Limonin precursor, enzymes

Recently, a nonspecific enzyme capable of degrading the precursor was extracted from Navel orange albedo (63). Nicol and Chandler (63) used a proximate assay for the degrading enzyme that is, the substrate was not identified except as a limonin precursor. The crude extracts from albedo contained degradation activity that was concentrated by 40 to 60% saturation with (NH J SO. Instability of the enzyme prevented further purification. 

Nicol, K. J. Chandler, B. V. The extraction of the enzyme degrading the limonin precursor in citrus albedo. J. Sci. 

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). 

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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