The results indicate that a primary ionization on either the A or the B chain of intact ricin was sufficient to inactivate the A chain. Possible conduits for the transfer of destructive energy from the B chain to the A chain are either the connecting disulfide bond or the noncovalent bonds that join the two chains. The former seems more probable because studies with other enzymes such as glutamate dehydrogenase (12) and other enzymes (2) clearly have shown that destructive energy does not pass between the subunits even if they are closely associated by noncovalent bonds. However, the most convincing argument that the disulfide bond was the conduit is the fact that destructive energy did not pass from the B chain to the A chain when the interchain disulfide bond of ricin was reduced prior to irradiation. The reduction of the interchain disulfide bond in ricin probably disrupts many of the intrachain disulfide bonds of the B chain and thus causes conformational changes. Nonetheless, under these reducing conditions the A and B chains do not dissociate, thus indicating that the noncovalent interactions between the two chains are at least partially intact.
The clear conclusion from these studies is that the interchain disulfide bond between the two subunits of ricin can transmit a significant amount of energy deposited by ionizing radiation. These results are not unexpected because the S-S bond is chemically similar to C-C and C-N bonds in proteins, which have previously been shown to conduct destructive energy (2-4, 13). The radiation sensitivity of disutfide bonds should be a chemical property of the bond rather than a property of the, secondary or tertiary structure of the polymer. It is likely therefore that the energytransferring properties of the interchain disulfide bond of ricin shown here is representative of disulfide bonds in other proteins and nonbiological polymers.
We thank Dr. Daniel Cawley for providing ricin. This work was supported in part by Grant CA 31279 from the U.S. Public Health Service and by the American Cancer Society.