Ornithine decarboxylase properties: is there a role for a microsome-bound inactivating activity?

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 ODC 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 vitro. A possible involvement of this microsomal activity in an autoregulatory pathway of ODC is suggested by the fact that it is induced by the administration of polyamines. However, inhibition of ODC activity by alpha-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 ODC in vivo, as evaluated after cycloheximide administration, does not appear to correlate with the microsomal ODC-inactivating capacity content and the stability properties of ODC in vitro.