The effects of phospholipids on the activation of glutamate dehydrogenase by L-leucine

Purification of glutamate dehydrogenase from liver and brain

Two alternative procedures are described for the purification of the major form of glutamate dehydrogenase (L-glutamate-NAD(P)+ oxidoreductase (deaminating), EC 1.4.1.3: GDH) from ox liver and brain. The first involves affinity chromatography on a column of the allosteric inhibitor GTP bound to Sepharose, whereas the other uses a bifunctional ligand (bis-NAD+) composed of two NAD+ molecules linked together by a spacer arm to precipitate the enzyme in the presence of the substrate analogue glutarate. In both procedures the affinity steps are preceded by ammonium sulfate precipitation and ion exchange chromatography on DEAE cellulose. Procedures for the synthesis of GTP-Sepharose and bis-NAD+ are described and the ancillary procedures, including the assay of GDH activity and the determination of protein concentration, are also presented.

Conformational flexibility of a model protein upon immobilization on self-assembled monolayers

The present study reports on the retention of conformational flexibility of a model allosteric protein upon immobilization on self-assembled monolayers (SAMs) on gold. Organothiolated SAMs of different compositions were utilized for adsorptive and covalent attachment of bovine liver glutamate dehydrogenase (GDH), a well-characterized allosteric enzyme. Sensitive fluorimetric assays were developed to determine immobilization capacity, specific activity, and allosteric properties of the immobilized preparations as well as the potential for repeated use and continuous catalytic transformations. The allosteric response of the free and immobilized forms towards ADP, L-leucine and high concentrations of NAD(+), some of the well-known activators for this enzyme, were determined and compared. The enzyme immobilized by adsorption or chemical binding responded similarly to the activators with a greater degree of activation, as compared to the free form. Also loss of activity involving the two immobilization procedures were similar, suggesting that residues essential for catalytic activity or allosteric properties of GDH remained unchanged in the course of chemical modification. A recently established method was used to predict GDH orientation upon immobilization, which was found to explain some of the experimental results presented. The general significance of these observations in connection with retention of native properties of protein structures upon immobilization on SAMs is discussed.

The sulphydryl groups of ox brain and liver glutamate dehydrogenase preparations and the effects of oxidation on their inhibitor sensitivities

Glutamate dehydrogenase preparations from several sources have been shown to have suffered limited proteolysis during purification. This proteolysis has been previously shown to involve removal of the N-terminal tetrapeptide and to result in changes in the regulatory properties of the enzyme. In the present work the previously unidentified N-terminal residue of the unproteolysed enzyme from ox brain and liver is shown to be cysteine. The thiol group of this residue is masked in the native enzyme but it becomes accessible after reduction. Exposure of solutions of the unproteolysed enzyme to air oxidation causes large changes in its sensitivity to inhibition by the antipsychotic drug perphenazine, GTP and by high concentrations of NADH. No such changes occurred in the behaviour of preparations of the enzyme that had suffered proteolysis during purification under these conditions.

Interaction of phospholipids with proteins, peptides and amino acids. New advances 1987-1989

1. The review deals with the recent achievements in the study of the various interactions of phospholipids with proteins, peptides and amino acids. The interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. The effect of the interaction on the structure and biological activity of the interacting biomolecules is discussed.

Inhibition of ox brain glutamate dehydrogenase by perphenazine

Factors affecting the inhibition of ox brain glutamate dehydrogenase (GDH) by the antipsychotic drug perphenazine have been studied. Inhibition was found to be of mixed type with respect to 2-oxoglutarate and competitive towards NADH. However, the data indicate that perphenazine binds to a site distinct from the catalytic site to which NADH binds. Perphenazine also enhanced the high-substrate inhibition by these two substrates. Inhibition by perphenazine was not affected by the allosteric effector GTP but it was enhanced by increasing pH, in the range of 6.3 to 7.6, and diminished by increasing ionic strength. Low concentrations of perphenazine relieved the inhibition of GDH by phosphatidylserine and cardiolipin. However, at higher concentrations phosphatidylserine did not interfere with the inhibition by perphenazine whereas cardiolipin relieved it. The possible significance of these interactions in terms of the behaviour of this antipsychotic drug in vivo are discussed.

Interactions between proteins, peptides and amino acids. New advances 1986-1989

The paper deals with the recent achievements in the study of the various interactions between proteins, peptides and amino acids. The interactions are classified according to the hydrophilic, hydrophobic or mixed character of the interactive forces. The effect of the interaction on protein (peptide) association, structure and biological activity as well as the role of individual amino acid residues in the hydrophobic and hydrophilic interactions are discussed.