[en] Recently, antineutrophil cytoplasmic antibody (ANCA) that is one of autoimmune antibodies has been paid attention since the antibody was found in patients with articular rheumatism. Molecular structure of myeloperoxidase (MPO), which has been regarded as the antigen of ANCA was investigated in this study. Peripheral neutrophil was exposed to γ-ray at 10 and 30 Gy followed by addition of cytochalasin B. The extracellular and intracellular activities of MPO were determined to estimate the effects of the radiation. Moreover, MPO released to the culture medium was purified from the crude extract of the medium and investigated by Western blot analysis to confirm the occurrence of molecular cleavage in MPO. The releasing activity of MPO was decreased by γ-ray exposure at a dose ranging from 0.1 to 1.0 Gy and it tended to increase with 10 or 30 Gy. On the other hand, the activity of the released enzyme was increased by exposure at 0.1-3.0 Gy and became the normal level with 10 or 30 Gy. There were no changes in the SDS-PAGE pattern for the proteins from neutrophils. Western blotting revealed that 30 kDa fragment was included in the proteins released from neutrophils exposed to 0.1-0.3 Gy. The evidence that a low dose exposure induced such fragmentation of MPO molecule would be utilized to evaluate the effects of low-dose radiation. These results suggested that the MPO molecule fragmented by radiation exposure is highly reactive with the autoantibody in neutrophils, but correlation between MPO and its ANCA- related autoimmune disease has not yet demonstrated. (M.N.)