Chemical Structure of Zearalenone

Zearalenone, 6-(10-hydroxy-6-oxo-trans-1-undecenyl)-b-resorcylic acid lactone, is a non-steroidal mycotoxin with oestrogenic properties. It was first isolated from extracts of fungus Gibberella zeae (Fusarium graminearum) by Stob et al. (1962). This component is believed to act as an endogenous regulator of the sexual stage of development of their producer fungi. In the organisms of warm-blooded animals, the lactones mimic endogenous 17 P-estradiol, i.e. they stimulate the growth of muscle tissue and affect the functions of the reproductive system (Burkin et al. 2002). Its chemical structure was determined by Urry et al. (1966), and its name is derived from G. zeae, the name of the first studied organism that produces it; resorcylic acid lactone, the generic name for this group of natural products; ene, the standard suffix indicating the presence of the C-1' to C-2' double bond; and one, the standard suffix indicating the presence of the C-6' ketone (Fig. 15.1). Nowadays zearalenone is produced commercially by fermentation (Hidy et al. 1977) for use in the manufacture of zeranol (zearalanol) by catalytic hydrogenation (Hodge et al. 1966). It is a secondary fungal metabolite produced by several species of Fusarium, mainly by F. graminearum and F. culmorum. These species are known to colonize maize, barley, oats, wheat and sorghum (Eppley et al. 1974; Mirocha et al. 1974; Jemmali et al. 1978; Bennett and Shotweli 1979; Farnworth and Neish 1980; Kuiper-Goodman et al. 1987; Kuiper et al. 1988; Tanaka et al. 1988; Bennett and Klich 2003) and tend to develop during prolonged cool, wet growing and harvest seasons in the temperate and warm regions of the world (Velluti et al. 2000). Of numerous zearalenone derivatives that can be produced by Fusarium spp., only trans-a-zearalenol has been found to occur naturally in cereal grains (Richardson et al. 1985). After consumption of zearalenone, the two stereoisomeric metabolites, a- and P-zearalenole (Fig. 15.1), are produced in mammals by reduction of the keto-group at C-6'. Another structurally similar compound is zearalanol (zeranol, Ralgro), which is synthetically produced from zearalenone and is used as a growth promoter in animals and has been banned in the European Union since 1985 (Hagler et al. 2001; Nsahlai et al. 2002). Zearalanol is distinguished from zearalenone by lack of a C-1'-C-2' double bond. This substance can also be formed in vivo from zearalenone and a-zearalenole, which can be carried over from contaminated feed stuff to animals. Zearalenone and zearalenoles (a and P) act as estrogens because they can adopt a conformation which sufficiently resembles 17 P-estradiol and other natural estrogens to enable binding to the estrogen receptor (King et al. 1978; Miksicek 1994). The physiological effects of zearalanol are similar to those of zearalenone, but zearalanol is generally considered to produce estrogenic effects five to ten times greater than those of zearalenone (Schollenberger et al. 2006).

Owing to their frequent occurrence, zearalenone and zearalenoles are an important class of endocrine disrupters. Their estrogenic potential is comparable to that of the naturally occurring estrogens estrone and estriol and is several orders of magnitude higher than those of well-known environmental estrogens, e.g. organochlorine pesticides (Mirocha et al. 1971, 1974; Krska and Josephs 2001; Dai et al. 2004).

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