P1114 Aims: Recent studies have shown that nitric oxide (NO) synthases,particularly inducible nitric oxide synthase (i-NOS),are massively induced in acute rejection episodes following heart,liver,pancreas and kidney allotransplantation. Furthermore, tissue and cellular injury in grafts has been demonstrated to be mediated by peroxynitrite anion (ONOO−), a metabollite of NO as well as a potent biological oxident. On the other hand, in chronic rejection of human renal allografts, ONOO− has been implicated in nitration and inactivation of manganese superoxide dismutase (MnSOD), a radical scavenger. However, a detailed relationship between NO, i-NOS, ONOO− and MnSOD in acute rejection episodes remains elusive. Methods: As models of renal transplantation in rats, allografts (Brown-Norway to Lewis), isografts (Lewis to Lewis), and allografts treated with aminoguanidine (AG), an i-NOS inhibitor, at a daily dose of 400 mg/kg of bodyweight,i.p., were used. Blood urea itrogen (BUN), serum creatinine (SCr) and urinary and serum nitrosocompounds (NOx) were measured on days 2,4 and 7 post transplant. Western blot analyses of i-NOS, MnSOD and nitrotyrosine-immunoprecipitated MnSOD protein expression, and measurement of i-NOS and MnSOD activities were performed in grafts harvested on day 7, along with immunohistochemical and histopathological examinations. Results: 1) In the allograft group, both BUN and SCr levels increased markedly on day 7, in parallel with a sharp increase in NOx. A band stained by anti-rat i-NOS antibody was detected at around 130 kDa, along with high levels of i-NOS activity and diffusely distributed i-NOS-positive cells (macrophages). Histologically, acute rejection episode was confirmed (Grade 3 according to Banff classifications). In the AG group, reduced renal function and graft injury were significantly less severe than in the allograft group. 2) In the allograft and isograft groups, a band stained by anti-rat MnSOD antibody was detected with equal intensity at around 23 kDa. In the allograft group, another strongly stained band was detected at around 25 kDa (inactivated tyrosine-nitrated MnSOD protein), along with significantly lower MnSOD activity than in the isograft group (p=0.0012). Increased nitrotyrosine reactivity detected by anti-rat nitrotyrosine antibody, was localized to inside tubular epithelial cells with no glomerular involvement. Conclusions: 1) In the allograft group, markedly increased levels of serum NOx were observed, along with enhanced tissue i-NOS activity, together resulting in graft injury. 2) In the allograft group, MnSOD activity was significantly decreased compared to the isograft group, in parallel with increased inactivated tyrosine-nitrated MnSOD, thus leading to enhanced accumulation of ONOO− products (nitrotyrosine) in tubules accounting for promotion of severe graft injuries. 3) AG administration suppressed the increase in serum NOx levels, with concomitant mitigation of tissue injury and renal function impairement.