Interaction of the 2,6-dimethoxysemiquinone and ascorbyl free radicals with Ehrlich ascites cells: a probe of cell-surface charge.

The rate of quenching by Ehrlich ascites cells of anionic 2,6-dimethoxy-p-semiquinone and ascorbyl free radicals is investigated as a function of cell concentration, the blocking of cell-surface sulfhydryl groups by N-ethylmaleimide, and the reduction of cell-surface charge by neuraminidase. The rate of quenching is found to be proportional to cell viability and to the number of free cell-surface sulfhydryl groups. The enzymatic action of neuraminidase results in an increase of the free radical quenching rate, indicating that this rate can be used as a probe of cell-surface charge. Measurements as a function of the ionic strength of the suspending electrolyte gave a value of -1.22 microC X cm-2 for the charge density at the ascites cell surface. This is equivalent to a surface membrane potential of -14 mV for a 150 mM NaCl electrolyte and is a value in good agreement with published electrophoresis data.