Castor bean (Ricinus communis
) is an oilseed crop that belongs to the family Euphorbiaceae, which is composed of 6,300 species including crops such as cassava (Manihot esculenta
), rubber tree (Hevea brasiliensis
), and physic nut (Jatropha curcas
), as well as the invasive weed leafy spurge (Euphorbia esula
) and ornamental poinsettias (Euphorbia pulcherrima
). The castor bean plant is a tropical perennial shrub originated in Africa and cultivated in many tropical and subtropical regions around the world. It can be self- and cross-pollinated and worldwide studies showed low genetic diversity among castor bean germplasm1,2
Castor bean seed oil contains 90% of the unusual hydroxy-fatty acid ricinoleic acid3
. Because of the nearly uniform content of ricinoleic acid in castor oil, and the unique chemical properties that this fatty acid confers to the oil, castor bean is a highly valued oilseed crop for lubricant, cosmetic, medical, and specialty chemical applications. Furthermore, castor bean is a potential biodiesel source, due to its high seed oil content4
and because it can be cultivated in unfavorable environments, making it an appealing crop in tropical developing countries. It is believed that castor oil was first used as an ointment 4,000 years ago in Egypt, from where it spread to other parts of the world, including Greece and Rome, where it was used as a laxative 2,500 years ago5
An important problem of castor bean as a crop is the high seed content of ricin, an extremely toxic protein6
. Ricin is considered one of the deadliest natural poisons when administered intravenously or inhaled as fine particles. Ricin was first isolated more than a century ago7
. It has been reportedly used as a weapon6
and attempts to using ricin as a specific immunotoxin for therapeutic purposes in different cancers have been reported8,9
. Its biochemical activity has been characterized as a type 2 ribosome-inactivating protein (RIP), composed by two subunits linked by a disulfide bond: a 32 kDa ricin toxin A (RTA) chain that harbors the ribosome-inactivating activity, and a 34 kDa ricin toxin B (RTB) chain, with a galactose-binding lectin domain. RTA is an N-glycosidase that depurinates adenine in a specific residue of the 28S ribosomal RNA10,11
. The RTB chain, allows ricin to enter eukaryotic cells by binding to cell surface galactosides and subsequent endocytosis. Other RIPs are common in plants, although they are not toxic because they are usually monomeric and lack a lectin domain. These proteins constitute the type 1 RIPs12
Ricin is synthesized as a precursor encoding both subunits in the endoplasmic reticulum of endosperm cells, and is translocated and accumulated in protein bodies13
. The precursor is proteolytically processed in the endoplasmic reticulum and in the protein bodies, where it is stored as the mature heterodimer.
Ricin is very similar to the Ricinus communis
agglutinin or RCA14
. However, while ricin is a weak hemagglutinin, RCA has low toxicity and a strong hemagglutinin activity. In addition, RCA is a tetrameric protein composed of two RTA- and two RTB-like subunits.
Purification of ricin can be achieved through a relatively simple process, raising biosafety concerns. For this reason, the United States does not extensively produce castor oil, and it is among the world's largest importers of castor oil and its derivatives. Therefore, knowledge of the castor oil metabolism is important to advance towards using castor oil as biofuel, as well as to enable metabolic engineering to obtain safe sources of hydroxy-fatty acids without the complications of ricin.