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J Biol Chem, Vol. 273, Issue 14, 8099-8105, April 3, 1998

Coenzyme Q₆ and Iron Reduction Are Responsible for the Extracellular Ascorbate Stabilization at the Plasma Membrane of Saccharomyces cerevisiae

Carlos Santos-Ocaña, Francisco Córdoba^¶, Frederick L. Crane, Catherine F. Clarke^**, and Plácido Navas

From the  Departamento de Biología Celular, Facultad de Ciencias, Universidad de Córdoba, Avenida San Alberto Magno, s/n, 14004 Córdoba, Spain, the ^¶ Departamento de Ciencias Agroforestales, Universidad de Huelva, 21819 Huelva, Spain, the  Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907, and the ^** Department of Chemistry and Biochemistry, Molecular Biology Institute, University of California at Los Angeles, Los Angeles, California 90095

Yeast plasma membrane contains an electron transport system that maintains ascorbate in its reduced form in the apoplast. Reduction of ascorbate free radical by this system is comprised of two activities, one of them dependent on coenzyme Q₆ (CoQ₆). Strains with defects in CoQ₆ synthesis exhibit decreased capacity for ascorbate stabilization compared with wild type or with atp2 or cor1 respiratory-deficient mutant strains. Both CoQ₆ content in plasma membranes and ascorbate stabilization were increased during log phase growth. The addition of exogenous CoQ₆ to whole cells resulted in its incorporation in the plasma membrane, produced levels of CoQ₆ in the coq3 mutant strain that were 2-fold higher than in the wild type, and increased ascorbate stabilization activity in both strains, although it was higher in the coq3 mutant than in wild type. Other antioxidants, such as benzoquinone or -tocopherol, did not change ascorbate stabilization.

The CoQ₆-independent reduction of ascorbate free radical was not due to copper uptake, pH changes or to the presence of CoQ₆ biosynthetic intermediates, but decreased to undetectable levels when coq3 mutant strains were cultured in media supplemented with ferric iron. Plasma membrane CoQ₆ levels were unchanged by either the presence or absence of iron in 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 CoQ₆ is the central electron carrier, whereas the remainder is independent of CoQ₆ and other antioxidants but is dependent on the iron-regulated ferric reductase complex.

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