Limonoid aglycones

Biochemical studies carried out at the Pasadena laboratory have shown how, where and when limonoids are biosynthesized and accumulate in citrus. The recent discovery of limonoid glucosides has cleared an unanswered question of how limonoid aglycones disappear at late stages of fruit growth and maturation. 

Fmit tissues do not normally contain bitter limonin, but instead contain the nonbitter precursor of limonin, limonoate A-ring lactone (2). Limonoate A-ring lactone is the predominant limonoid aglycone present in fmit tissues of most citms species, and it is gradually converted to limonin after the juice is extracted. This conversion proceeds under acidic conditions below pH 6.5 and is accelerated by the... 

Thirty-six limonoid aglycones have been isolated from Citrus and its hybrids. These aglycones are present in intact fruit tissues as open D-ring forms such as limonoate A-ring lactone (2), while in seeds they are present in both open and closed forms. It appears that these citrus limonoids are biosynthesized through at least four different limonoid pathways the limonin pathway, the ichangensin pathway, the calamin pathway and the 7-acetate limonoid pathway (12). The products of the limonin pathway are found in all citrus species. The products of the other three pathways are only found in certain species of citrus and its close relatives. 

In fruit tissues and seeds, the limonoid aglycones are converted to 17- 3-D-... 

Limonoids are a group of chemically related triterpene derivatives found in the Rutaceae and Meliaceae families. There are two groups of limonoids the aglycones and their corresponding glucosides. A total of 36 limonoid aglycones have been isolated from Citrus and its hybrids (Table 1). Five of them are bitter in taste -limonin, nomilin, nomilinic acid, obacunoic acid and ichangin (Fig. 1). In citrus juices, limonin is the major cause of delayed bitterness. 

Limonoid aglycones have been shown to possess certain biological activities such as the inhibition of cancerous tumour growth in laboratory animals (Lam et al. 1989 a,b Miller et al. 1989) and antifeedant activities against insects (Alford and Bentley 1986 Liu et al. 1990). Recently, the glucoside of limonin, limonin 17-P-d-glucopyranoside, was found to possess anticancer activities as well (Miller et al. 1992). 

TLC is the easiest and most economical technique of detecting limonoid aglycones. Unlike other methods, TLC can be used for a variety of tissue samples. TLC solvent systems were well developed and an appropriate system can be found for a particular type of tissue sample with just a few trials. The major disadvantage is the use of subjective visual comparison with standards for qualitation and quantitation. 

In the food industry, high-performance liquid chromatography (HPLC) is the most widely used technique for the determination of limonin levels in citrus juices, since it is accurate and reliable. Most of the HPLC methods developed are for the analysis of limonin in citrus juices. These methods may not work for other bitter limonoid aglycones and/or tissues. 

Limonin has recently become commercially available from several chemical companies including Aldrich and Sigma. Nomilin is also available from Takasago (Toyko, Japan). Other limonoid aglycones and limonoid glucosides are currently only obtained by isolation from citrus seeds. See Herman et al. (1992) for extraction and isolation methods. 

Breksa, A.P. and lharra, P. 2007. Colorimetric Method for the Estimation of Total Limonoid Aglycones and Glucoside Contents in Citrus Juices. Journal of Agricultural andFood Chemistry, 55, (13), 5013-5017. 

Figure 7.29 Citrus fruit are a rich source of terpenoids including the Cio diterpenes (+)- and (—)-limonene and a number of more complex limonoids which are distinctive in that the glucosides are tasteless while the aglycones have a bitter taste.

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