Nomilin

XXXV) 1 3 - imi the fall poison pierotoxiain (XXXVI), ae well as coriamyrtin1 73 and related substanoes the citrus bitter principle limonin (XXXVII) and the related substances nomilin and obaou-none 4" and the terpenoid substances clerodin1 7.1 and oedre-kne.im.iMa... 

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). 

Hasegawa (54) reported on properties of the lactonase. The enzyme purified from grapefruit seeds hydrolyzed limonoids that have the D-ring intact but differ from limonin in the vicinity of the A and A -rings, namely, obacunone, nomilin and ichangin. 

Isolation of nomilin, a new bitter principle from the seeds of oranges and lemons. J. Am. Chem. Soc., 19A8, 7Q, 5A5-5A9. 

Bitterness, caused by naringin, is also removed by an enzyme, naringinase. Another possibility would be to eliminate one or more of the enzymes in the limonin biosynthetic pathway by using recombinant DNA techniques. Limonin, a triterpenoid, is probably synthesized via the mevalonate pathway, as are the monoterpenoid flavor compounds. It appears that nomilin, a precursor of limonin, is synthesized in the stems and roots of citrus and then the precursor transported to the fruit where it is converted by several enzymes to limonin and other bitter limonoids (46). 

Several limonoids are known to be bitter principles of citrus (Rutaceae). A typical example is limonin. Although fresh juice does not elicit a bitter taste, sometimes it becomes bitter after heating or storage. This is explained by the formation of bitter-tasting limonin by deglycosylation and further cyclization from limonin glucoside, which is present in citrus fruit tissue and seeds and does not exhibit bitterness.146 Recently, it was reported that limonin had antitumor activity.147 Besides limonin, nomilin and obakunone, which are considered to be... 

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). 

Limonin and nomilin are two bitter limonoids present in citrus juices. Systematic organoleptic tests showed that the bitterness threshold is 6 ppm (5) for limonin, and 6 ppm (6) or 3 ppm (7) for nomilin. The bitterness due to limonin develops gradually in juices after extraction from certain varieties of oranges, grapefruit, lemon, Natsudaidai, mandarin and some other minor citrus such as Iyokan and Ponkan. This phenomenon is generally referred to as "delayed bitterness". 

The intact fruits do not contain bitter limonin, but rather a nonbitter precursor, limonoate A-ring lactone (8). When juice is extracted, this nonbitter precursor is gradually converted to limonin under acidic conditions and the conversion is accelerated by the action of limonin D-ring lactone hydrolase, which has been isolated from citrus (9). The bitterness due to nomilin in juices most likely develops in a manner similar to that of limonin bitterness, but this has not yet been directly proven. However, the contribution of nomilin to juices is minor. It occurs mainly In grapefruit juices (4). 

Preparations of Radioactive Substrates. Young citrus seedlings are excellent tools for the preparation of 14-C nomilin from labeled acetate. When fed to stems of lemon seedlings, up to 5% of the labeled acetate was incorporated into nomilin (10). Incorporation of acetate was the best followed by mevalonate and farnesyl pyrophosphate. This high value of incorporation supports the fact that nomilin is one of the major compounds which accumulate in seedlings. 

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. 

Stems are the major site of nomilin biosynthesis from acetate in citrus (26). Analysis of the phloem, the cortex and the inner core regions of the stem showed that the phloem region is the site of nomilin biosynthesis from acetate (27). Root tissues also have this capacity. Leaves, fruits and seeds are either incapable of biosynthesizing limonoids from acetate or have a very low capacity. However, these tissues are capable of biosynthesizing limonoids from nomilin. Nomilin is translocating from the stem to other locations, where it is further biosynthesized to other limonoids (26). 

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. 

Table 1. Inhibition of Nomilin Biosynthesis by Indoleacetic Acid in Young Lemon Seedlings...

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