Electron Spin Resonance Studies of Spin-Labeled Mammalian Cells by Detection of Surface-Membrane Signals

Lipid-soluble spin labels were incorporated into human lymphocytes and mouse L-cells and the resulting electron spin resonance spectra were compared with spectra obtained from similarly labeled human erythrocytes. Spin labels were found in all subcellular fractions of the nucleated cells that contained membranes. Spin-labeled cells remained viable and capable of replicating in vitro. Electron spin resonance signals from spin-labeled nucleated cells underwent a time- and temperature-dependent decay that was reversed by bathing the cells in K3Fe(CN)6. The demonstration of a relative cell impermeability to ferricyanide, as measured by both colorimetric and radioisotopic label methods, indicated that only spin-labeled molecules in the surface membrane were reactivated when ferricyanide was added to spin-labeled cells after the electron spin resonance signal had decayed.