Protein disulphide-isomerase: a homologue of thioredoxin implicated in the biosynthesis of secretory proteins.

and have a tendency to aggregate, indicating that formation of a second disulphide in the molecule is accompanied by denaturation of the structure. Reduction of oxidized thymus thioredoxin can be achieved by dithiothreitol or by NADPH and thioredoxin reductase representing a possible autocatalytic control mechanism. Another major difference between the mammalian and the E. coli thioredoxin system is the properties of thioredoxin reductase. E. coli thioredoxin reductase has a subunit size of 33 000 and the native M , is 66 000 (Thelander, 1968). In contrast, thioredoxin reductase from calf thymus or rat liver (Luthman & Holmgren, 1982) have a subunit size of 58000 and a native M , of 116000. Furthermore, the mammalian thioredoxin reductase has a broader substrate specificity and will in addition to thioredoxin-S, also reduce 5,5’-dithiobis-( 2-nitrobenzoic) acid (DTNB), vitamin K, and alloxan. We have recently found a reactivity of the thioredoxin system with an element of great interest. Thus, sodium selenite is a substrate for calf thymus thioredoxin reductase, leading to massive oxidation of NADPH (S. Kumar & A. Holmgren, unpublished work). The reactivity between thioredoxin reductase and selenite represents a novel interaction between selenium compounds and pyridine nucleotides of great potential interest. During these studies we also observed that thioredoxin rapidly reduced selenite (S. Kumar & A. Holmgren, unpublished work). It has previously been known that selenite shows an interaction with glutathione, forming selenotrisulphides. The reactivity between selenite and the thioredoxin system may be of importance in explaining some of the actions of selenium. Since selenite has been shown previously to react with glutathione, relative contributions and reactivity of the thioredoxin and the glutaredoxin system with selenite is a matter for future experiments.