Involved in the cell growth and proliferation. Catalyzes the irreversible transfer of a propylamine group from the amino donor S-adenosylmethioninamine (decarboxy-AdoMet) to putrescine (1,4-diaminobutane) to yield spermidine. Spermidine cannot be used as an aminopropyl acceptor. putrescine + S-adenosyl 3-(methylsulfanyl)propylamine = H(+) + S-methyl-5'-thioadenosine + spermidine Inhibited by methylglyoxal bis(cyclopentylamidinohydrazone)(MGBCP). Amine and polyamine biosynthesis; spermidine biosynthesis; spermidine from putrescine: step 1/1. Homodimer. Belongs to the spermidine/spermine synthase family.