Norpseudopelletierine-N-oxyl (NPPN) has been shown to increase the sensitivity of B. megaterium spores when irradiated in anoxic buffered suspensions. The maximum sensitization achieved by NPPN alone corresponded to a 58% increase over the simple anoxic response. This compares with a 40% increase produced by paranitroacetophenone (PNAP) and 115% increase shown by oxygen. Like PNAP, NPPN is capable of producing a small measure of sensitization by interfering with water radiolysis intermediates (indirect affects) when used at low concentrations. At high concentrations, at which NPPN exerts its maximum degree of sensitization, it is concluded that the compound operates by increasing the lethality of the direct effects of radiation on the cellular target. The use of mixtures of NPPN and PNAP produces a greater degree of sensitization than can be achieved by either alone, at least a 91% increase over the anoxic sensitivity being observed. It is concluded that these two sensitizers, chosen as representatives of the electron-affinic class (PNAP) and of the free radical nitroxyl class (NPPN) of sensitizers do act through different mechanisms.
The basic anoxic radiosensitivity of B. megaterium spores irradiated in buffered aqueous suspension can be increased by up to 40% by the addition of PNAP to the suspension. At the highest concentrations, above 6× 10 -4 M PNAP, this increased sensitivity is constant and cannot be reduced by the addition of t-butanol as an OH scavenger. This does not hold, however, at lower concentrations at which PNAP exerts a submaximal sensitizing action since in this range t-butanol can eliminate the effect of PNAP. Nitrous oxide produces radiosensitization in the spore system that can be completely reversed by the presence of t-butanol. PNAP, however, can apparently only partially remove the sensitization produced by nitrous oxide. These results are explained in terms of indirect, or water radical-mediated, sensitization by nitrous oxide or low concentrations of PNAP. High concentrations of PNAP, however, are believed to enhance the lethality of the direct effects of radiation on the cellular target and this effect is independent of the presence of nitrous oxide.