The sensitization of anoxic bone marrow stem cells irradiated in vitro in the presence of the free radical norpseudopelletierene-N-oxyl (NPPN) has been demonstrated. No sensitization, however, was obtained following administration to animals which were then killed before irradiation so as to render the stem cells anoxic. Electron spin resonance measurements demonstrated a rapid disappearance of NPPN from blood which could explain the failure to sensitize anoxic stem cells in intact animals. Measurements in vitro showed that NPPN is degraded by red blood cells. The rate of degradation was not sufficient to explain the loss in vivo indicating that NPPN is probably being degraded by some other tissue. The in vitro reaction kinetics suggest that an enzymatic process may be involved. Another nitroxyl, triacetoneamine, was found to be degraged much more slowly by blood in vitro.

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