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The rate of scavenging by Ehrlich ascites cells of anionic ascorbyl and 2,6-dimethoxy-p-semiquinone free radicals has been investigated by electron spin resonance spectroscopy both for viable cells and for subcellular fractions obtained by differential centrifugation. The scavenging activity is concluded to be associated with an NAD(P)H enzyme containing an active sulfhydryl group. Attempts to identify the enzyme with the reported properties of either semi-dehydro-ascorbate reductase or DT-diaphorase have not been successful. The overall free-radical scavenging activity for viable cells is dextrose dependent and is controlled by the coulombic barrier associated with the cell-surface charge. The cytotoxicity of the mixture of ascorbic acid with 2,6-dimethoxy-p-benzoquinone is concluded to result from a loss of NAD(P)H reducing power in the cells.

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.

Standard midpoint potentials have been determined for p-benzoquinone, methoxy-p-benzoquinone and 2,3-, 2,5-, and 2,6-dimethoxy-p-benzoquinones in aqueous solution. ESR studies have been made of the ascorbate and semiquinone radicals produced when these quinones interact with sodium ascorbate. Direct correlations are found between the electrochemical potentials, generated semiquinone lifetimes, and cytotoxic action in Ehrlich ascites-bearing mice.