Tetrahydrobiopterin generate BH4 or BH3 radical and BH2, respectively. Research shows that ascorbic acid (also known as ascorbate or vitamin C) can reduce BH3 radical into BH4, preventing the BH3 radical from reacting with other free radicals (superoxide and peroxynitrite specifically). Without this recycling process, uncoupling of the endothelial nitric oxide synthase (eNOS) enzyme and reduced bioavailability of the vasodilator nitric oxide occur, creating a form of endothelial dysfunction. Ascorbic acid is oxidized to dehydroascorbic acid during this process, although it can be recycled back to ascorbic acid. Other than PKU studies, tetrahydrobiopterin has participated in clinical trials studying other approaches
Chemistry of ascorbic acid the presence of free metal ions can not only promote but also initiate free radical reactions, thus making it a potentially dangerous pro-oxidative compound in certain metabolic contexts. Ascorbic acid and its sodium, potassium, and calcium salts are commonly used as antioxidant food additives. These compounds are water-soluble and, thus, cannot protect fats from oxidation: For this purpose, the fat-soluble esters of ascorbic acid with long-chain fatty acids (ascorbyl palmitate or ascorbyl stearate) can be used as food antioxidants. It creates volatile compounds when mixed with glucose and amino acids in 90 °C. It is a cofactor in tyrosine oxidation.