Evidence for the existence of several enzyme-specific thioredoxins in plants

Abstract

Thioredoxin, a small molecular weight protein, functioning as hydrogen carrier in DNA synthesis has recently been identified to be an indispensable factor in the light activation of certain regulatory enzymes of plant chloroplasts [l] . In the presence of thioredoxin and ferredoxin-thioredoxin reductase, photochemically-reduced ferredoxin is able to activate enzymeglike NADP malate dehydrogenase [2] and fructose 1,6-bisphosphatase (FBPase) [3] , both enzymes restricted to the chloroplasts [3,4]. In vitro reduced ferredoxin and ferredoxin-thioredoxin reductase can be replaced by the non-physiological sulfhydryl reagent dithiothreitol (DTT), thus eliminating the need for light, but not the need for thioredoxin [3,5,6] . During our efforts to isolate and purify thioredoxin from different plant species (spinach, sorghum and French beans) and plant materials (leaves and roots) we have realized that there are several forms of thioredoxin which are specific for the activation of either NADP malate dehydrogenase or FBPase. Similar findings have recently been reported by [7] . We describe here a simple method for the separation of different forms of thioredoxin which is applicable to various plant materials. In addition our results indicate that the different forms of thioredoxin are enzyme specific. However a given form of thioredoxin isolated from one plant species can activate the respective enzyme isolated from another plant species and vice versa.