Affinity labeling of purified ornithine decarboxylase by alpha-difluoromethylornithine

Ornithine decarboxylase (L-ornithine carboxy-lyase, EC 4.1.1.17) purified from rat liver was affinity-labeled by alpha-[5-14C]difluoromethylornithine. On analysis by SDS-polyacrylamide gel electrophoresis, the radioactivity migrated as a single major peak that coincided with a single protein band of Mr 50,000. Calculation from bound radioactivity indicated that ornithine decarboxylase has two active sites, one for each subunit, and that pure enzyme should have a specific activity of about 1.4 x 10(6) nmol CO2/h per mg protein.