Glutamate déshydrogénase

The pig liver glutamic dehydrogenase has been studied by light scattering and optical dispersion as a function of protein concentration. At infinite dilution, the molecular weight of pig liver glutamate dehydrogenase is about 310,000, and the unit obtained is a hexamer. When the concentration increases these hexamers associate to form a polymer having an elongated shape and indefinite length. The Steiner calculations applied to association equilibria show that the successive constants of polymerization K 2 , K 3 , K 4 … are quite identical; this shows that the polymerization is continuous and indefinite. The glutamate dehydrogenase of pig and beef liver have been compared. Their properties are very similar. The dissociation equilibria of pig and beef liver glutamate dehydrogenase are identical and these two enzymes, from different sources may form hybrid polymers in a statistical manner; the contact areas of these two enzymes are therefore the same. A thorough study of glutamate dehydrogenase rotatory properties has been done. The optical rotatory dispersion shows a positive maximum in the visible range of the spectrum. This maximum may be described by the Moffitt parameters a 0 and b 0 . The rotatory dispersion undergoes modifications by protein dilution. The rotatory power amplitude decreases and the maximum shifts to longer wavelenghts. Nevertheless, the amplitude of Cotton effect at 233 nm, which is typical of the secondary structure in an α‐helix, is not modified during the association. This is reflected by the variation of Moffitt parameters: only the a 0 coefficient decreases, while remaining positive, whereas the b 0 coefficient reflecting the organization of the secondary structure, is not modified. The variation of a 0 is therefore proportional to the rotatory power variation. We have shown that this property can be correlated with the association of the polymer into hexamers. It has been found that the variation of [α] is neither proportional to M̄ w , nor to the mean number molecular weight ā n , but to the ratio (ā n − 1)/ā n : it follows that this ratio is proportional to the number of association areas per open or linear polymer. The rotatory properties have also been studied in the presence of a dissociating agent, dioxane. The increase of dioxane concentration favours the association of the protein into hexamers and involves a decrease of the Moffitt rotatory parameter a 0 . At dioxane concentrations of 20% (v/v), the dissociation of polymers into hexamers is complete and the a 0 value is therefore characteristic of the hexamer in aqueous solution. The dioxane effect only decreases the interaction energy between the hexamers and so facilitates the dissociation.