Studies of Glutamate Dehydrogenase.

The association-dissociation equilibrium of beef-liver glutamate dehydrogenase was investigated in phosphate buffer of different ionic strengths and in the presence of the organic solvents ethylene glycol and dioxane. For some of the solvents the temperature dependence of the equilibrium constant was analyzed. The dependence of the apparent weight-average molecular weight on enzyme concentration shows always the same general shape as that found earlier in 0.067 M phosphate buffer at pH 7.6, Each curve can be fitted assuming an open association-dissociation equilibrium with one single equilibrium constant for all steps and with one single value for the second virial coefficient. Within the limit of the experimental error, the second virial coefficient is found to be the same for all systems (8-9 nmol × 1 × g^-2), whereas the equilibrium constants vary by orders of magnitude. At low ionic strength a very small temperature dependence was found with a reduced association at higher temperatures. At high ionic strength a temperature maximum of the equilibrium constant is observed. Preferential phosphate binding is indicated by the high values of the refractive index increment. The organic solvents investigated here (ethylene glycol, dioxane) cause a dissociation into the unimers (oligomers). The equilibrium constant decreases from 9 × 10⁵ M^-1 in aqueous phosphate buffer to 5.2 × 10³ M^-1 in 6 M ethylene glycol and to 2.4 × 10⁵ M^-1 in 2 M ethylene glycol. The effects obtained with 5% dioxane (0.57 M) are similar to that with 2 M ethylene glycol. In both cases a sharp decrease of the association with increasing temperature is observed. The reaction enthalpy and the reaction entropy of the association reaction decrease with decreasing temperature in all solvents, the enthalpy being positive in the absence and negative in the presence of organic solvents. The free enthalpy of reaction, however, increases with increasing temperature in buffers of high and low ionic strengths but decreases in the presence of organic solvents. The association reaction at high ionic strength appears mainly to be entropydriven whereas in the presence of the organic solvents the reaction seems to be energy-driven at all temperatures. It is suggested that the organic solvents interfere with the hydrophobic interactions between the unimers, The positive reaction enthalpy in the presence of high salt concentrations indicates that the attractive electrostatic forces are largely shielded, whereas shielding is imperfect for the repulsive forces which even seem to be enhanced. This could be due to a preferential binding of anions which would tend to increase the negative net charge and could explain the difference observed between the effects of phosphate and chloride solutions of identical ionic strength. [ABSTRACT FROM AUTHOR]