FRACTIONATION OF CARBON ISOTOPES (C-13 C-12) IN ENZYMATIC DECARBOXYLATION OF GLYCINE BY PLANT LEAF MITOCHONDRIA IN-VITRO/

The fractionation of carbon isotopes (C-13/C-12) in the course of glyc ine decarboxylation was studied in mitochondria from a number of plant species (pea, alfalfa, wheat, barley, spinach, cabbage, and Wolffia). Mitochondria were isolated from leaves by differential centrifugation and incubated with glycine (20 mM) and cofactors of glycine decarboxy lase, The CO2 released from the pea mitochondria was enriched with C-1 2 by 8 parts per thousand as compared to glycine carbon. In the rest o f the plant species, the CO2 evolved was enriched with C-13 by 5-16 pa rts per thousand. It was concluded that carbon isotope fractionation m ay take place in glycine decarboxylation during photorespiration, resu lting in a shift of the isotope ratio in the plant biomass relative to the carbon of the assimilated CO2. The magnitude of the isotope effec t was shown to vary greatly, up to the inversion of the sign, which wa s accounted for by changes in the contribution of the rate-limiting st ages in the enzyme reaction to its overall rate due to pH shifts induc ed by the NH3 release occurring in the course of the reaction.