Purification of Glutamate Dehydrogenase Isoenzymes and Characterization of Their Substrate Specificities

Abstract

Glutamate dehydrogenase (GDH) isoenzymes were purified from control, and ribonucleoside triphosphate (NTP)-treated peanut seedlings. GDH purification was by preparative-scale, free solution isoelectric focusing, followed by native PAGE, and the cryoelectrophoretic elution of the isoenzymes from the gel. SDS-PAGE of the purified GDH isoenzymes, followed by either silver staining of the gel, or western analysis using anti-GDH antibody, gave identical GDH polypeptide (a, α, and b) bands, thus, confirming the purity of the isoenzymes. The substrate specificities in the aminating activity of the GDH isoenzymes, or disaggregated polypeptides were determined by photometry, but the substrate specificities in the RNA synthesis activity were determined in cocktails containing 0.06–0.8 mM of each of UTP, ATP, GTP, and CTP, 0–100.0 mM NH4Cl, 0–50.0 mM α-ketoglutaratr (α-KG), 0–0.2 mM NADH, 0–10.0 mM CaCl2 5 units of DNase 1, antibiotics, and ∼5 µg pure GDH isoenzymes or polypeptides at pH 8.0, and overnight at 16°C. The GDH polypeptides were active only in amination reaction, but the GDH isoenzymes were active in both amination and RNA synthesis. Whereas, NADH, NH4Cl and α-KG served as the substrates for the amination reaction, and as modulators in the RNA synthetic reaction, ATP, GTP, UTP, and CTP served as substrates for the isoenzymes in RNA synthesis reaction. The product RNA was up to 2 µg µg−1 GDH, and consisted of RNA species in the size ranges of 26, 16, and 5 S rRNAs. DNAse 1 in the assay cocktail ruled out transcription as the mechanism of the RNA synthesis. Addition of [α-32P] NTP led to the production of labeled RNA, thus confirming the specificity of NTPs as substrates, and that the RNA was not pre-existing in the reaction cocktail.