Proc. Nat. Acad. Sci. USA Vol. 68, No. 5, pp. 916-919, May 1971 Covalent Attachment of Diethylstilbestrol to Glutamate Dehydrogenase: Implications for Allosteric Regulation JOHN KALLOS* AND K. P. SHAW The Research Laboratory, Department of Pathology, St. Mary's Hospital, Montreal, Canada Communicated by C. B. Anfinsen, December 17, 1970 ABSTRACT An affinity labeling reagent for the estro- glutamate dehydrogenase by means of site-specific reagents genic-binding site of bovine liver L-glutamate dehydro- and to be published). The present paper deals genase (EC 1.4.1.3) was prepared by conversion of diethyl- (9, manuscript stilbestrol to its alkylating analogue, bromoacetyldiethyl- with the covalent attachment of diethylstilbestrol to GDH stilbestrol. Under standard assay conditions, the analogue and its implication in allosteric transition. With regard to the acted as a reversible allosteric ligand with regulatory problem of allosteric transition, the present paper describes activity much like that of diethylstilbestrol. However, experiments pertinent to the following question: (a) Since incubation of the enzyme with the alkylating agent in the presence of DPNH resulted in a permanent decrease in allosteric ligands exert a controlling function, without them- glutamate (X form) and an increase in alanine (Y form) selves participating in the reaction, will covalent attachment activities, and in covalent attachment of diethylstilbestrol of ligand to the protein produce the same, similar, or different in the ratio of 1 mol per subunit (of particle weight effects as the reversible ligand-protein interaction? (b) What is 52,000). The brominated analogue behaved as an affinity the degree of conformational stabilization resulting from label that mimicked the allosteric effects of diethyl- stilbestrol. Diethylstilbestrol protection of the enzyme permanently freezing the protein in one form as a result of the against alkylation by bromoacetylated sterol suggested chemical attachment of the ligand to the protein? competition for the same binding site, while ADP protec- Preliminary studies with various DES derivatives indicated tion indicated a shift of protein equilibrium into the X that one of the two hydroxyl groups can be modified by acet- form. The diethylstilbestrol-enzyme compound was de- sensitized (relative to the native enzyme) to allosteric ylation without loss of the capacity to bind to GDH. This ob- reagents such as ADP and GTP. The results were con- servation prompted us to attach a bromoacetyl group to one of sistent with conformational freezing of the modified the hydroxyl groups of the DES molecule. The close structural protein molecule into the Y form. analogy between bromoacetyl and acetyl DES led us to the expectation that a reversible ligand-protein interaction should the structure the Estrogenic hormones alter and catalytic produce a reversible allosteric effect, and that such a complex activity of crystalline glutamate dehydrogenase (EC 1.4.1.3) could be induced to form a covalent bond (by elimination of (GDH); they simultaneously inhibit glutamate and stimulate bromoacetic acid) and attach the DES molecule to the protein. alanine dehydrogenase activity of the enzyme (1-3). ADP can reverse the effect of these steroids (4). The action of regulatory molecules, such as diethylstilbestrol (DES), GTP, and ADP, is MATERIALS AND METHODS explained by a shift of the equilibrium between two different forms (X=Y) of the enzyme. The X form possesses high Bovine liver -glutamate dehydrogenase (type 1, ammonium glutamate activity; the Y form possesses alanine dehydro- sulfate suspension), DPNH, ADP, and GTP were obtained genase activity. Estrogen and GTP favor the Y form, while from Sigma Chemical Co. The enzyme suspension was usually ADP shifts the equilibrium to the X form. Such behavior has centrifuged before use, and the crystals were redissolved in an been rationalized in terms of allosteric interaction by Monod appropriate buffer. The enzyme solution was equilibrated with et al. (5) and Koshland (6); the term allosteric is now generally buffer either by gel filtration through Sephadex G-25 or by employed in connection with regulatory effects that are due to dialysis. Protein concentration was measured at 278 nm on the conformational changes in the protein molecule induced by a assumption that 1 mg/ml has an absorbance of 1 in a 1.0-cm ligand (7, 8). The precise molecular mechanism by which the cuvette. DES was obtained from Mann Research Laboratories allosteric ligand acts remains obscure. Clearly, more specific (New York, N.Y.). Bromoacetyldiethylstilbestrol (BADES) information is needed about the chemistry of the regulatory was prepared in our laboratory and dissolved in acetonitrile- and active sites before allosteric regulation is understood. water-ethylene glycol 2: 1: 1 before use. During the past several years we have been engaged in a Tritium-labeled BADES was prepared in our laboratory systematic chemical study of the active and regulatory sites of from ['HIDES (400 Ci/mol, Amersham/Searle Corp.). All radioactive materials were mixed with Bray's solution or toluene-2,5-diphenyloxazole solution and counted in a Nu- Abbreviations: DES, diethylstilbestrol; BADES, bromoacetyl- clear-Chicago liquid scintillation counter. All the buffer salts diethylstilbestrol; GDH, glutamate dehydrogenase. used were of analytical grade. in a * Present address: Department of Cell Biology, Faculty of Medi- Enzyme assays were performed at room temperature cine, Centre Hospitalier Universitaire, Sherbrooke, P.Q., Canada. Bausch and Lomb 505 spectrophotometer; the disappearance To whom request for reprints should be addressed. of DPNH at 340 nm was measured in 1-cm quartz cuvettes. 916 Downloaded by guest on September 25, 2021 Proc. Nat. Acad. Sci. USA 68 (1971) Modification of Glutamate Dehydrogenase 917 c 0 0.3 c f._ 0 a c a E 0.5 Alanine Reaction E 4 an < E 0 E 04 0.2 >, t. a.o 0-E -U 0 -U 0.3 0 0 -a <t 0) 0.1 a -ft0 4. on Glutamate Reaction a ay 0.1I CL Concn of steroid FIG. 1. Effects of concentration of DES and BADES on GDH activity. GDH activity was measured in the assay mixture described in Methods; the GDH concentration was 0.2 jug/ml. The mixture for determining alanine dehydrogenase activity contained 0.1 M Tris (pH 8.6) and 5 X 10-2 M pyruvate in lieu of the a-ketoglutarate; the enzyme concentration was 5.15 pg/ml. Activity is expressed as A340 units per cuvette per 2 min. The reactions were started by the addition of the enzyme; Finally, it was dissolved in 0.25 ml of Nuclear-Chicago 2-min incubations were used for calculations of enzymatic solubilizer and counted in a liquid scintillation spectrometer. activity. GDH activity was measured, in the direction of a-keto- Hydroxylamine treatment of ['HJDES-GDH glutarate reductive amination, with a solution containing 1 ml Three samples, 1 ml each, were incubated at 220C; they con- of 0.05 M phosphate (pH 7.8); 1 X 10-4 M EDTA; 2 X tained [8H]DES-GDH (0.38 mg/ml), 0.15 M hydroxyl- 10-4 M DPNH; 0.2 M NH4Cl; and 2.5 X 10-2 M a-ketc- amine-HCl, 0.025 M Tris, and 0.1 M KCl (pH 7.8). The reac- glutarate. Alanine dehydrogenase activity was measured, tion was stopped by the addition of ice-cold 2 M HCl to pH 3 in the direction of pyruvate reductive amination, with a and the free [3HIDES was extracted with ether. The ether solution containing 1 ml of 0.1 M Tris * HCl (pH 8.6); 1 X 10-4 layer was washed with diluted NaHCO3, dried over MgSO4, M EDTA; 2 X 10-4 M DPNH; 0.2 M NH3Cl; and 5 X 10-2 M pyruvate. I - 100 100 > 0-e 0 Chemical reaction of GDH with BADES 5' 0 GDH (0.3 mg) was incubated in 1 ml of a reaction mixture _ 80 80 c containing 0.05 M phosphate buffer (pH 7.6), 5 X 10-5 M BADES, and 1 X 10-3 M DPNH at room temperature. The iO rate of irreversible inhibition was determined by assay of '04 60 60 o 0~ 0 aliquots. The reaction was terminated by gelfiltration through to )) a G-25 column X 40 the product was 0 Sephadex (1.5 cm); 0' C eluted with 0.05 M phosphate buffer (pH 7.6). 0C Preparation of [SH]DES-GDH .Ad 20 0 20 GDH (6 mg) was incubated in a 10-ml reaction mixture con- E taining 0.05 M phosphate (pH 7.6), 8 X 10-5M bromoacetyl- 0 O-I ['H]DES (536,000 cpm/mg), and 1 X 10-3 M DPNH: after url 30 60 90120-c 2 hr of incubation, an aliquot (5 ml) of the reaction mixture Time in Minutes was passed through a Sephadex G-25 (fine) column (2.5 X 40 to remove the excess For an FrG. 2. Progress of the chemical reaction of GDH with cm) reagent. counting, aliquot (1 BADES. GDH (0.3 mg/ml) was incubated in phosphate buffer ml) of modified protein was mixed with 10 ml of Bray's solution (0.05 M, pH 7.6) with 5 X 10-4 M BADES in the presence of 1 X and counted. Another aliquot (2 ml) of modified protein was 10-3 M DPNH. Aliquots were taken at various time intervals precipitated with 4 ml of trichloroacetic acid (30%), then and passed through a column of Sephadex G-25, and the alanine centrifuged at 5000 rpm for 15 min. The precipitate was and glutamate activities were determined. 0, Alanine; A, gluta- washed four times with absolute alcohol and once with ether. mate. Downloaded by guest on September 25, 2021 918 Biochemistry: Kallos and Shaw Proc.