Two constituent polypeptide chains of ricin D, a highly toxic protein from castor bean seeds, were cleaved with cyanogen bromide (CNBr). Four CNBr fragments, α-CB I, α-CB II, α-CB III, and α-CB IV, were isolated from the lie (α) chain by gel filtration through BioGel P-30 and characterized. N-Terminal sequence analyses revealed that the alignment of these fragments was (α-CB I)-(α-CB IV)-(α-CB II)-(α-CB III). CNBr cleavage of Ala rø chain, followed by reduction and S-carboxymethylation, yielded four fragments, β-CB I, β-CB II, β-CB III, and β-CB IV. Methionine residue located at 5th position from N-terminus of Ala chain was found to be resistant to CNBr cleavage. Sequence analyses of these fragments revealed that fragment β-CB IV was originated from fragment β-CB III by acid cleavage, not by CNBr cleavage of as party 1-prolyl bond d uring CNBr treatment, and the alignment of CNBr fragments was established to be (β-CB II)-(β-CB I)-(β-CB III).
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