Abstract
Abrin and ricin are highly toxic proteins isolated and crystallized from Abrus precatorius1 and Ricinus communis respectively2–4. The LD50s of abrin and ricin are 0.020 0.012 mg/kg body weight of mice respectively. Both have a molecular weight of 65,000, but a different chemical structure as judged by the analysis of N and C terminal amino-acid residues. For abrin these are leucine5, while the N-termini of ricin are alanine and isoleucine, and the C-termini are phenylalanine and serine2,6. The biological activities of the two proteins are alike; both cause similar toxic symptoms and have no effect on mitochondrial respiration7,8. The inhibitory effect of abrin and ricin on Ehrlich ascites tumour has been demonstrated by simultaneous injection of a sublethal dose with the tumour cells. Total inhibition of tumour growth in mice was demonstrated by measurements of body weight and total cell numbers in the peritoneal cavity. The therapeutic index of abrin was found to be 4. These inhibitory effects can be eliminated by heating the phytotoxin solution at 100° C for 30 min9,10, which indicates that the anti-tumour activity of phytotoxins is due to protein.
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