Fruits Produce Branched-Chain Esters Primarily from Newly Synthesized Precursors

Inhibitors of acetohydroxyacid synthase (also known as acetolactate synthase), the common enzyme of branched-chain amino acid biosynthesis, were used as tools to discern the contribution of newly synthesized precursors (i.e., branched-chain amino acids and α-ketoacids) to branched-chain ester formation in ripening apple (Malus ×domestica Borkh.), banana (Musa spp.), and flowering quince (Chaenomeles ×superba) fruits. After treatment, anteiso- and iso-branched-chain esters (i.e., those related to isoleucine, and valine and leucine, respectively) universally decreased in content by at least 90%. Among free amino acids, only the branched-chain amino acids, with correspondingly reduced branched-chain esters, had a lesser concentration following treatment with the inhibitor. Branched-chain ester production recovered after subsequent feeding with precursor compounds. Our results ultimately reject the hypothesis that anteiso- and iso-branched-chain esters of ripening fruits are primarily derived from preexisting sources and instead support the hypothesis that these esters are largely the product of de novo precursor biosynthesis.