Conversion of exogenous spermidine into putrescine after administration to rats

Administration of large, but non-toxic doses of spermidine (0.4-1.25 mmol/kg) led to a substantial increase in putrescine in liver, kidney and a number of other tissues including muscle. The increase in putriscine peaked at 6 h after treatment and was completely prevented by administration of cycloheximide 3 h after the spermidine suggesting that the induction of a new protein was required. This protein is likely to be spermidine N¹-acetyltransferase which was induced by the treatment with spermidine and increased 3-4-fold in liver and kidney within 6 h. N¹-Acetylspermidine was detected in tissues at this time after spermidine treatment and experiments in which labeled spermidine was given indicated that a substantial fraction of the administered spermidine was converted into N¹-acetylspermidine and into putrescine. These results suggest that the rise in putrescine after spermidine treatment is brought about by the production of N¹-acetylspermidine which is converted into putrescine by the action of polyamine oxidase. The limiting step in this conversion is the activity of the acetylase which is induced in response to the rise in spermidine content. The acetylase/oxidase pathway, therefore, provides a means by which polyamine levels can be regulated and excess polyamine disposed of.