Metabolite pools and metabolic branching as factors of in-vivo isotope discriminations by kinetic isotope effects

Inter- and intra-molecular non-statistical isotope distributions do not onl y require the existence of a kinetic isotope effect on a defined enzyme cat alyzed reaction, but also the prerequisite that this reaction is located at a metabolic branching point. Furthermore a metabolic and isotopic balance demand that the extent of the isotopic shift is reciprocal to the products' yields. On this base the C-13-enrichment of L-ascorbic acid in position C- 1 and the depletion of glycerol in C-1 are interpreted. The C-13-pattern of natural malic acid is discussed as a consequence of isotope effects on the carboxylation of pyruvate and PEP and on the pyruvate dehydrogenase reacti on. The patterns of natural products synthezised by transfer of "active ace taldehyde" is proposed to be due to an isotope effect on the thiamine pyrop hosphate containing lyase reaction. An isotope effect on the reduction of " active formaldehyde" to "active methyl" and the existence of corresponding pools is responsible for C-13-enrichments and depletions of natural product s in positions bearing these intermediates. Finally a model for the main ni trogen pools and for isotope discriminations between alpha-amino, omega-ami no-N and amide pools in plants is proposed.