The biosynthesis of erythroascorbate in Saccharomyces cerevisiae and its role as an antioxidant.

This study investigated the ability of the yeast Saccharomyces cerevisiae to synthesize ascorbate and its 5-carbon analogue erythroascorbate from a variety of precursors, and their importance as antioxidants in this organism. Studies of ascorbate and analogues in micro-organisms have been reported previously, but their function as antioxidants have been largely ignored. Ascorbate and erythroascorbate concentrations in yeast extracts were measured spectrophotometrically, and their levels and identity were checked using liquid chromatography-electrospray mass spectrometry. The yeast was readily able to synthesize ascorbate from L-galactono-1,4-lactone or erythroascorbate from D-arabinose and D-arabino-1,4-lactone, whereas L-gulono-1,4-lactone was a much poorer substrate for ascorbate biosynthesis. In untreated cells, the concentration of ascorbate-like compounds was below the level of detection of the methods of analysis used in this study (approximately 0.1 mM). Intracellular ascorbate and erythroascorbate were oxidized at high concentrations of tert-butylhydroperoxide, but not hydrogen peroxide. Their synthesis was not increased in response to low levels of stress, however, and preloading with erythroascorbate did not protect glutathione levels during oxidative stress. This study provides new information on the metabolism of ascorbate and erythroascorbate in S. cerevisiae, and suggests that erythroascorbate is of limited importance as an antioxidant in S. cerevisiae.