About the Lectin
Two lectins with varying degrees of toxicity have been isolated from the seeds of the castor bean. Both of the lectins are composed of two similar subunits an α and a β chain. These lectins are commonly referred to as the agglutinin (RCA-I) and the toxin (RCA-II), based on their relative biological activities. RCA-I is a tetramer composed of two α and two βchains. RCA II is a dimer composed of only a single α and βchain. When comparing the relative toxicity of the lectins, RCA-II (also called ricin) is about 20 times more potent than RCA-I 1 . Originally it was believed that toxicity was due to the hemagglutinating activity of the lectin but it has been determined that toxicity and hemagglutination are distinct properties of the lectins, the agglutination activity residing in the β-chain, whereas the α-chain is an N-glycanase that specifically attacks ribosomal RNA 2 .
RCA-I (or RCA 120 ) exhibits a specificity for βgalactose residues, with a preference for terminal sugars. Carbohydrate inhibition studies indicate that a β(1,4)-linkage is important for binding since lactose is a potent inhibitor; Gal β(1,3)glucose is only about one-third as inhibitory as lactose. N-acetyl-D-galactosamine is a very poor inhibitor of the agglutinin 3 . Specific binding studies have been performed with a number of synthetic glycosides, indicating that RCA-I reacts more strongly with branched cluster glycosides than with a monosaccharide 4 . RCA-I was originally believed to be non-toxic but when studies similar to those used for determining the toxicity of RCA-II were applied to RCA-I, the agglutinin was shown to be toxic as well. RCA-I was found to be only slightly toxic to mice 2 , but very toxic to cells in culture1. The RCA-I α-chains can inhibit protein synthesis in a cell free system, but not in intact cells. The RCA-I β-chains are required for cell binding, but they are not considered toxic by themselves. Both an α and a β-chain are required to inhibit protein synthesis in intact cells. Isoforms of RCA-I (designated as RCL-I and RCL-II in some literature) are found in some samples of castor bean seeds. They are identical in size and affinity, and differ only in their ion-exchange properties 5 .
RCA-II (or RCA60) is similar, with respect to carbohydrate specificity, to RCA-I, although RCA-II reacts strongly with GalNAc, while RCA-I does not. RCA-II usually reacts weaker with carbohydrates than RCA-I. It is also a weaker hemagglutinin, but is at least 20 times more toxic, than RCA-I. The dimeric structure of RCA-II may explain its weaker carbohydrate reactivity relative to the tetrameric RCA-I. Although monomeric for the carbohydrate-binding β-chain, the β-chain appears to have at least 3 binding sites, accounting for the agglutinating ability of RCA-II 6 . While RCA-I is only slightly toxic to mice, RCA-II is 1000-2000 times more toxic, with an LD50=0.8µg/100g body weight. As with RCA-I, the βchain of RCA-II is responsible for cell binding, while the αchain is the toxic fragment 7 . In cell culture, RCA-II is only about 20 times more toxic than RCA-I. RCA-II occurs in at least 3 isoforms having similar molecular structure but different pI values 8 . A slightly heavier α-chain variant is also known 7 . An X-ray crystal structure ricin RCA-II has been determined 9 . The agglutinin and toxin isoforms are members of a multigene family of at least 8 genes 10. Hybrid immunotoxins composed of ricin αchain and specific monoclonal antibodies have been used in therapeutic applications, as have hybrid molecules using intact ricin 11-14 .
ALL FORMS OF RCA ARE CONSIDERED EXTREMELY TOXIC. ONLY QUALIFIED, TRAINED PERSONNEL SHOULD HANDLE THESE PRODUCTS.
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|Buffer||0.01M Phosphate – 0.15M NaCl, pH 7.2-7.4.|
|Activity||Less than 1 μg/ml of RCA-I will agglutinate type O human erythrocytes. Less than 10 μg/ml of RCA-II will agglutinate type O human erythrocytes. Both lectins will agglutinate neuraminidase treated red blood cells at a concentration of less than 0.5 μg/ml.|
|Inhibitory Carbohydrate||Lactose will inhibit both RCA-I and RCA-II. Galactose will inhibit RCA-I. GalNAc will inhibit RCA-II.|
|Molecular Weight||Aggregate MW=120,000 for RCA-I. For RCA‑II the MW=60-65,000. By SDS-PAGE three bands of MW=30,000, 32,000, and 60‑65,000 can appear for both lectins.|