Involved in the protection against polyamine toxicity by regulating their concentration. Catalyzes the transfer of an acetyl group from acetyl coenzyme A (AcCoA) to the primary amino groups of spermidine to yield N(1)- and N(8)-acetylspermidine. It can use polyamines such as spermine and N(1)-acetylspermine, but not putrescine or cadaverine. acetyl-CoA + an alkane-alpha,omega-diamine = an N-acetylalkane-alpha,omega-diamine + CoA + H(+) acetyl-CoA + spermidine = CoA + H(+) + N(1)-acetylspermidine acetyl-CoA + spermidine = CoA + H(+) + N(8)-acetylspermidine acetyl-CoA + spermine = CoA + H(+) + N(1)-acetylspermine Allosterically regulated by polyamines (PubMed:25623305, PubMed:26410587). Polyamines trigger conformational changes and induce the symmetric closed dodecameric state of the protein when they bind to their allosteric sites (PubMed:26410587). kcat is 16.8 sec(-1) for acetyltransferase activity with AcCoA as substrate (in the presence of 500 uM spermine at pH 9 and 37 degrees Celsius). kcat is 6.79 sec(-1) for acetyltransferase activity with spermine as substrate (in the presence of 100 uM AcCoA at pH 9 and 37 degrees Celsius). kcat is 6.58 sec(-1) for acetyltransferase activity with spermidine as substrate (in the presence of 100 uM AcCoA at pH 9 and 37 degrees Celsius). Amine and polyamine degradation; spermidine degradation. Amine and polyamine degradation; spermine degradation. Homododecamer; dimer of hexamers (PubMed:25623305, PubMed:26410587). Exists in solution in a variety of protein homooligomeric states including dodecamers in an open state. The presence of the polyamines spermidine or spermine shifts the equilibrium to dodecamers and induces the formation of the closed, symmetric dodecamers (PubMed:26410587). Belongs to the acetyltransferase family.