Thioredoxin system is a major player in glutathione metabolism, due to the demonstrated absence of a glutathione reductase. Functionally interacts with the Sod/Cat reactive oxidation species (ROS) defense system and thereby has a role in preadult development and life span. Lack of a glutathione reductase suggests antioxidant defense in Drosophila, and probably in related insects, differs fundamentally from that in other organisms. [thioredoxin]-dithiol + NADP(+) = [thioredoxin]-disulfide + H(+) + NADPH Binds 1 FAD per subunit. Measurements were conducted with isoform A unless noted otherwise. Optimum pH is 7.6 for isoform A with Trx-2 and NADPH as substrates. Homodimer. During embryogenesis, expression is seen in germ cell progenitors, developing midgut, hindgut and proventriculus. Expressed both maternally and zygotically during all stages of development, highest expression during adult stages. The active site is a redox-active disulfide bond. Can partially substitute for the cytoplasmic enzyme activity. Unable to compensate for the loss of the mitochondrial enzyme activity. Produced by alternative initiation at Met-106 of isoform B. Belongs to the class-I pyridine nucleotide-disulfide oxidoreductase family. Was originally thought to be a glutathione reductase. Chimeric cDNA. Chimeric cDNA originating from chromosomes X and 3.