Determination of K_m for oxygen of nitric oxide synthase isoforms

Biosynthesis of nitric oxide (NO) requires L-arginine and molecular oxygen. Although the apparent K_m values for L-arginine of NO synthase isoforms have been reported, the apparent K_m values for oxygen are unknown. Low oxygen tension has been shown to attenuate NO synthase activity and NO-dependent vascular relaxation. We investigated the effect of different concentrations of oxygen on NO synthase activity of bovine brain, cultured bovine aortic endothelial cells and RAW 264.7 macrophages. The apparent K_m values for oxygen were 23.2 ± 2.8, 7.7 ± 1.6 and 6.3 ± 0.9 μM for the brain, endothelial and macrophage NO synthases, respectively. This suggests that pathophysiological conditions involving a decrease in tissue oxygen concentration may attenuate NO production.