SOD1 and systemic sclerosis: The presence of sildenafil alone induced a significant increase in SOD activity in both cell types compared with unstimulated cells (healthy, ctr vs. S: 754.9 ± 41.3 vs. 1521.2 ± 11.7, p < 0.01 and SSc, ctr vs. S: 473.8 ± 6.2 vs. 839.8 ± 24.9, p < 0.05).