Effects of the myeloperoxidase system on fluorescent probes of granulocyte membrane potential

Activation of normal or myeloperoxidase-deficient human granulocytes by phorbol myristate acetate resulted in an initial membrane depolarization as indicated by an increase in fluorescence of the lipophilic cation probe of membrane potential, 3,3'-dipropylthiodicarbocyanine. A subsequent apparent hyperpolarization (decrease in fluorescence) was observed in normal but not myeloperoxidase-deficient cells. Addition of purified myeloperoxidase restored a normal pattern of fluorescence changes to the enzyme-deficient granulocytes. The secondary decrease in fluorescence in normal cells was markedly blunted by addition of azide, cyanide, or catalase. In a cell-free system, the fluorescence of 3,3'-dipropylthiodicarbocyanine, but not that of 3,3'-dipentyloxadicarbocyanine, was rapidly eliminated by myeloperoxidase in the presence of hydrogen peroxide and a halide; this loss of fluorescence was inhibited by azide, cyanide, or catalase. These findings indicate that secretion of myeloperoxidase and hydrogen peroxide by activated granulocytes results in decreased fluorescence of 3,3'-dipropylthiodicarbocyanine, probably by thioether oxidation. While the determination of initial rates of depolarization using this probe is unaffected by the myeloperoxidase system, measurement of extent of depolarization and any subsequent membrane potential changes requires the addition of inhibitors. In the absence of inhibitors, the secondary decrease in fluorescence can be used as an indicator of secretion of myeloperoxidase and hydrogen peroxide.