NADPH-Dependent Thioredoxin Reductase and a New Thioredoxin from Wheat

An albumin fraction extracted from wheat flour contains thioredoxin reductase (Mr = 65,000) and a heat-stable thioredoxin (Mr = 15,000) which are separated on DEAE cellulose and further purified by gel filtration. W heat thioredoxin stimulates E. coli ribonucleotide reductase but not chloroplast fructose-bis-phosphatase. The enzyme is NADPH-dependent (Km = 3.2 X 10 -6 m) . In presence of the thioredoxin it slowly reduces other proteins like insulin or ribonuclease. Therefore it is most likely identical with a protein disulfide reductase (of unknown specificity) previously described in wheat. This new thioredoxin system is a counterpart of the ferredoxin-dependent system found in photosynthetic plant cells, suggesting different, specific mechanisms for regeneration of reduced thioredoxins in germinating seeds and green plants.