Kinetics of nitroxide spin label removal in biological systems: an in vitro and in vivo ESR study

A systematic study on the disappearance of the electron spin resonance (ESR) signal of nitroxides based on six-or five-membered ring and bearing various charges was carried out in vitro and in vivo. The second-order kinetic rate constants of the reaction of spin probes with ascorbate were measured in vitro at various temperatures in phosphate buffered saline, and the relative activation energies were calculated. Clearance rates of the nitroxide radicals in rat brain homogenates and in blood indicate that the ascorbate contribution to nitroxide removal is about 50-70% in brain and 50-90% in blood. These rates can be easily calculated on the basis of the ascorbate concentration and of the second-order kinetic rate constants measured in phosphate buffered saline. ESR spectra acquired in vivo in rat head and tail, by an L-band resonator, indicated that the nitroxide decay rate is a first-order kinetic process in both domains and that the positively charged nitroxides are not retained in the brain, whereas the anionic and uncharged nitroxides are. Once nitroxides with piperidine ring enter the brain, their decay appears controlled mainly by ascorbate, while the ascorbate has a negligible influence on disappearance in brain of five-membered ring proxyl nitroxides.