The polyamine oxidase from lycophyte Selaginella lepidophylla (SelPAO5), unlike that of angiosperms, back-converts thermospermine to norspermidine.

In the phylogeny of plant polyamine oxidases (PAOs), clade III members from angiosperms, such as Arabidopsis thaliana PAO5 and Oryza sativa PAO1, prefer spermine and thermospermine as substrates and back-convert both of these substrates to spermidine in vitro. A clade III representative of lycophytes, SelPAO5 from Selaginella lepidophylla, also prefers spermine and thermospermine but instead back-converts these substrates to spermidine and norspermidine, respectively. This finding indicates that the clade III PAOs of lycophytes and angiosperms oxidize thermospermine at different carbon positions. We discuss the physiological significance of this difference.