Plant seeds contain several thioredoxins of regular size

Thioredoxin systems composed of several thioredoxin isoproteins and a NADPH: thioredoxin reductase are contained in the albumin-globulin fraction of wheat and soy-bean seed proteins. Two wheat thioredoxins I and II were separated on CM-cellulose whereas soy-bean extracts could be resolved into three thioredoxins I, II, and III on DEAE-cellulose. These proteins were purified to apparent homogeneity and were shown by sodium dodecylsulfate/polyacrylamide gel electrophoresis to possess the molecular weight Mr identical to 12000 typical of the single bacterial and animal thioredoxin. In contrast, gel filtration runs may yield erroneous estimates of thioredoxin molecular weights. The seed thioredoxins can serve as ribonucleotide reductase (Escherichia coli) substrates. They stimulate spinach NADP: malate dehydrogenase but are inactive towards chloroplast fructose-bisphosphatase. These results demonstrate that the number of thioredoxins in nongreen plant tissues approaches that of leaves; additional explanations must therefore be sought for the multiple thioredoxin profiles of plants besides diversification for light-dependent and light-independent functions.