Tetrahydrobiopterin (BH4) is a cofactor for aromatic amino acid hydroxylases and an essential cofactor for nitric oxide synthase (NOS), which is used in the study of endothelial dysfunction such as hypertension, hypercholesterolemia, and diabetes.

3-Hydroxyanthranilic acid also known as 2-amino-3-hydroxy-benzoate or 3-ohaa belongs to the class of organic compounds known as hydroxybenzoic acid derivatives. 3-Hydroxyanthranilic acid has been found in human epidermis and bladder tissues and has also been detected in multiple biofluids such as urine and blood.

Glutaric acid is a simple five-carbon linear dicarboxylic acid. Glutaric acid is naturally produced in the body during the metabolism of some amino acids including lysine and tryptophan. Glutaric acid may cause irritation to the skin and eyes. When present in sufficiently high levels glutaric acid can act as an acidogen and a metabotoxin.