Thioredoxin-like activity of thylakoid membranes. Thioredoxin catalyzing the reductive inactivation of glucose-6-phosphate dehydrogenase occurs in both soluble and membrane-bound form

The inactivation of pea leaf chloroplast glucose-6-phosphate dehydrogenase by dithiothreitol can be catalyzed by thioredoxin-like molecules that are present in chloroplasts. This thioredoxin activity occurs predominantly as a soluble species, but washed thylakoid membranes also exhibit some thioredoxin-like activity. The membrane-associated thioredoxin can be extracted by treatment with the detergent Triton X-100. The solubilized thioredoxin appears to have a molecular size similar to that of the soluble thioredoxin which catalyzes the same reaction. The thylakoid-bound activity constitutes only about 5% of the total chloroplast thioredoxin activity. The thioredoxin occurring in the membrane fraction cannot, however, be ascribed to the trapping of stroma since less than 0.1% of three stromal marker enzymes are found in the same thylakoid extract.