Limonin biosynthesis

Limonin and nomilin cause bitter flavor in citrus products. The bitterness is a major problem for the industry. Hasegawa et al. (this volume) have identified three target enzymes involved in limonoid biosynthesis for development of transgenic citrus free of limonin and nomilin bitterness. They are linoleate dehydrogenase, UDP glucose transferase and nomilin deacetylesterase. The isolation of the genes for these enzymes is currently being conducted in order to eventually insert them into cultured citrus cells where they will convert the bitter compounds to non-bitter derivatives. From the cultured cells mature citrus plants will be produced and these plants should produce fhiit free of bitter flavor. 

Biosynthesis and metabolism of limonin and its closely related limonoids... 

Radioactive tracer work has shown that nomilin (3) is biosynthesized by the terpenoid pathway from acetate in the phloem region of stems and translocated to other parts of plants such as leaves, fruit tissues and seeds (13,14). At those locations, nomilin is further biosynthesized to other limonoids. Limonoid biosynthesis occurs at each location independently, thus the composition of limonoids in fruit tissues, seeds and leaves are different from each of the others. Limonin is biosynthesized from nomilin via obacunone (4), obacunoate (5) and ichangin (6) (15-17) (Fig. 2)... 

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). 

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