The in vivo nitrogen isotope discrimination among organic plant compounds

The bulk delta(15)N-value of plant (leaf) biomass is determined by that of the inorganic primary nitrogen sources NO3-, NH4+ and N-2, and by isotope discriminations on their uptake or assimilation. NH4+ from these is transferred into organic N mainly by the glutamine synthetase reaction. The involved kinetic nitrogen isotope effect does not become manifest, because the turnover is quantitative. From the product glutamine any further conversion proceeds in a closed system, where kinetic isotope effects become only efficient in connection with metabolic branching. The central, and most important corresponding process is the GOGAT-reaction, involved in the de novo nitrogen binding and in recycling processes like the phenylpropanoid biosynthesis and photorespiration. The reaction yields relatively N-15-depleted glutamate and remaining-glutamine, source of N-15-enriched amide-N in heteroaromatic compounds. Glutamate provides nitrogen for all amino acids and some other compounds with different N-15-abundances. An isotope equilibration is not connected to transamination; the relative delta(15)N-value of individual amino acids is determined by their metabolic tasks. Relative to the bulk delta(15)N-value of the plant cell, proteins are generally N-15-enriched, secondary products like chlorophyll, lipids, amino sugars and alkaloids are depleted in N-15. Global delta(15)N-values and N-15-patterns of compounds with several N-atoms can be calculated from those of their precursors and isotope discriminations in their biosyntheses. (C) 2002 Elsevier Science Ltd. All rights reserved.