Ammonium metabolism stimulation of glucose-6P dehydrogenase and phosphoenolpyruvate carboxylase in young barley roots

The effect of ammonium metabolism on the alternative pathways to glycolysis in young barley roots was investigated through measurements of enzyme activities and changes in amino acid levels. 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 glucose-6P dehydrogenase [EC 1.1.1.49] levels were measured, suggesting the activation of the pentose 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 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 glutamine and asparagine levels within 48 h. The results obtained seem to indicate the enhancement, by nitrogen assimilation, of both the dark CO2 fixation and the oxidative pentose phosphate pathway, which synthesise metabolic precursors for amino acid synthesis via transaminases. An involvement of anaplerotic CO2 fixation and of the isoforms of glucose-6P in the roots during ammonia assimilation is discussed.