title: Branched-chain and aromatic amino acid catabolism into aroma volatiles in Cucumis melo L. fruit.
abstract: The unique aroma of melons (Cucumis melo L., Cucurbitaceae) is composed of many volatile compounds biosynthetically derived from fatty acids, carotenoids, amino acids, and terpenes. Although amino acids are known precursors of aroma compounds in the plant kingdom, the initial steps in the catabolism of amino acids into aroma volatiles have received little attention. Incubation of melon fruit cubes with amino acids and alpha-keto acids led to the enhanced formation of aroma compounds bearing the side chain of the exogenous amino or keto acid supplied. Moreover, L-[(13)C(6)]phenylalanine was also incorporated into aromatic volatile compounds. Amino acid transaminase activities extracted from the flesh of mature melon fruits converted L-isoleucine, L-leucine, L-valine, L-methionine, or L-phenylalanine into their respective alpha-keto acids, utilizing alpha-ketoglutarate as the amine acceptor. Two novel genes were isolated and characterized (CmArAT1 and CmBCAT1) encoding 45.6 kDa and 42.7 kDa proteins, respectively, that displayed aromatic and branched-chain amino acid transaminase activities, respectively, when expressed in Escherichia coli. The expression of CmBCAT1 and CmArAT1 was low in vegetative tissues, but increased in flesh and rind tissues during fruit ripening. In addition, ripe fruits of climacteric aromatic cultivars generally showed high expression of CmBCAT1 and CmArAT1 in contrast to non-climacteric non-aromatic fruits. The results presented here indicate that in melon fruit tissues, the catabolism of amino acids into aroma volatiles can initiate through a transamination mechanism, rather than decarboxylation or direct aldehyde synthesis, as has been demonstrated in other plants.
authors.affiliation: Institute of Plant Sciences, Newe Ya'ar Research Center, Agricultural Research Organization, PO Box 1021, Ramat Yishay 30095, Israel.
authors.forename: Itay
authors.initials: I
authors.lastname: Gonda
authors.forename: Einat
authors.initials: E
authors.lastname: Bar
authors.forename: Vitaly
authors.initials: V
authors.lastname: Portnoy
authors.forename: Shery
authors.initials: S
authors.lastname: Lev
authors.forename: Joseph
authors.initials: J
authors.lastname: Burger
authors.forename: Arthur A
authors.initials: AA
authors.lastname: Schaffer
authors.forename: Ya'akov
authors.initials: Y
authors.lastname: Tadmor
authors.forename: Shimon
authors.initials: S
authors.lastname: Gepstein
authors.forename: James J
authors.initials: JJ
authors.lastname: Giovannoni
authors.forename: Nurit
authors.initials: N
authors.lastname: Katzir
authors.forename: Efraim
authors.initials: E
authors.lastname: Lewinsohn
mesh_terms: D024322:Amino Acids, Aromatic / Q000096:biosynthesis* / Q000737:chemistry; D000597:Amino Acids, Branched-Chain / Q000096:biosynthesis* / Q000737:chemistry; D028463:Cucumis melo / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D018506:Gene Expression Regulation, Plant; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism; D014835:Volatilization
chemical_list: D024322:Amino Acids, Aromatic; D000597:Amino Acids, Branched-Chain; D010940:Plant Proteins