Effect of proteolysis on the electron spin resonance spectra of maleimide spin labeled erythrocyte membrane

The ratio of low-field amplitudes of weakly and strongly immobilized signals of ESR spectra of a maleimide spin label bound to erythrocyte membranes (h_w/h_s) increases progressively during incubation at 37δC. This increase is due to the 'self-digestion' of membrane proteins by endogenous proteinases and is attenuated by proteinase inhibitors. Digestion of membranes with chymotrypsin also increases the h_w/h_s, ratio. These results suggest a need for a careful interpretation of data from spin-labeled membrane proteins, especially in experiments involving prolonged incubations of membrane preparations when the proteolytic effects may be significant.