Nitric oxide (eNOS)

Nitric oxide (NO) is a gaseous signaling molecule produced in vascular endothelium by endothelial NO synthase (eNOS, NOS3), converting L-arginine and O2 to NO and L-citrulline, requiring cofactor tetrahydrobiopterin (BH4). NO diffuses into smooth-muscle cells, activates soluble guanylyl cyclase, raises cyclic GMP, and triggers relaxation — the principal mechanism of flow-mediated vasodilation. eNOS-derived NO also inhibits platelet aggregation, suppresses leukocyte adhesion, and supports progenitor cell migration and angiogenesis. With age, eNOS activity declines: reduced phosphorylation at Ser-1177, increased phosphorylation at Thr-495, lower BH4, and oxidative uncoupling cause eNOS to generate superoxide instead of NO; the SIRT1-eNOS positive-feedback loop is also attenuated (Puca et al. 2012). The nitrate-nitrite-NO pathway offers an eNOS-independent route: dietary inorganic nitrate (concentrated in leafy vegetables and beetroot) is reduced to nitrite by oral commensal bacteria and further to NO under hypoxic, acidic conditions, partially compensating for age-related eNOS decline (Rocha 2021). In a randomized trial, 12 weeks of oral sodium nitrite (80 mg/day) improved flow-mediated dilation in older adults (mean age ~68 years; inclusion range 50–79 years), attributed to suppression of mitochondria-derived oxidative stress rather than direct eNOS upregulation (Rossman et al. 2021). Evidence is largely mechanistic or from small human trials; whether sustained NO restoration extends healthspan in larger randomized cohorts remains open.