The reaction of this enzyme occurs in three steps: (i) NAD+-dependent dehydrogenation of spermidine, (ii) transfer of the 4-aminobutylidene group from dehydrospermidine to putrescine, (iii) reduction of the imine intermediate to form homospermidine. Hence the overall reaction is transfer of a 4-aminobutyl group. This enzyme is more specific than EC 2.5.1.44, homospermidine synthase, which is found in bacteria, as it cannot use putrescine as donor of the 4-aminobutyl group. Forms part of the biosynthetic pathway of the poisonous pyrrolizidine alkaloids of the ragworts (Senecio).
