In the course of our screening program for inhibitors of thioredoxin (TRX) system, a strain of Actinoplanes, A40644, isolated from a soil sample collected at Ohita Prefecture, Japan, was found to produce the first potent inhibitor of TRXsystem from a natural source. The structure of this novel compound, BE-40644, was elucidated as shown in Fig. 1. BE-40644 showed strong inhibitory activity against TRXsystem, while it showed weak inhibitory activity against glutathione reductase. This paper describes the strain, production, isolation, physico-chemical properties, structure elucidation and activity of this new inhibitor of TRXsystem. Assay ofTRX system essentially followed the modified method of Krause et al.1] and was carried out in the reaction mixture of 470/il of 0.1 m Tris-HCl (pH 7.5) containing 2mM EDTA, 15 jA of0.25 mg/ml E. coli TRX (IMCO Co.), 20jA of0.023 mg/ml E. coli TRX reductase (IMCO Co.), 10jal of 10mg/ml bovine insulin (Sigma Chem. Co.) as substrate of TRX and 5jA of dimethyl sulfoxide with or without test sample. Thereaction was started by adding 15/A of 0.2mM NADPHsolution at room temperature and after 30minutes optical density decrease of NADPH consumption was read at 340nm. Assay of human TRXsystem was carried out by the same method described above but the reaction mixture consisted of S0/A of 0.1 m Tris-HCl (pH 7.5) containing 2mMEDTA, 10/A of 75mg/ml human TRX produced by expressing as a fusion protein with glutathione S-transferase in E. coli, 10 jA of0.214mg/ml human TRX reductase (IMCO Co.), 2/A of 10mg/ml bovine insulin (Sigma Chem. Co.) as substrate of TRXand 1^1 of dimethyl sulfoxide with or without test sample and 3 jA of NADPHsolution. To investigate the selectivity of test samples, inhibition against glutathione/glutathione reductase system was carried out. This counter assay was performed by the modified method of Brehe et al.2) in a reaction mixture comprised of 750 jA of 0.1 m Tris-HCl (pH 7.5), 30 fi\ of oxidative dimer of glutathione (GSSG, 30.6 mg/ml), 30 jA of water, 10 /d of yeast GSSG reductase (Sigma Chem. Co., 5IU/ml) and 20^1 of dimethyl sulfoxide with or without test sample. Twominutes after addition of 80^1 of 0.8mg/ml NADPH solution, inhibitory activity was measured in the same way described above in TRXassay system. Characterization of the strain followed the method adopted by the International Streptomyces Project (ISP)3). Strain A40644 had a brownish vegetative mycelium and bottle shaped sporangia on the surface of the agar media which had hair-like structures on their surfaces (Fig. 2). The spores were rod shaped or ellipsoidal and motile by a polar tuft of flagella. Chemotaxonomic analysis of strain A40644 revealed raestf-diaminopimelic acid and glycine as distinguishing components of the cell wall. Xylose and arabinose were the major sugars in the whole-cell hydrolysate. These results indicated that the strain A40644 has a type IID

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