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The function of glutamate dehydrogenase was studied in cultured sweetpotato (Ipomoea batatas) nodal explants. The glutamate dehydrogenase was fractionated to charge isomers. Supplementation of the growth medium with either naphthaleneacetic acid or benzyladenine in the presence of 20 mM NH4NO3 induced normal growth (type 1 sweetpotato). The basic and acidic charge isomers of glutamate dehydrogenase were not suppressed. Combined supplementation with 70 mM NH4+ and either 1 mg ∙ L−1 benzyladenine or 0.1 mg ∙ L−1 naphthaleneacetic acid caused growth retardation (type 2 sweetpotato) and the suppression of the basic charge isomers. Combined supplementation with 45 mM NH4+ and either 1 mg ∙ L−1 benzyladenine or 0.1 mg ∙ L−1 naphthaleneacetic acid induced normal growth (type 3 sweetpotato), but the acidic charge isomers were suppressed. Combined supplementation with benzyladenine and naphthaleneacetic acid suppressed all the charge isomers and abolished amination by the enzyme, thereby causing severe growth retardation. The type 1 and type 3 sweetpotato glutamate dehydrogenases were more aminating (Michaelis constant Km = 12.5 and 14.8 mM NH4Cl, respectively) than type 2 sweetpotato glutamate dehydrogenase (Km = 82.6 mM NH4Cl). The differential growth retardations which accompanied the three phases of the suppression of the aminating charge isomers are evidence that the enzyme is aminating in vivo and that it employed that activity in the regulation of sweetpotato growth and differentiation. Key words: glutamate dehydrogenase, amination depression, sweetpotato retardation.

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, alpha, 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 alpha-ketoglutaratr (alpha-KG), 0-0.2 mM NADH, 0-10.0 mM CaCl2 5 units of DNase 1, antibiotics, and approximately 5 microg pure GDH isoenzymes or polypeptides at pH 8.0, and overnight at 16 degrees 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 alpha-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 theisoenzymes in RNA synthesis reaction. The product RNA was up to 2 microg microg(-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 [alpha-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.