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.