Overproduction of nitric oxide (NO) by inducible nitric oxide synthase (iNOS) has been implicated in the pathogenesis of many disorders. iNOS is notably distinguished from constitutive NOSs by its production of large amounts of NO for a prolonged period; hence, it was termed the high-output NOS. Understanding how cells regulate iNOS is a prerequisite for strategies aimed at modulating NO synthesis. iNOS is thought to be regulated primarily at the transcriptional level in response to cytokines and inflammatory mediators. In this study, we report a posttranslational regulatory mechanism for control of iNOS expression through a rapid cellular rate of turnover. Unexpectedly, iNOS cellular half-life was found to be relatively short. In primary bronchial epithelial cells, iNOS half-life was 1.6 ؎ 0.3 h. A similar half-life was found for iNOS in several cell lines. This fast rate of turnover is in sharp contrast to that reported for the constitutive NOS isoforms. iNOS half-life was not affected by intracellular depletion of tetrahydrobiopterin, a critical cofactor required for iNOS activity. Further, iNOS monomers and dimers had a similar half-life. Importantly, we discovered a previously unrecognized cotranslational down-regulation mechanism by which the newly discovered pyrimidineimidazole-based allosteric dimerization inhibitors of iNOS lead to reduced iNOS expression. This study provides insights into the cellular posttranslational mechanisms of iNOS and has important implications for design of selective iNOS inhibitors and their use in therapeutic strategies.degradation ͉ proteasome ͉ half-life N itric oxide (NO) is an important signaling and cytotoxic molecule that is synthesized from L-arginine by isoforms of nitric oxide synthase (NOS) (1-3). As a signaling molecule, NO is produced by two constitutive calcium (Ca 2ϩ )-dependent isoforms, neuronal NOS and endothelial NOS (or NOSI and NOSIII, respectively). Ca 2ϩ -activated calmodulin binds to and transiently activates constitutive NOS dimers (3). Because of the transient nature of elevated Ca 2ϩ levels, these isoforms produce small amounts of NO for short periods of time. As an agent of inflammation and cell-mediated immunity, NO is produced by a Ca 2ϩ -independent cytokine-inducible NOS (iNOS or NOSII) that is widely expressed in diverse cell types under transcriptional regulation by inflammatory mediators (2, 4). Calmodulin is tightly bound to iNOS even at basal Ca 2ϩ levels; therefore, iNOS is notably distinguished from the constitutive isoforms by its prolonged production of a relatively large amount of NO (5). iNOS has been implicated in the pathogenesis of many diseases including Alzheimer's disease, tuberculosis, asthma, transplant rejection, stroke, glaucoma, inflammatory bowel disease, arthritis, and septic shock (6, 7). Such wide implication has produced a corresponding intense interest in understanding the regulation of NO synthesis by iNOS with the goal of developing therapeutic strategies aimed at selective modulation of iNOS activity (8). Understan...