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 (<i>Malus</i> ×<i>domestica</i> Borkh.), banana (<i>Musa</i> spp.), and flowering quince (<i>Chaenomeles</i> ×<i>superba</i>) fruits. After treatment, <i>anteiso</i>- and <i>iso</i>-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 <i>anteiso</i>- and <i>iso</i>-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 <i>de novo</i> precursor biosynthesis.