Determination of the melatonin content of different varieties of tomatoes (Lycopersicon esculentum) and strawberries (Fragaria ananassa)

Abstract

Melatonin has recently been detected in various plants and foods. However, data regarding the food composition of melatonin are too scarce to evaluate dietary intake. This paper aims to identify melatonin unequivocally using LC–MS in a wide set of varieties of tomatoes (Lycopersicon esculentum) and strawberry (Fragaria ananassa). Furthermore, a validated LC fluorescence was developed.

This is the first time melatonin has been identified in Bond, Borsalina, Catalina, Gordal, Lucinda, Marbone, Myriade, Pitenza, Santonio, Perlino, Platero, and RAF varieties of tomatoes, as well as in strawberry (Fragaria ananassa): Camarosa, Candonga, Festival, and Primoris. Melatonin concentration was shown to vary greatly depending on the tomato varieties and harvests (2009, 2010), ranging from 4.11 ng/g to 114.52 ng/g fresh weight. However, the four varieties of strawberries collected during the two harvests showed greater similarity in melatonin (1.38–11.26 ng/g fresh weight).

Introduction

It is widely recognised that dietary intake of fruit and vegetables is important in order to maintain good health. Programmes such as “Five a day” have been introduced to increase consumer consumption of these natural products and their bioactive ingredients (Geeroms, Verbeke, & Kenhove, 2008). One of these bioactive compounds that has recently come under focus is melatonin (N-acetyl-5-methoxytryptamine), shown in Fig. 1. This molecule is a biogenic indoleamine, which performs an important role in the regulation of circadian rhythm (Srinivasan et al., 2010). Furthermore, melatonin is proven to be a potent free radical scavenger and a broad-spectrum antioxidant (Reiter et al., 1999). Due to its amphipathic nature, melatonin is able to permeate all tissues and subcellular compartments. It has been identified in membranes, mitochondria, nucleoli and cytosol (Martin et al., 2000, Menendez-Pelaez et al., 1993). In this regard, melatonin is an effective protector for lipid membrane structures, proteins and DNA against free radical oxidation (Bubenik et al., 2000, Reiter, 1998). In addition, this molecule stimulates the production of several antioxidative enzymes such as glutathione peroxidase and reductase (Pablos et al., 1998).

Melatonin was first detected in 1995, in mono- and also di-cotyledon edible plant families (Dubbels et al., 1995) including edible seeds such as rice or corn (Hattori et al., 1995). Higher levels of melatonin were identified in traditional medicinal herbs (Chen et al., 2003) such as in St. John´s wort (Hypericum parthenium), in quantities of 1.75–4.39 μg/g (Murch, Simmons, & Saxena, 1997). Plant leaves and seeds generally contain the highest amounts of melatonin, although wide variation (pg/g – μg/g) has been observed across the different plant materials (Dubbels et al., 1995, Hattori et al., 1995, Manchester et al., 2000). Due to the high levels of melatonin in, for instance, tomatoes seeds (cultivar Micro-Tom) or feverfew seedlings, it can be hypothesised that melatonin might play a role in the antioxidative defence system of the plant. Its role could therefore include protecting the germ and reproductive tissue from biological and environmental assaults (Manchester et al., 2000, Okazaki and Ezura, 2009).

Some research articles suggest that melatonin might also have the function of countering high levels of free radicals that may be generated by metabolic activities. Likewise, direct exposure to sunlight or UV light that promotes photooxidation could also have an influence on the biosynthesis of melatonin in plants, increasing its production levels (Murch et al., 2000, Tan et al., 2007). Additionally, the influence of yeast has been reported in fermented products as wine (Rodriguez-Naranjo, Gil-Izquierdo, Troncoso, Cantos-Villar, & Garcia-Parrilla, 2011).

Tomatoes are one of the most extensively-produced crops, with nearly 130 million tonnes having been produced worldwide in the year 2008 (http://www.faostat.fao.org ). Melatonin detected in tomatoes (Lycopersicon pimpinellifolium and Lycopersicon esculentum Mill.) has ranged from 2.2 pg/g to 1.1 ng/g (Badria, 2002, Dubbels et al., 1995, Hattori et al., 1995, Okazaki and Ezura, 2009, Pape and Lüning, 2006, Van Tassel et al., 2001). Some studies point to a correspondence between the state of ripeness and the melatonin content (Dubbels et al., 1995).

Strawberries (Fragaria ananassa) represent another important crop, with their production at over 4 million tonnes per year (http://www.faostat.fao.org ). Spain is the second largest producer of strawberry of the world behind the USA (FAO Data). Melatonin has been detected in strawberries (Fragaria magna) in a low concentration: 12.4–136.6 pg/g (Badria, 2002, Hattori et al., 1995).

At present, the available scientific data regarding melatonin and its quantity in different foods is scarce. It is therefore impracticable to evaluate dietary intake. Additionally, there are knowledge deficits concerning the bioavailability and metabolism of melatonin. Research intends to understand its role in plant physiology as well as its metabolic properties (Arnao & Hernández-Ruiz, 2009; Garcia-Parrilla, Cantos, & Troncoso, 2009) For this purpose, suitable analytical methods for determining melatonin in food matrices are required.

The aim of the present study was to explore melatonin content in different varieties of tomatoes and strawberries. These crops are selected for our study as they are consumed with seeds and the contribution to dietary intake is expected to be higher. The extraction procedure was optimised for the respective plant material. To confirm the presence of melatonin in all extracts, LC–electrospray ionisation multiple mass spectrometry (LC–ESI-MSⁿ) was performed. For routine analysis, a liquid chromatography (LC) using the fluorescence detection method was set up and validated to quantify melatonin in tomatoes and strawberries. Different cultivars of two consecutive harvests (2009, 2010) were analysed.