COENZYME Q(6) AND IRON REDUCTION ARE RESPONSIBLE FOR THE EXTRACELLULAR ASCORBATE STABILIZATION AT THE PLASMA-MEMBRANE OF SACCHAROMYCES-CEREVISIAE

Yeast plasma membrane contains an electron transport system that maint ains ascorbate in its reduced form in the apoplast, Reduction of ascor bate free radical by this system is comprised of two activities, one o f them dependent on coenzyme Q(6) (CoQ(6)), Strains with defects in Co Q(6) synthesis exhibit decreased capacity for ascorbate stabilization compared with wild type or with atp2 or cor1 respiratory-deficient mut ant strains, Both CoQ(6) content in plasma membranes and ascorbate sta bilization were increased during log phase growth, The addition of exo genous CoQ(6) to whole cells resulted in its incorporation in the plas ma membrane, produced levels of CoQ(6) in the coq3 mutant strain that were 2-fold higher than in the wild type, and increased ascorbate stab ilization activity in both strains, although it was higher in the coq3 mutant than in wild type, Other antioxidants, such as benzoquinone or alpha-tocopherol, did not change ascorbate stabilization. The CoQ(6)- independent reduction of ascorbate free radical was not due to copper uptake, pH changes or to the presence of CoQ(6) biosynthetic intermedi ates, but decreased to undetectable levels when coq3 mutant strains we re cultured in media supplemented with ferric iron, Plasma membrane Co Q(6) levels were unchanged by either the presence or absence of iron i n wild type, atp2, or cor1 strains, Ascorbate stabilization appears to be a function of the yeast plasma membrane, which is partially based on an electron transfer chain in which CoQ(6) is the central electron carrier, whereas the remainder is independent of CoQ(6) and other anti oxidants but is dependent on the iron-regulated ferric reductase compl ex.