Figure 8.

Polyamine oxidation pathways in peroxisomes.

(A) Interconversion pathway. Spermine and spermidine are substrates for peroxisomal polyamine oxidases (PAO2-4). PAO4 acts specifically on spermine to oxidize it to spermidine. PAO3 catalyses the sequential back-conversion of spermine to spermidine and spermidine to putrescine, and is subjected to feedback inhibition by its end product, putrescine. The specific substrate for PAO2 has not been established.

(B) Speculative pathway for terminal catabolism. Peroxisomal copper amine oxidase (CuAO) oxidatively deaminates putrescine to produce ammonia and H₂O₂. The reaction product 4-aminobutyraldehyde spontaneously isomerizes to Δ1-pyrroline, which is a substrate for peroxisomal betaine aldehyde dehydrogenase (BADH) that catalyzes its conversion to γ-aminobutyric acid (GABA) and releases H₂O₂ as a by-product.