Figure 21.10 depicts synthesis of putrescine, spermidine, and spermine from ornithine.The polyamines are polycationic substances that stabilize intracellular conformations of negatively charged nucleic acids. Polyamines bind to phosphates on both strands of a duplex nucleic acid, thereby stabilizing double-stranded DNA or a duplex region of RNA.

Some proteins covalently bind polyamines, with the nitrogens linked to glutamate--carboxyl groups.

Putrescine is a precursor to spermidine, which is subsequently converted to spermine through the AdoMet-mediated transfer of active propylamino groups (Figure 21.10).

Polyamine biosynthesis is closely related to the proliferative state of the cell. When nucleic acid synthesis is activated, so is polyamine synthesis.