Acetyl-CoA: Alcohol Acetyltransferase Activity and Aroma Formation in Ripening Melon Fruits Moshe Shalit, Nurit Katzir, Yaakov Tadmor, Olga Larkov, Yosef Burger, Fernond Shalekhet, Elena Lastochkin, Uzi Ravid, Orit Amar, Menahem Edelstein, Zvi Karchi, and Efraim Lewinsohn* Department of Vegetable Crops, Agricultural Research Organization (ARO), Newe Ya’ar Research Center, P.O. Box 1021, Ramat Yishay 30-095, Israel Melon varieties (Cucumis melo L.) differ in a range of physical and chemical attributes. Sweetness and aroma are two of the most important factors in fruit quality and consumer preference. Volatile acetates are major components of the headspace of ripening cv. Arava fruits, a commercially important climacteric melon. In contrast, volatile aldehydes and alcohols are most abundant in cv. Rochet fruits, a nonclimacteric melon. The formation of volatile acetates is catalyzed by alcohol acetyltransferases (AAT), which utilize acetyl-CoA to acetylate several alcohols. Cell-free extract derived from Arava ripe melons exhibited substantial levels of AAT activity with a variety of alcohol substrates, whereas similar extracts derived from Rochet ripe melons had negligible activity. The levels of AAT activity in unripe Arava melons were also low but steadily increased during ripening. In contrast, similar extracts from Rochet fruits displayed low AAT activity during all stages of maturation. In addition, the benzyl- and 2-phenylethyl-dependent AAT activity levels seem well correlated with the total soluble solid content in Arava fruits. Keywords: Aroma; volatile acetates; biosynthesis; alcohol acetyltransferase; melons (Cucumis melo. L) INTRODUCTION Contemporary melon cultivars differ in shape, size, color, surface netting, sweetness, flavor, and storability (1-3) and can be divided into two groups, climacteric and nonclimacteric, according to their ripening patterns. The climacteric mode of ripening is characterized by a significant increase in the levels of respiration and release of ethylene, triggering profound and rapid changes in a range of attributes, such as flavor (sweet- ness and aroma), as well as in other physical parameters such as firmness of the fruits and a typical slip area produced around the peduncle in climacteric melons. Some of the climacteric varieties of melons are consid- ered to be highly aromatic [such as Galia, Charentais, and Ananas (Cucumis melo var. reticulatus)], whereas the nonclimacteric varieties are normally considered to be less aromatic, that is, Casaba-type melons such as Rochet (Cucumis melo var. inodorus). Two of the most important parameters that determine the quality and consumer preferences of the fruits are their sweetness and aroma properties. The aroma of fruits is ordinarily composed of complex mixtures of volatile compounds present in the headspace (4). Vola- tile aldehydes, alcohols, and especially the large quanti- ties of esters present in their headspace are likely to be the key contributors to the unique aromas of melons (3, 5-11). Although the aromas of some of the individual volatile compounds often resemble melon or fruity notes, the peculiar aroma of melons cannot be related to any single compound. Many of the volatile aldehydes present in the head- space of fruits are formed by degradation of fatty acids (12), whereas the respective alcohols are derived from aldehydes by the action of alcohol dehydrogenases or as a result of degradation of amino acids (13). Volatile esters, important contributors to the aroma of many fruits, are formed by esterification of alcohols and carboxylic acids, normally utilizing a CoA ester as the acyl donor. The ability to esterify alcohols has been previously noted in many melon cultivars by incubating fruit slices with isobutyl alcohol (14). The nonaromatic inodorus types had low esterification potential, and other cucurbits such as cucumber (Cucumis sativus L.), squash (Cucurbita maxima Duch.), and watermelon (Citrullus lanatus Matsumu.) lacked the ability to esterify isobutyl alcohol (14). The mechanism of ester formation has been studied in microorganisms, where a group of enzymes termed alcohol acetyltransferases (AAT) have been identified (15, 16). AAT catalyze the transfer of an acetyl moiety from acetyl-CoA into the corresponding alcohol, forming an ester and free CoA (Figure 1). The first attempt to identify AAT activity in fruits was carried out in banana slices (17). In this study, isoamyl acetate was synthesized from isoamyl alcohol and acetyl-CoA in partially purified cell-free protein extracts. AAT activities have been also detected in apple (18), strawberry (19, 20), and melon fruits (14, 21, 22). In all cases, an alcohol is acetylated; the acyl donor is acetyl-CoA. The partially purified AAT from oriental sweet melons mainly esterified isobutyl alcohol to form isobutyl acetate (21). Other alcohols were also esterified, but benzyl alcohol, the putative precursor of benzyl * Author to whom correspondence should be addressed (fax 972-4-983-6936; telephone 972-4-953-9552; e-mail twefraim@netvision.net.il). 794 J. Agric. Food Chem. 2001, 49, 794-799 10.1021/jf001075p CCC: $20.00 © 2001 American Chemical Society Published on Web 01/06/2001