GCH1 and cardiovascular disorder: In rats, treatment with human CRP at concentrations achievable in patients with cardiovascular disease impairs endothelium-dependent vasomotor function linked with uncoupling of eNOS due to reduction in dimerization of the enzyme, as well as inhibition of GTP cyclohydrolase I (GTPCH1), the rate-limiting enzyme in BH4 biosynthesis, and decrease in BH4 levels (86).