Differentiation of Six Distinct Thioredoxins in Seeds of the Soybean

Abstract

The thioredoxin profile of non-photosynthetic plant tissues, in particular of seeds, is much less known than that of green leaves. We have systematically fractionated and assayed the low molecular weight, heat-stable proteins present in extracts from soybean seeds by a variety of chromatographic and enzymatic methods. A multiplicity of six thioredoxins has been established, closely comparable to the pattern previously observed in soybean leaves (Häberlein,1991). Purification to homogeneity of the six proteins was achieved by FPLC. Thioredoxin activities in vitro towards soybean chloroplast NADP-MDH, fructosebisphosphatase, NADPH thioredoxin reductase, and E. coli ribonucleotide reductase have been measured. Some of the seed thioredoxins exhibit higher molecular weights, stronger binding to anion exchange columns, and much lower specificity in enzyme assays than their counterparts isolated from leaves, suggesting the presence of unspecialized storage forms in the dry seed.