DEHYDROASCORBATE AND DEHYDROASCORBATE REDUCTASE ARE PHANTOM INDICATORS OF OXIDATIVE STRESS IN PLANTS

In many physiological studies dehydroascorbate (DHA) reductase is rega rded as one of the chloroplast enzymes involved in the protection agai nst oxidative stress, Here, evidence is presented that plant cells do not possess a specific DHA reductase, The DHA reductase activities mea sured in plant extracts are due to side reactions of proteins containi ng redoxactive dicysteine sites, Native gel electrophoresis combined w ith specific activity staining revealed three different proteins with DHA reductase activity in leaf and chloroplast extracts. These protein s have been identified as thioredoxins and trypsin inhibitors (Kunitz type) by Western blot analysis. The essential regulatory functions of thioredoxins in chloroplast metabolism are strongly inhibited in the p resence of as little as 50 mu M DHA, Thus, the intacellular DHA concen tration should be kept below 50 mu M but not all proteins with DHA red uctase activity are effective enough for this purpose, A specific DHA reductase is frequently demanded as part of the enzymatic equipment to avoid oxidative stress, We argue that this is not necessary because i n chloroplasts DHA does not accumulate to any significant extent due t o the high activities of monodehydroascorbate reductase and of reduced ferredoxin. (C) 1997 Federation of European Biochemical Societies.