The formation of isovaleraldehyde from ℓ-leucine and isobutyraldehyde from ℓ-valine catalyzed by banana polyphenoloxidase and peroxidase

Possible mechanisms for the formation of isovaleraldehyde from l-leucine and isobutyraldehyde from l-valine in bananas are described. These aldehydes are key intermediates for the development of the branched-chain alcohols and esters, which are prominent components of banana flavor volatiles. When amino acids were incubated with dopamine and banana enzyme extract, these aldehydes were formed. Greater amounts of aldehydes were produced when hydrogen peroxide was included in the reaction mixture. It is proposed that the reaction mechanism involves oxidation of dopamine to o-quinones catalyzed by polyphenoloxidase and peroxidase, followed by the nonenzymic reaction of the quinone with the amino acid to form the corresponding aldehyde. At levels of leucine, polyphenoloxidase (PPO) and dopamine tested, aldehyde production increased with leucine concentration (up to 75 mM) and tended to peak at relatively low levels of PPO (0.1 units/ml) and dopamine (0.5 mM).