Investigation of Structure and Rate of Synthesis of Ornithine Decarboxylase Protein in Mouse Kidney

An immunoblotting technique was used to study the forms of ornithine decarboxylase present in androgen-in-duced mouse kidney. Two forms were detected which differed slightly in isoelectric point but not in subunit molecular weight (~55 000). Both forms were enzymatically active and could be labeled by reaction with radioactive a-(difluoromethyl)-ornithine, an enzyme-activated irreversible inhibitor. On storage of crude kidney homogenates or partially purified preparations of ornithine decarboxylase, the enzyme protein was degraded to a smaller size (M_r ~ 53 000) without substantial loss of enzyme activity. The synthesis and degradation of ornithine decarboxylase protein were studied by labeling the protein by intraperitoneal injection of [³⁵S]methionine and immunoprecipitation using both monoclonal and polyclonal antibodies. The fraction of total protein synthesis represented by renal ornithine decarboxylase was increased at least 25-fold by testosterone treatment of female mice and was found to be about 1.1% in the fully induced androgen-treated female. Both forms of the enzyme were rapidly labeled in vivo, and the immunoprecipitable ornithine decarboxylase protein was almost completely lost after 4-h exposure to cycloheximide, confirming directly the very rapid turnover of this enzyme. Treatment with 1,3-diaminopropane which is known to cause a great reduction in ornithine decarboxylase activity did not greatly selectively inhibit the synthesis of the enzyme. However, 1,3-diaminopropane did produce an increase in the rate of degradation of ornithine decarboxylase and a general reduction in protein synthesis. These two factors, therefore, appear to be responsible for the loss of ornithine decarboxylase activity and protein in response to 1,3-diaminopropane.