Interaction of ricin and its constituent polypeptides with dipalmitoylphosphatidylcholine vesicles

The interaction of ricin and of its constituent polypeptides, the A- and B-chain, with dipalmitoylphosphatidylcholine (DPPC) vesicles was investigated. The A- and B-chain were individually associated with DPPC vesicles, although the intact ricin was not associated. The maximum binding and association constants were evaluated to be 154 micrograms per mg of DPPC and Ka = 2.30 X 10(5) M-1 for the A-chain, and 87 micrograms per mg of DPPC and Ka = 14.5 X 10(5) M-1 for the B-chain, respectively. The A-chain could induce the phase transition release of carboxyfluorescein from DPPC vesicles to a greater extent than the B-chain, whereas the release induced by the intact ricin was negligible. The evidence indicated that the hydrophobic regions on the A-chain and on the B-chain were buried inside when the two chains constituted the intact ricin molecule through one interchain disulfide bond, and that the A-chain caused perturbation of the DPPC bilayer at the phase transition temperature with consequent leakage of carboxyfluorescein.