AMMONIUM METABOLISM STIMULATION OF GLUCOSE-6P DEHYDROGENASE AND PHOSPHOENOLPYRUVATE CARBOXYLASE IN YOUNG BARLEY ROOTS

The effect of ammonium metabolism on the alternative pathways to glyco lysis in young barley roots was investigated through measurements of e nzyme activities and changes in amino acid levels. The activities of m ost glycolytic enzymes did not change either before or after the suppl y of ammonium to young barley plants. By contrast, increases in phosph oenolpyruvate carboxylase [EC 4.1.1.31] and glucose-6P dehydrogenase [ EC 1.1.1.49] levels were measured, suggesting the activation of the pe ntose phosphate 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 t he activities of aspartate aminotransferase [EC 2.6.1.1], alanine amin otransferase [EC 2.6.1.2] and asparaginase [EC 3.5.1.1], and an increa se in glutamine and asparagine levels within 48 h. The results obtaine d seem to indicate the enhancement, by nitrogen assimilation, of both the dark CO2 fixation and the oxidative pentose phosphate pathway, whi ch synthesise metabolic precursors for amino acid synthesis via transa minases. An involvement of anaplerotic CO2 fixation and of the isoform s of glucose-6P in the roots during ammonia assimilation is discussed.