Formation of putrescine in rat liver

Two pathways exist for the formation of putrescine in rat liver. Putrescine can be produced by the action of L-ornithine decarboxylase, an inducible enzyme which can be irreversibly inhibited by the drug, alpha-difluoromethylornithine. A method for quantitating the amount of active ornithine decarboxylase protein present in the liver under various conditions by measuring the binding of [5-14C]alpha-difluoromethylornithine is described. The results indicated that, even when maximally induced, less than 0.0002% of the liver cytosol protein is ornithine decarboxylase. A second pathway for the production of putrescine occurs in the liver by means of the acetylation of spermidine to N1-acetylspermidine and its oxidation to putrescine and N-acetyl-3-aminopropionaldehyde by polyamine oxidase. This pathway is controlled by the activity of spermidine N1-acetyltransferase which is induced by hepatotoxins. Both ornithine decarboxylase and spermidine N1-acetyltransferase turn over rapidly as indicated by the loss of activity in response to cycloheximide. Following treatment with either carbon tetrachloride or thioacetamide, changes in spermidine N1-acetyltransferase activity precede those in ornithine decarboxylase and experiments with appropriate inhibitors indicate that the early enhancement of hepatic putrescine levels if brought about by the acetylase/oxidase pathway. Subsequently, enhanced ornithine decarboxylase activity maintains the putrescine levels and restores the depleted spermidine content of the liver.