الفهرس 
AcknowledgementOnly 14 pages are availabe for public view
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Abstract
Since 1888, researchers have intensively studied a hemeagglutinating, cytotoxic protein they termed ricin. Ricin was extracted from the seeds of Ricinus communis, the castor bean, family Euphorbiaceae. Studies on both, the cytotoxic and hemeagglutinating activities revealed that they were attributed to the existence of two distinct chains within the protein, the A-chain and the B-
chain respectively. These two chains are differ in their nature and function and are connected
with a disulfide bond. The cytotoxic A-chain was found to be a ribosome inactivating protein,
while the B-chain was characterized as a plant lectin. Although the A-chain is the cytotoxic part
of the molecule, still, the presence of the B-chain is a limiting factor for protein toxicity. The fact
that the B-chain is of lectin-nature, enables the molecule to bind to cell surfaces and thus be
introduced to the cells and exert its ribosome inactivating function. The introduction into the cell
is not the only mechanism of the B-chain activity. A lipase active site at the interface between
both chains has been discovered. The lipase active site enables a translocation step within the cell
which is crucial for cytotoxicity. Researchers have widely investigated the use of ricin in the
treatment of cancer. The use of ricin in both, in vivo and in vitro has been intensively studied.
Recently, ricin has been extracted from the seeds of Ricinus sanguine us. Still, little has been
known about the degree of similarity between ricin from Ricinus sanguineus and Ricinus
communis. The nucleotide, the amino acid sequence and tertiary structure of the A-chain
originating from Ricinus sanguineus has been recently revealed. In this study, the nucleotide
sequence, amino acid sequence, tertiary structure of the B-chain and the quaternary structure of
ricin isolated from Ricinus communis have been identified. Additional studies on the behavior of
the newly discovered ricin revealed the degree of similarity of ricin from both sources regarding
its effect on the quaternary structure and lipase active site in order to show differences and
similarities in the rate-limiting step of cytotoxicity within mal functional cells.