NAD⁺‐aminoaldehyde dehydrogenase candidates for 4‐aminobutyrate (GABA) and β‐alanine production during terminal oxidation of polyamines in apple fruit

The last step of polyamine catabolism involves the oxidation of 3‐aminopropanal or 4‐aminobutanal via aminoaldehyde dehydrogenase. In this study, two apple ( Malus x domestica ) AMADH genes were selected ( MdAMADH1 and MdAMADH2 ) as candidates for encoding 4‐aminobutanal dehydrogenase activity. Maximal activity and catalytic efficiency were obtained with NAD + and 3‐aminopropanal, followed by 4‐aminobutanal, at pH 9.8. NAD + reduction was accompanied by the production of GABA and β‐alanine, respectively, when 4‐aminobutanal and 3‐aminopropanal were utilized as substrates. Md AMADH2 was peroxisomal and Md AMADH1 cytosolic. These findings shed light on the potential role of apple AMADHs in 4‐aminobutyrate and β‐alanine production.