The 26S proteasome, a eukaryotic ATP-dependent, dumb-bell shaped, protease complex with a molecular mass of approx 20kDa consists of a central 20S proteasome,functioning as a catalytic machine, and two large V-shaped terminal modules, having possible regulatory roles,composed of multiple subunits of 25- 110 kDa attached to the central portion in opposite orientations. It is responsible for degradation of abnormal intracellular proteins, including oxidatively damaged proteins, and may play a role as a component of a cellular anti-oxidative system. Expression of catalytic core subunits including PSMB5 and peptidase activities of the proteasome were elevated following incubation with 3-methylcholanthrene. The 20S proteasome comprises a cylindrical stack of four rings, two outer rings formed by seven alpha-subunits (alpha1-alpha7) and two inner rings of seven beta-subunits (beta1-beta7). Two outer rings of alpha subunits maintain structure, while the central beta rings contain the proteolytic active core subunits beta1 (PSMB6), beta2 (PSMB7), and beta5 (PSMB5). Expression of PSMB5 can be altered by chemical reactants, such as 3-methylcholanthrene.