Stimulation of confluent serum-starved SV-3T3 cells by serum produces a transient enhancement of ornithine decarboxylase activity which fluctuates at least 20-fold over a 12-h period. The mechanism by which this fluctuation is produced was investigated. Two techniques for assaying the amount of enzyme protein in these cells were utilized: radioimmunoassay and titration with α-(difluoromethyl) [5-3H]ornithine. The radioimmunoassay was carried out by using a specific antiserum prepared in rabbits against homogeneous mouse kidney ornithine decarboxylase and by using α-(difluoromethyl)[5-3H]ornithine-labeled kidney ornithine decarboxylase as the tracer ligand. An exact correlation between the amount of enzyme protein and the amount of enzyme activity was seen during the rise and fall of ornithine decarboxylase activity after serum stimulation. Similarly, the amount of protein which was labeled covalently by reaction with α-(difluoromethyl)[5-3H]ornithine (a specific enzyme-activated irreversible inhibitor) correlated with the enzyme activity. Investigation of the protein labeled in this way by Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate indicated that no change in the size of the protein which had a molecular weight of 53 000 occurred during this time period. These results indicate the alteration in ornithine decarboxylase activity can be accounted for entirely by changes in the amount of enzyme protein rather than by posttranslational modifications, activators, or inhibitors.
All Science Journal Classification (ASJC) codes