1. Administration of spermidine or sym-norspermidine decreased the activity of AdoMet (S-adenosylmethionine) decarboxylase in extracts prepared from rat liver, Kidney, psoas, diaphragm, soleus and small intestine, but not heart. The decline in psoas, diaphragm and soleus was much greater than that in liver and kidney. The difference in sensitivity to spermidine could not be explained by changes in the uptake and accumulation of the polyamine, because much higher contents were found in liver and kidney that in diaphragm and psoas. 2. Spermidine administration also led to a substantial increase in putrescine in all tissues examined. However, the rise in putrescine was not responsible for the decline in AdoMet decarboxylase activity, since norspermidine, which cannot form putrescine, also produced the decline. Also, administration of putrescine or 1,3-diaminopropane did not decrease AdoMet decarboxylase. 3. The decline in skeletal-muscle AdoMet decarboxylase activity in response to spermidine may be due to an increased rate of degradation of the enzyme protein. The t1/2 (half-time) for the decline in activity after inhibition of protein synthesis by cycloheximide was almost halved in the psoas of spermidine-treated rats. Spermidine treatment did not change the t1/2 in liver. 4. These results raise the possibility that there are at least two different forms of AdoMet decarboxylase and that the enzyme from psoas or diaphragm differs from that in liver. Additional support for this hypothesis was obtained by comparing the activation by putrescine of AdoMet decarboxylase from these tissues. The liver enzyme was stimulated 10-fold, but the muscle enzyme was stimulated 30-fold.

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