FRACTIONATION OF CARBON (C-13 C-12) ISOTOPES IN GLYCINE DECARBOXYLASEREACTION/

Fractionation of carbon isotopes (C-13/C-12) by glycine decarboxylase (GDC) was investigated in mitochondrial preparations isolated from pho tosynthetic tissues of different plants (Pisum, Medicago, Triticum, Ho rdeum, Spinacia, Brassica, Wolffia). 20 mM glycine was supplied to mit ochondria, and the CO2 formed was absorbed and analyzed for isotopic c ontent, CO2 evolved by mitochondria of Pisum was enriched up to 8 part s per thousand in C-12 compared to the carboxylic atom of glycine, CO2 evolved by mitochondria of the other plants investigated was enriched by 5-16 parts per thousand in C-13. Carbon isotope effects were sensi tive to reaction conditions (pH and the presence of GDC cofactors), Th eoretical treatment of the reaction mechanism enabled us to conclude t hat the value and even the sign of the carbon isotope effect in glycin e decarboxylation depend on the contribution of the enzyme-substrate b inding step and of the decarboxylation step itself to the overall reac tion rate, Therefore, the fractionation of carbon isotopes in GDC reac tion was revealed which provides essential isotopic effects in plants in addition to the well-known effect of carbon isotope fractionation b y the central photosynthetic enzyme, ribulose-1,5-biphosphate carboxyl ase.