Decarboxylation of glycine contributes to carbon isotope fractionation in photosynthetic organisms

Carbon isotope effects were investigated for the reaction catalyzed by the glycine decarboxylase complex (GDC; EC 2.1.2.10). Mitochondria isolated fro m leaves of pea (Pisum sativum L.) and spinach (Spinacia oleracea L.) were incubated with glycine, and the CO2 evolved was analyzed for the carbon iso tope ratio (delta C-13). Within the range of parameters tested (temperature , pH, combination of cofactors NAD(+), ADP, pyridoxal 5-phosphate), carbon isotope shifts of CO2 relative to the C-1-carboxyl carbon of glycine varied from +14 parts per thousand to -7 parts per thousand. The maximum effect o f cofactors was observed for NAD(+), the removal of which resulted in a str ong C-12 enrichment of the CO2 evolved. This indicates the possibility of i sotope effects with both positive and negative signs in the GDC reaction. T he measurement of delta C-13 in the leaves of the GDC-deficient barley (Hor deum vulgare L.) mutant (LaPr 87/30) plants indicated that photorespiratory carbon isotope fractionation, opposite in sign when compared to the carbon isotope effect during CO2 photoassimilation, takes place in vivo. Thus the key reaction of photorespiration catalyzed by GDC, together with the key r eaction of CO2 fixation catalyzed by ribulose-1,5-bisphosphate carboxylase, both contribute to carbon isotope fractionation in photosynthesis.