• ••••• AL.F. PI ....... , ••• .., © 1998 by Gustav Fischer Verlag, Jena Ammonium Metabolism Stimulation of Glucose-6P Dehydrogenase and Phosphoenolpyruvate Carboxylase in Young Barley Roots SERGIO ESPOSITO, PETRONIA CARILLO, and SIMONA CARFAGNA Dipartimento di Biologia Vegetale, Universita di Napoli «Federico II», Via Foria, 223, 80139 Napoli, Italy Received June 12, 1997 . Accepted August 15, 1997 Summary The effect of runmonium metabolism on the alternative pathways to glycolysis in young barley roots was investigated through measurements of enzyme activities and changes in amino acid levels. Introduction The activities of most glycolytic enzymes did not change either before or after the supply of ammonium to young barley plants. By contrast, increases in phosphoenolpyruvate carboxylase [EC 4.1.1.31] and glu- cose-6P dehydrogenase [EC 1.1.1.49] levels were measured, suggesting the activation of the pentose phos- phate and anaplerotic pathways during ammonium assimilation. Electrophoretic analysis indicated at least two different isoforms of glucose-6P dehydrogenase in barley roots, one of which was increased by ammonium supply. Ammonium supply caused a significant increase in the activities of aspartate aminotransferase [EC 2.6.1.1], alanine aminotransferase [EC 2.6.1.2] and asparaginase [EC 3.5.1.1], and an increase in glut- amine and asparagine levels within 48 h. The results obtained seem to indicate the enhancement, by nitrogen assimilation, of both the dark CO 2 fixation and the oxidative pentose phosphate pathway, which synthesise metabolic precursors for amino acid synthesis via transaminases. An involvement of anaplerotic CO 2 fixation and of the isoforms of glu- cose-6P in the roots during ammonia assimilation is discussed. Key words: Hordeum vulga", ammonia assimilation, carbo- AlAT = alanine aminotransferase (2-oxoglutarate + alanine::::} glutamate + pyruvate) [EC 2.6.1.2]; &AT = aspartate aminotransferase(2-oxoglutatrate + aspartate::::} glutamate + oxalacetate) [EC 2.6.1.1]; ASNase = asparaginase (asparagine::::} aspartate + NH 3 ) [EC 3.5.1.1]; DTE = dithioerythreitol; EDTA = ethylenediaminetetraacetic acid; HEPES = (N-[2-Hydroxyethyl]piperazine-N'[2- ethanesulphonic acid]); HK = Hexokinase (glucose + ATP::::} glucose-6P + ADP) [EC 2.7.1.1]; G6PDH = glucose-6P dehydrogenase (glucose-6P + NADP+ ::::} 6P-gluconic acid + NADPH + H+) [EC 1.1.1.49]; MSO = methionine sulfoximine; PEP carboxylase = Phosphoenolpyruvate carboxylase (PEP + HC0 3 - ::::} oxalacetate) [EC 4.1.1.31]; PFK = phosphofructokinase [EC 2.7.1.11] (fructose-6P + ATP ::::} fructose- 1,6bP + ADP); PFP = pyrophosphate: fructose-6P phosphotransferase (fructose-6P + Ppi ::::} Fructose- 1,6bP + Pi) [EC 2.7.1.90]. studies have concerned the effects of NH4 + on respiration (Emes and Bowsher, 1991; Huppe and Turpin, 1994). The relationship between carbon metabolism and nitrogen assimilation has been studied for the last 20 years, and many Carbon flow through respiratory pathways is affected by nitrogen availability in higher plants (Champignyand Foyer, J Plant PhysioL WlL 153. pp. 61-66 (1998)