Catalyzes the oxidation 4-aminobutanal (gamma-aminobutyraldehyde) to 4-aminobutanoate (gamma-aminobutyrate or GABA) (PubMed:3510672, PubMed:16023116). This is the second step in one of two pathways for putrescine degradation, where putrescine is converted into 4-aminobutanoate via 4-aminobutanal, which allows E.coli to grow on putrescine as the sole nitrogen source (PubMed:3510672, PubMed:22636776). Also functions as a 5-aminopentanal dehydrogenase in a a L-lysine degradation pathway to succinate that proceeds via cadaverine, glutarate and L-2-hydroxyglutarate (PubMed:30498244). Can also oxidize n-alkyl medium-chain aldehydes, but with a lower catalytic efficiency (PubMed:15381418, PubMed:16023116). 4-aminobutanal + H2O + NAD(+) = 4-aminobutanoate + 2 H(+) + NADH 5-aminopentanal + H2O + NAD(+) = 5-aminopentanoate + 2 H(+) + NADH Optimum pH is 5.4. Amine and polyamine degradation; putrescine degradation; 4-aminobutanoate from 4-aminobutanal: step 1/1. Amino-acid degradation. Homotetramer. Up-regulated under nitrogen starvation conditions. Cells lacking this gene show a high decrease in gamma-aminobutyraldehyde dehydrogenase activity, but are still able to grow with putrescine as the sole nitrogen source. However, a mutant lacking both patD and puuC cannot grow with putrescine as the sole nitrogen source. Can be used in the industrial production of the value-added compound 5-aminovalerate. 4-aminobutanal can spontaneously cyclize to 1-pyrroline, and 5-aminopentanal to 1-piperideine. Belongs to the aldehyde dehydrogenase family. Gamma-aminobutyraldehyde dehydrogenase subfamily.