Evaluation of spin labels for in-cell EPR by analysis of nitroxide reduction in cell extract of Xenopus laevis oocytes

► Reduced lifetime of spin labels under in-cellin vivo conditions. ► Reduction of nitroxides in cellular environment has enzymatic origin. ► Reduction kinetics described using the Michaelis-Menten model. ► Five member ring nitroxides are much more stable than six member ring nitroxides.