Limonoids in citrus

Ethylphenoxy)triethylamine and 2-(3,4-dimethoxyphenoxy)triethylamine markedly reduce the biosynthesis of limonoids in citrus leaves, presumably by inhibition of cyclase activity. Radio-tracer studies have revealed that limonoids are synthesized in the leaves of citrus and transported to the fruit. The fruit tissue does not appear to be capable of the de novo synthesis of limonoids from acetate or mevalonate. 

Manners GD, Breksa AP III, Schoch TK, Hidalgo MB. Analysis of bitter limonoids in citrus juices by atmospheric pressure chemical ionization and electrospray ionization liquid chromatography-mass spectrometry. J Agric Food Chem 2003 51(13) 3709-3714. 

More recently Maier et al. (3) published a comprehensive review of the limonoid constituents of Citrus and the impact of limonin bitterness on juice quality. This paper summarizes the chemical, biochemical and juice quality aspects of limonoids in Citrus (and related genera) and presents relevant advances since previous reviews. 

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

Citrus trees are capable of translocating limonoids from leaves to fruit tissues (12). When 20,000 cpm of XIV, randomly labeled, was fed to a leaf adjacent to a 5-g-size lemon, about 13.3% of the total activity was translocated to the fruit during 20 hrs. These findings of Hasegawa et al. (12,13) show that limonoids in citrus fruit tissues are synthesized in leaves and translocated to the fruit. 

Inhibition of Biosynthesis. Triethylamine derivatives such as 2-(4-ethylphenoxy)triethylamine and 2-(3,4-dimethylphenoxy)-triethylamine markedly inhibit the accumulation of limonoids in citrus leaves (15). For example, young lemon leaves sprayed with 500 ppm of 2-(4-ethylphenoxy)triethylamine contained only 27 ppm of XIV 8 days after the treatment, whereas the control contained 344 ppm. Similarly, those sprayed with 300 ppm of the compound contained 0.3 times as much XIV as the control. 

S. Hasegawa, Biochemistry of Limonoids in Citrus. In Citrus Limonoids, Functional Chemicals in Agriculture and... 

This review paper summarizes the biochemical and biological aspects of limonoids in citrus and presents possible preharvest approaches to reduce bitter limonoids in citrus fruits. 

Thus, nomilin acetyl-lyase appears to play a key role in the regulatory system which controls the biosynthesis and accumulation of limonoids in citrus. 

Recently, deacetylnomilinate was found to be the initial precursor of all the limonoids known to be present in citrus (13), When 14-C labeled deacetylnomilinate was fed to the stem of a lemon seedling, it was converted mainly to nomilin. According to the previously proposed pathways (17), nomilin and obacunone were thought to be biosynthesized from deacetylnomilinate via nomilinate and/or deacetylnomilin. However, recent work has clearly demonstrated that deacetylnomilinate is the initial precursor of all the known citrus limonoids. This finding well established the biosynthetic pathways of the major limonoids in citrus. 

Figure 2. Biosynthetic pathways of limonoids in Citrus lchangensis...

Tetranortriterpenoids.—The occurrence of limonoids in Citrus species has been reviewed. ... 

Hasegawa, S., R. D. Bennett, and V. P. Maier, Biosynthesis of limonoids in Citrus seedlings. Phytochemistry, 23, 1601-1603... 

Hasegawa and Hoagland (1977) have recently carried out tracer studies on the biosynthesis of limonoids in citrus. These studies showed that young leaves were capable of synthesizing limonoids and su ested that the limonoids were translocated from this biosynthetic site to the fruits. 

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. 

A variety of methods have been proposed for the detection and quantification of bitter limonoids in citrus. Most methods were developed specifically for the quantification of limonin in citrus juices. The early techniques included colorimetry, fluorimetry, and gas chromatography. 

Hahlbrock K, Scheel D (1989) Physiology and molecular biology of phenylpropanoid metabolism. Annu Rev Plant Physiol Plant Mol Biol 40 347-369 Hasegawa S, Herman Z (1992) Biosynthesis of limonoids in Citrus. In Petroski RJ, McCormick SP (eds) Secondary-metabolite biosynthesis and metabolism. Plenum Press, New York, pp 305-317... 

Hasegawa S, Bennett RD, Verdon CP (1980) Limonoids in citrus seeds origin and relative concentration. J Agric Food Chem 28 922-925... 

Hasegawa S, Herman Z, Orme E, Ou P (1986) Biosynthesis of limonoids in Citrus sites and translocation. Phytochemistry 25 2783-2785... 

Hendrickson R, Edwards GJ (1958) Ultraviolet absorption technique to determine the naringin content of grapefruit juice. Proc Fla State Hortic Soc 71 194-198 Herman Z, Hasegawa S, Fong CH, Ou P (1989) Limonoids in Citrus ichangensis. J Agric Food Chem 37 850-851... 

Rouseff RL, Fisher JF (1980) Determination of limonin and related limonoids in citrus juices by high performance liquid chromatography. Anal Chem 52 1228-1233 Rouseff RL, Mansell RL (1982) Comparison of high performance liquid chromatographic and radioimmunoassay limonin values from commercial grapefruit juice. Proc Fla State Hortic Soc 95 249-252... 

Rouseff RL, Nagy S (1982) Distribution of limonoids in citrus seeds. Phytochemistry 21 85-90 Seitz U, Oefner PJ, Nathakarnkitkool S, Popp M, Bonn GK (1992) Capillary electrophoretic analysis of flavonoids. Electrophoresis 13 35-38... 

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