Studies on mechanisms of ornithine decarboxylase activity regulation in regenerating liver

Rat liver (hydrocortisone-induced) ornithine decarboxylase has been shown to be stable when the cytosolic fraction is incubated alone at 37°C, although there is a very rapid and drastic loss of activity after addition of microsomes to the incubation medium. The present paper is concerned with the behaviour of ornithine decarboxylase induced in rat liver by a growth stimulus (partial hepatectomy); comparative studies have been carried out on the enzyme induced by sham operation, or by hydrocortisone. Results show that ornithine decarboxylase from regenerating liver is more stable when incubated with microsomes (from the same source); this higher stability depends both on a lower microsome-bound inactivating capacity and a limited susceptibility of the enzyme to the inactivation. A critical role in modulating the microsome-dependent inactivation appears to be played by low molecular weight cytosolic factors, whose greater content in regenerating liver is likely to be included with the factors above in determining the relative stability of ornithine decarboxylase.