NOS3 and diabetes mellitus: The guanosine triphosphate cyclohydrolase I (GTPCH I)/tetrahydrobiopterin (BH4) pathway was shown to potentially regulate NO production and impair EPC mobilization and function by endothelial nitric oxide synthase (eNOS) uncoupling in diabetes [35], and it was shown to be involved in declined EPC function in postmenopausal women with overweight [36].