GCH1 and atherosclerosis: BH4 deficiency in atherosclerosis could be caused by at least two different mechanisms, i.e., by (i) BH4 oxidation as a consequence of increased ROS levels (cf. Figure 3), and (ii) impaired BH4 synthesis due to decreased levels/activity of the GTP cyclohydrolase 1 (Gch1), which is the rate-limiting enzyme in tetrahydrobiopterin (BH4) biosynthesis (36, 37).