Limonoid acidic

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 III. Relative Concentrations of Acidic Limonoids in Citrus...

While most of the limonoids of Citrus have been isolated from seeds, several occur in detectable amounts in other parts of the fruit. Minor amounts of deacetylnomilin, nomilin, obacunone, de-acetylnomilinic acid and nomilinic acid were identified in extracts of navel orange peel (37). 17-Dehydrolimonoate A-ring lactone was isolated from peel and juice of navel oranges (38) and nomilin has been reported to occur in grapefruit juice and juice vesicles (39). The only Citrus limonoids known to be bitter are limonin, VI, XII, obacunoic acid and IX (3). 

Azadirachtin, meliacin, azadironic acid bitter limonoids (Nortriterpenoids). Nimbibin, nimbin, salanin. Sesquiterpenes. Limonoids,... 

Limonoids are a group of chemically related triterpene derivatives found in the plant families, Rutaceae and Meliaceae. Among 37 limonoids reported to occur in citrus and hybrids, four are known to be bitter. They are limonin, nomilin, ichangin and nomilinic acid. Limonin is the major cause of citrus juice bitterness and is widely distributed. Nomilin is also known to contribute to limonoid bitterness in grapefruit juice (4). 

Inhibition of Limonoid Biosynthesis by Auxins. Auxins are potent inhibitors of nomilin biosynthesis in citrus seedlings (31). For instance, up to 91% inhibition was observed when 10 ppm of indoleacetic acid was fed to the stem of a lemeon seedling two days prior to and two days following feeding of 25 pCi of 14-C acetate (Table 1). Other auxins tested include 1-naphthaleneacetic acid (NAA), indolepropionic acid, indolebutyric acid, 3-indole acetonitrile, ethyl indole-3-acetate, 3-indoleacrylic acid, 3-(2-hydroxyethyl)indole, indole-2-carboxylic acid and 2,3,4-trichlorophenoxyacetic acid. They were all very effective. 

The technical feasibility of an immobilized cell system was demonstrated on a laboratory scale for use in reducing limonoid bitterness of citrus juices. The treatment caused no significant adverse effect on the other juice constituents as judged by analysis of ascorbic acid, citric acid, malic acid, sucrose, glucose and fructone. 

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... 

The isolation of obacunol (68) from Lovoa trichilioides provides a second example of the occurrence of a ring-A-cleaved tetranortriterpenoid in the Meli-aceae (see Vol. 1, p. 176). The acid limonoids present in grapefruit seeds include" " nomilinic acid (69) and deacetylnomilinic acid (70) together with the known compounds, iso-obacunoic acid (71) and epi-iso-obacunoic acid (72). The full paper on spathelin (73) has appeared. ... 

The structure of phragmalin (75), a complex limonoid from Entandrophragma caiidatum, has been determined by X-ray analysis." It is closely related to utilin (see Vol. I, p. 177) and occurs in combination with nicotinic and isobutyric acids. 

Calodendrolide (76), a novel degraded terpenoid, has been isolated from Calodendron capense (Rutaceae). It was readily converted into pyroangolen-solide (77) by hydriodic acid. Calodendrolide co-occurs with other limonoids... 

Full papers have appeared on odoratin (84)/° a partial synthesis of mexicanol-lide and the acid-catalysed rearrangement of limonoid epoxides. The Eu(dpm)3 shift reagent has been utilized for the assignment of the methyl resonances of a number of limonoids. An unexpected temperature effect has been... 

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). 

To use a more sophisticated example, we can look to the products of the neem tree (Azadirzchta indica), a tropical plant that is known for its pesticidal properties. The seed of this tree is abundant with limonoids and simple terpenoids that are responsible for its biological activity. One particular limonoid found in the seed is Azadirachtin (2.134). The bioactivity of Azadirachtin potentially leads to a wide range of applications in herbal medicine and healthcare products for the treatment of malaria and tuberculosis and in anti-worm, clotting, and blood-detoxification preparations. These uses of Azadirachtin as a biopesticide or herbal medicine is limited due to solubility constraints in water and its instability as a result of its propensity to undergo complicated, irreversible rearrangements under acidic, basic and photolytic conditions. Consequently, there has been much research in the structural modification of Azadirachtin to overcome its solubility constraints to increase stability. This process normally involves many protection and deprotection synthetic steps and chromatographic separations. 

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. 

Nomilin the general biosynthetic precursor of all limonoids, is biosynthesized via the terpenoid pathway from acetate and/or mevalonate in stems. Nomilin is then translocated to other plant tissues, including leaves, seeds, and fruit tissues, where it is converted to the other limonoids independently (Hasegawa et al. 1986 Ou et al. 1988). In most citrus species and hybrids, nomilin is converted to obacunone and then to obacunoate. Obacunaote is then converted to limonin probably via the intermediate ichangin. Nomilinic acid is the predominant acidic limonoid in citrus, but it does not seem to be directly involved in the biosynthetic pathways of major limonoids. 

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