Ornithine decarboxylase activity in Helianthus tuberosus

Ornithine decarboxylase (ODC, EC 4.1.1.17) was studied in crude extracts of parenchyma slices of dormant tubers activated for 12 h, tuber shoots and shoot apices. It was highest in shoot apices. The enzyme activity was measured by the production of 14CO2 from labelled ornithine; Vmax was 450 nmol (mg protein)‐1h‐1, Km for ornithine and pyridoxal phosphate were, respectively, 30 mM and 5μM. Only when partially purified, the 14CO2 production was inhibited by α‐difluoromethylornithine, while in crude extracts dithiothreitol was inhibitory. Ornithine and arginine decarboxylase (ADC, EC 4.1.1.19) activities from parenchyma tubers were not greatly altered by exogenously supplemented ornithine, even though its endogenous pool increased. Exogenously supplemented arginine enhanced ornithine decarboxylase activity, whereas putrescine decreased it slightly. The possibility of artifactual activities in the crude extract is also discussed.