Pseudomonas aeruginosa PAO1 has two possible catabolic pathways of spermidine and spermine; one includes the spuA and spuB products with unknown functions and the other involves spermidine dehydrogenase (SpdH; EC 1.5.99.6) encoded by an unknown gene. The properties of SpdH in P. aeruginosa PAO1 were characterized and the corresponding spdH gene in this strain identified. The deduced SpdH (620 residues, calculated Mr of 68,861) had a signal sequence of 28 amino acids at the amino terminal and a potential transmembrane segment between residues 76 and 92, in accordance with membrane location of the enzyme. Purified SpdH oxidatively cleaved spermidine into 1,3-diaminopropane and 4-aminobutyraldehyde with a specific activity of 37 units (mg protein)(-1) and a Km value of 36 microM. The enzyme also hydrolysed spermine into spermidine and 3-aminopropanaldehyde with a specific activity of 25 units (mg protein)(-1) and a Km of 18 microM. Knockout of spdH had no apparent effect on the utilization of both polyamines, suggesting that this gene is minimally involved in polyamine catabolism. However, when spdH was fused to the polyamine-inducible promoter of spuA, it fully restored the ability of a spuA mutant to utilize spermidine. It is concluded that SpdH can perform a catabolic role in vivo, but P. aeruginosa PAO1 does not produce sufficient amounts of the enzyme to execute this function.