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 ( Ho rdeum vulgare L.) mutant (LaPr 87/30) plants indicated that photorespirator y carbon isotope fractionation, opposite in sign when compared to the carbo n isotope effect during CO2 photoassimilation, takes place in vivo. Thus th e key reaction of photorespiration catalyzed by GDC, together with the key reaction of CO2 fixation catalyzed by ribulose-1,5-bisphosphate carboxylase , both contribute to carbon isotope fractionation in photosynthesis.