INTERACTION OF SELENOPROTEIN-PA AND THE THIOREDOXIN SYSTEM, COMPONENTS OF THE NADPH-DEPENDENT REDUCTION OF GLYCINE IN EUBACTERIUM-ACIDAMINOPHILUM AND CLOSTRIDIUM-LITORALIS

Purification of protein P(A) of the glycine reductase complex from Eubacterium acidaminophilum and Clostridium litoralis was monitored by a new spectrophotometric assay. The procedure depended on a specific two- to threefold stimulation of a dihydrolipoamide dehydrogenase activity that is elicited by the interaction of a thioredoxin reductase-like flavoprotein and thioredoxin from both organisms. Protein P(A) isolated from E. acidaminophilum by Se-75 labeling and monitoring of the dithioerythritol-dependent glycine reductase activity was identical in its biochemical, structural, and immunological properties to the protein isolated by using the stimulation assay. Proteins P(A) from both organisms were glycoproteins of M(r) about 18,500 and exhibited very similar N-terminal amino acid sequences. Depletion of thioredoxin from crude extracts of E. acidaminophilum totally diminished the NADPH-dependent but not the dithioerythritol-dependent glycine reduction. The former activity could be fully restored by adding thioredoxin. Antibodies raised against the thioredoxin reductase-like flavoprotein or thioredoxin inhibited to a high extent NADPH-dependent but not dithioerythritol-dependent glycine reductase activity. These results indicate the involvement of the thioredoxin system in the electron flow from reduced pyridine nucleotides to glycine reductase.