Evolution of antioxidant mechanisms: Thiol-Dependent peroxidases and thioltransferase among procaryotes |
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Authors: | Alfred R Sundquist Robert C Fahey |
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Institution: | (1) Department of Chemistry, University of California-San Diego, 92093-0506 La Jolla, California, USA |
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Abstract: | Summary Glutathione peroxidase and glutathione S-transferase both utilize glutathione (GSH) to destroy organic hydroperoxides, and
these enzymes are thought to serve an antioxidant function in mammalian cells by catalyzing the destruction of lipid hydroperoxides.
Only two groups of procaryotes, the purple bacteria and the cyanobacteria, produce GSH, and we show in the present work that
representatives from these two groups (Escherichia coli, Beneckea alginolytica, Rhodospirillum rubrum, Chromatium vinosum, andAnabaena sp. strain 7119) lack significant glutathione peroxidase and glutathione S-transferase activities. This finding, coupled
with the general absence of polyunsaturated fatty acids in procaryotes, suggests that GSH-dependent peroxidases evolved in
eucaryotes in response to the need to protect against polyunsaturated fatty acid oxidation. A second antioxidant function
of GSH is mediated by glutathione thiol-transferase, which catalyzes the reduction of various cellular disulfides by GSH.
Two of the five GSH-producing bacteria studied (E. coli andB. alginolytica) produced higher levels of glutathione thiol-transferase than found in rat liver, whereas the activity was absent in the
other three species studied. The halobacteria produced γ-glutamylcysteine rather than GSH, and assays for γ-glutamylcysteine-dependent
enzymes demonstrated an absence of peroxidase and S-transferase activities but the presence of significant thioltransferase
activity. Based upon these results it appears that GSH and γ-glutamylcysteine do not function in bactera as antioxidants directed
against organic hydroperoxides but do play a significant, although not universal, role in main-taining disulfides in a reduced
state. The function of GSH in the photosynthetic bacteria, aside from providing a form of cysteine resistant toward autoxidation,
remains a puzzle, as none of the GSH-dependent enzymes tested other than glutathione reductase were present in these organisms. |
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Keywords: | Glutathione peroxidase Glutathione S-transferase Thioltransferase Glutathione reductase Glutathione Bis-γ -glutamylcystine reductase γ -glytamylcysteine Procaryotes |
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