Catalyzes the aminotransferase reaction from putrescine to 2-oxoglutarate, leading to glutamate and 4-aminobutanal, which spontaneously cyclizes to form 1-pyrroline. This is the first step in one of two pathways for putrescine degradation, where putrescine is converted into 4-aminobutanoate (gamma-aminobutyrate or GABA) via 4-aminobutanal. Also functions as a cadaverine transaminase in a a L-lysine degradation pathway to succinate that proceeds via cadaverine, glutarate and L-2-hydroxyglutarate. 2-oxoglutarate + an alkane-alpha,omega-diamine = an omega-aminoaldehyde + L-glutamate 2-oxoglutarate + putrescine = 1-pyrroline + H2O + L-glutamate 2-oxoglutarate + cadaverine = 5-aminopentanal + L-glutamate Amine and polyamine degradation; putrescine degradation; 4-aminobutanal from putrescine (transaminase route): step 1/1. Belongs to the class-III pyridoxal-phosphate-dependent aminotransferase family. Putrescine aminotransferase subfamily.