Brief Introduction to Citrus Limonoids

Jiaxing Li

TAMU-College Station

TAMUK-Citrus Center

Bitterness problem in citrus industry

• Cause 1. The flavonoids e.g. naringin-- species related to pumello

• Cause 2. The limonoids e.g. limonin

Biochemistry of Limonoids in Citrus

• A group of highly oxygenated tetracyclic triterpenoids -- Citrus & related genera

• Tow forms in citrus:

1. Limonoid aglycones (LA) -- >50 isolated from the Rutaceae (36 from Citrus&related genera)

2. Limonoid glucosides (LG) -- 17 isolated

Biochemistry Cont.

• Relation between LA and LG

1. LA: bitter, insoluble in water

2. LG: non-bitter, water-soluble

3. LA glucosidated to LG--fruit maturation

--- Natural Debittering process

--- Occurs only in fruit tissues and seeds

Delayed bitterness

• PH<6.5

• Limonoid D-ring lactone hydrolase

• Closed D-ring is the key structure

An example of natural debittering and delayed bitterness

• Navel oranges and Valencia oranges in California

• Early to mid-season Navel juice has delayed bitterness, Valencia dose not

• Reason: Short maturation period of the Navel (2 months); 6 months for the Valencia allow:

LA LG

Structures of some of the major Limonoids in citrus

Structures Cont.

Biosynthesis of Citrus Limonoids

• Four groups of LA

1. Limonin group

2. Calamin group

3. Ichangensin group

4. 7α-acetate limonoid

• Biosynthetic pathways of each group of these limonoids have been elucidated

Example of Biosynthetic pathways: limonin group

Analytical Methods of Citrus Limonoids

• Thin-layer chromatography (TLC)

-- for limonoid detection

• Nuclear Magnetic Resonance (NMR)

-- determination of limonoid structure

• HPLC -- detection & quantification

• Radioimmunoassay --detection&quantif.

• HPLC-MS -- detection& quantification

Distribution of limonin in Citrus fruit and vegetative tissues -- by Radioimmunoassay

• Limonin levels in fruit and vegetative tissue (White Marsh)

High: Seeds

Pith

Lamella

Albedo

Flavedo

Low: Juice vesicles

Limonin distribution within grapefruit leaves

• More concentrated in areas adjacent to the conductive tissues

• flushing leaves > mature leaves

Biological activities of citrus limonoids

• Inducers of GST, a detoxifying enzyme in mice and rats:

-- to inhibit formation of chemically-induced neoplasia in forestomach, small intestine, colon, lung, skin and oral of lab animals;

-- to inhibit the proliferation of breast cancer cells in culture

Biological activities contd.

• Antifeedant activity

• insect repellents

• growth disrupters

• reproduction inhibitors

Limonoids and the Taxonomic Studies of Citrus

• Theoretical bases

1. Plants have incorporated many secondary metabolites into specialized physiological functions such as reproduction and intracelluar signaling

2. secondary metabolites are often functionally unique at the species level.

3. Different compounds are responsible for identical functions in different species.

Different Limonoid groups and the Chemotaxonomy of the true-citrus tree

species and hybrids

Future research

• Nutritional research: determination of the roles phytochemicals play in the long term health of humans.

-- Food additives for cancer prevention

The End

• Thank You

• References

Mark A. Berhow, Shin Hasegwa, and Gary D. Manners.Citrus Limonoids -- Functional Chemicals in Agriculture and Food/ 2000, American Chemical Society.