EXTRACELLULAR ASCORBATE STABILIZATION AS A RESULT OF TRANSPLASMA ELECTRON-TRANSFER IN SACCHAROMYCES-CEREVISIAE

The presence of yeast cells in the incubation medium prevents the oxid ation of ascorbate catalyzed by copper ions. Ethanol increases ascorba te retention. Pyrazole, an alcohol dehydrogenase inhibitor, prevents a scorbate stabilization by cells. Chelation of copper ions does not acc ount for stabilization, since oxidation rates with broken or boiled ce lls or conditioned media are similar to control rates in the absence o f cells. Protoplast integrity is needed to reach optimal values of sta bilization. Chloroquine, a known inhibitor of plasma membrane redox sy stems, inhibits the ascorbate stabilization, the inhibition being part ially reversed by coenzyme Q(6). Chloroquine does not inhibit ferricya nide reduction. Growth of yeast in iron-deficient media to increase fe rric ion reductase activity also increases the stabilization. In concl usion, extracellular ascorbate stabilization by yeast cells can reflec t a coenzyme Q dependent transplasmalemma electron transfer which uses NADH as electron donor. Iron deficiency increases the ascorbate stabi lization but the transmembrane ferricyanide reduction system can act i ndependently of ascorbate stabilization.