ABSTRACT
Present results concern a microsome-bound enzymatic system which has been recognized as responsible for the rapid inactivation in vitro of ornithine decarboxylase (ODC). Two different models have been investigated: a) rat liver after a single thioacetamide administration, and b) the 3924 A Morris hepatoma. In both these models we observed variations in the microsome-bound ODC-inactivating capacity. In parallel, changes in ODC properties were observed. The possibility of a causal relationship between the two events is discussed. The actual role of the microsome-bound ODC-inactivating system, in ODC activity regulation in vivo cannot be established, but it remains as a fairly plausible working hypothesis.
Subject(s)
Ornithine Decarboxylase/metabolism , Protein Processing, Post-Translational , Animals , Liver Neoplasms, Experimental/enzymology , Microsomes, Liver/enzymologyABSTRACT
Rat liver (hydrocortisone-induced) ornithine decarboxylase has been shown to be stable when the cytosolic fraction is incubated alone at 37 degrees 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.
Subject(s)
Liver Regeneration , Liver/enzymology , Ornithine Decarboxylase/metabolism , Animals , Cytosol/enzymology , Enzyme Induction/drug effects , Hepatectomy , Hydrocortisone/pharmacology , In Vitro Techniques , Male , Microsomes, Liver/enzymology , Ornithine Decarboxylase Inhibitors , Rats , Rats, Inbred StrainsABSTRACT
A very rapid and drastic microsome-dependent in vitro inactivation of the hydrocortisone-induced ornithine decarboxylase in rat liver was reported recently (M. F. Zuretti and E. Gravela, Biochim. Biophys. Acta. 742: 269-277, 1983). Present results show that ornithine decarboxylase from preneoplastic nodules and hepatomas, which have been induced in rats by N-2-fluorenylacetamide, is much more stable, its greater stability not being accounted for by a lower microsome-bound inactivating capacity. The possibility of a relationship between the in vitro enzyme stability and the increase of enzyme activity in neoplastic tissues is suggested.