Ornithine decarboxylase properties: Is there a role for a microsome‐bound inactivating activity?

Abstract

Liver microsomes have a strong ornithine decarboxylase (ODC) inactivating capacity in vitro. The present results suggest that this may be involved in regulation of ODC activity in vivo: (1) the O D C inactivating capacity of microsomes appears susceptible to in vivo modulation: a single administration of thioacetamide, which induces ODC, also causes a significant increase in the inactivating capacity of the microsomes; (2) under conditions leading to increased microsome-bound ODC-inactivating capacity (e.g. liver from thioacetamide-treated rates versus regenerating liver) ODC displays a greater thermal lability and inactivability in virro.A possible involvement of this microsomal activity in an autoregulatory pathway of O D C is suggested by the fact that it is induced by the administration of polyamines. However, inhibition of ODC activity by a-difluoromethylornithine does not prevent the increase of the microsomal activity caused by thioacetamide. Thus, polyamine biosynthesis does not appear to be an absolute requirement for induction of the microsomal ODC-inactivating capacity.The apparent half-life of O D C in vivo, as evaluated after cycloheximide administration, does not appear to correlate with the microsomal ODC-inactivating capacity content and the stability properties of O D C in vitro.

Rapid and regulated degradation of ornithine decarboxylase

“…The selective proteolytic machinery, however, appears to be more fragile than the protein synthesis system, and establishment of a cell-free ODC degradation system took a long time. Although several in vitro ODC inactivation systems have been reported, these are probably not related to physiological ODC degradation in cells [90][91][92]. Several years ago, Kahana and co-workers developed an in vitro ODC degradation system [93].…”