyl-2,4- pentanediol > sucrose > L-glutamate > glycerol = PEG-200 > betaine, with no apparent change in Km.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Animals; Biphenyl Compounds; Brain Chemistry; Cattle; Colchicine; Enzyme Activation; GTP Phosphohydrolases; Guanosine Diphosphate; Guanosine Triphosphate; Hydrogen-Ion Concentration; Kinetics; Magnesium; Microtubules; Polyethylene Glycols; Solvents; Tubulin

The locus of action of cosolvent additives in the activation of the tubulin-colchicine GTPase was investigated. The GDP off rates were slowed down by the cosolvents in a manner that parallels their specific viscosities, indicating that diffusion-controlled release of GDP may be rate-limiting under the conditions of these studies. Yet, the net effect of cosolvents was to increase the overall rate of GTP hydrolysis. Pre-steady-state kinetics of liganded tubulin in the presence of 1%, w/v, poly(ethylene glycol) 6000 (PEG-6000) exhibited a burst of inorganic phosphate release indicating that the cosolvents act at an early step in the process. A similar conclusion was drawn from measurements of the activation energy (Ea) of the reaction, which showed that 3.4 M glycerol decreased the value of Ea to 10.6 kcal mol-1 from 17.3 kcal mol-1 in its absence. The observed difference in apparent binding free energies of the colchicine analogues allo-colchicine (ALLO) and 2-methoxy-5-(2,3,4-trimethoxyphenyl)-2,4,6-cycloheptatrien-1-one (MTC, or des-ring B colchicine), when measured by fluorescence and enzyme activity titrations, identified the presence of a GTPase-activating protein conformational transition subsequent to the physicochemical binding of the ligands. The decrease of the apparent binding constant measured by enzyme activity in dilute buffer relative to that measured by fluorescence [for ALLO, Kb(fluor) = 1.46 x 10(6) M-1; Kb(enz act) = 1.1 x 10(5) M-1] yielded the value of the enzyme-activating conformational transition constant, K3 = 0.08.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Animals; Cattle; Colchicine; Glycerol; Glycols; GTP Phosphohydrolases; Guanosine Diphosphate; Guanosine Triphosphate; Hydrolysis; Polyethylene Glycols; Protein Conformation; Solvents; Spectrometry, Fluorescence; Thermodynamics; Tubulin