ID 2.5.1.45 DE homospermidine synthase (spermidine-specific). CA putrescine + spermidine = propane-1,3-diamine + sym-homospermidine. CC -!- A eukaryotic enzyme found in plants. CC -!- The reaction occurs in three steps, with some of the intermediates CC presumably remaining enzyme-bound: CC (1) NAD(+)-dependent dehydrogenation of spermidine to CC dehydrospermidine. CC (2) Attack by water forming 4-aminobutanal (and releasing propane- CC 1,3-diamine). CC (3) Condensation of 4-aminobutanal with purescine, which forms CC homospermidine and restores NAD(+). CC -!- This enzyme is more specific than EC 2.5.1.44, which is found in CC bacteria, as it cannot use putrescine as donor of the 4-aminobutyl CC group. CC -!- Forms part of the biosynthetic pathway of the poisonous pyrrolizidine CC alkaloids of the ragworts (Senecio). DR Q9SC13, HSS1_SENVE ; Q9M4B0, HSS1_SENVU ; P60038, HSS2_SENVE ; //