Limonoid neutral

Table I (8) shows the concentrations of limonoids in seeds of several citrus species, as well as the relative proportions of neutral and acidic limonoids in each. Tables II and III (8j show the relative amounts of the major individual neutral and acidic limonoids, respectively, in several species. The cases in which two different samples of the same species were analyzed show the...

Table II. Relative Concentrations of Neutral Limonoids in Citrus...

No evidence of limonoid biosynthesis in fruit or seed tissues exists, despite the fact that most of the limonins are found in the seeds of mature fruits. Limonoid synthesis occurs in the leaves and limonoids are transported into the fruits (Maier, 1983). In citrus tissues, the naturally occurring precursor of limonin is a salt of limonoic acid A-ring lactone (60) (Fig. 25.13) in which the A ring is closed and the D ring is open. This tasteless compound is stable only in the salt form (Maier, 1983). In the presence of acid or the enzyme citrus limonoate D-ring hydrolase, the D-ring lacton-izes to form limonin (19). The rate of lactonization is accelerated by pasteurization of the juice. In the fruit, the precursor appears to be located in a compartment of the cell where the pH is neutral or alkaline, probably the cytoplasm (Maier, 1983). 

Miyake et al. (1991) found that extraction of neutral limonoids from Natsudaidai seeds depended upon the solvents used, time of extraction, and methods of sample preparation prior to extraction. Since the seeds contain high concentrations of both dilactones and monolactones with dilactones dominating, direct organic solvent extraction method yields lower values than extraction with the Hasegawa method mentioned above. The optimum extraction method was determined to be grinding the seeds in 0.1 M Tris buffer at pH 7.0 and incubating the mixture for at least 20 h at room temperature. It was followed by acidification and extraction with methylene chloride. The organic phases can then be evaporated off to concentrate the samples for analysis. 

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