Biosynthesis of straight-chain ester volatiles in Red Delicious and GrannySmith apples using deuterium-labeled precursors

Biosynthesis of straight-chain ester volatiles by Granny Smith and Red Deli cious apples was investigated using deuterium-labeled fatty acids, C-6 alde hydes, and alcohols. Perdeuterated saturated and monounsaturated fatty acid s were metabolized to hexyl-d(11), hexanoate-d(11), heptanoate-d(13), and o ctanoate-d(15) esters, whereas perdeuterated linoleic acid produced only he xyl-d(11) and hexanoate-d(11) esters. Exposure of fruit to vapors of deuter ated 3Z-hexenal, 2E-hexenal, and hexanal identified the following biosynthe tic processes: (1) isomerization between 3E, 3Z, and 2E-hexenals; (2) reduc tion to 3E, 3Z, and 2E-hexenyl esters; (3) reduction to hexanol and hexyl e sters; (4) oxidation to hexanoic acid and formation of hexanoate esters; (5 ) beta-oxidation of hexanoic acid leading to butyl and butanoate eaters; an d (6) alpha-oxidation of hexanoic acid leading to pentyl and pentanoate est ers. Unsaturated straight-chain ester volatiles appear to arise only by the lipoxygenase pathway and may be useful indicators of lipoxygenase activity in fruit.