**1. Introduction**

Aldehyde dehydrogenases (ALDH, aldehyde:NAD(P)<sup>+</sup> oxidoreductases, EC 1.2.1) use nicotinamide adenine dinucleotide (NAD+) or nicotinamide adenine dinucleotide phosphate (NADP+) as a cofactor to convert aldehydes to their corresponding carboxylic acids plus NADH or NADPH. ALDH in plants are currently receiving considerable attention because they are involved in processing many aldehydes that serve as biogenic intermediates in a wide range of metabolic pathways [1]. They often function as an 'aldehyde scavenger',

**Citation:** Ueda, Y.; Zhao, W.; Ihara, H.; Imahori, Y.; Tsantili, E.; Wendakoon, S.K.; Chambers, A.; Bai, J. Functional Characteristics of Aldehyde Dehydrogenase and Its Involvement in Aromatic Volatile Biosynthesis in Postharvest Banana Ripening. *Foods* **2022**, *11*, 347. https://doi.org/ 10.3390/foods11030347

Academic Editor: Arun K. Bhunia

Received: 24 December 2021 Accepted: 23 January 2022 Published: 26 January 2022

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thus removing reactive aldehydes generated during the oxidative degradation, especially under environmental stress, such as exposure to salinity, drought, cold, and heat [1].

The highly abundant volatiles in fresh bananas are aldehydes, ketones, alcohols, carboxylic acids, and esters. It has been recognized that the production of straight chain alcohols, aldehydes, ketones, and acids in fruit is largely derived from α-oxidation, βoxidation, or the lipoxygenase pathway [2,3]. Branched chain volatiles are derived from branched chain amino acids [4,5]. Recently, Sugimoto et al. [6,7] proposed that some branched and straight chain alcohols and acids come from the citramalate pathway.

Many enzymes regarding the conversions between volatile alcohols, aldehydes/ketones, acids, and esters have been studied extensively in fruits and plants, and/or adapted from microorganism studies [8–10]. In banana fruit ripening, gene expression and enzyme activity of alcohol dehydrogenase (ADH, short and medium chains), which facilitates the interconversion between alcohols to aldehydes [11,12], acyl-CoA synthetase (ACS), which activates carboxylic acids to acyl-CoAs [13] thus can be used to biosynthesize esters, and alcohol acetyl transferase (AAT), which catalyzes ester biosynthesis, have been intensively studied (Figure 1) [14–17]. ALDH dehydrogenizes aldehydes to carboxylic acids in different plant tissues [1,18]. However, to our knowledge, there is no report on the role of ALDH in volatile flavor metabolism in fruits.

**Figure 1.** Role of aldehyde dehydrogenase (ALDH) in the dehydrogenation from aldehydes to carboxylic acids and the entire ester production pathways. ADH, alcohol dehydrogenase; ACS, acyl-CoA synthetase; AAT, alcohol acyl-CoA transferase; BCAT, branched-chain amino transferase; BCKD, branched-chain α-ketoacid decarboxylase; BCKDH, branched-chain α-ketoacid dehydrogenase.

Beekwilder, Alvarez-Huerta et al. [16] showed that when incubating petunia leaves with 3-methylbutanol vapor for 24 h, 3-methylbutyl 3-methylbutanoate was the dominant volatile, in addition to 3-methylbutanol in the headspace, indicating a strong dehydrogenization of 3-methylbutanol to 3-methylbutanal, further to 3-methylbutanoic acid, then passing through 3-methylbutanoyl-CoA, and finally synthesizing 3-methylbutyl 3 methylbutanoate, catalyzed by ADH, ALDH, ACS, and AAT, respectively [13–16].

The purpose of the current study was to determine the enzyme which converts volatile straight and branched aldehydes to carboxylic acids, to identify their involvement in volatile metabolisms in banana fruit (Figure 1), and to reveal the functional characteristics of ALDH. Due to both ADH and ALDH activities being determined by the change in NADH concentration, efforts were made to minimize the effect of ADH [19].
