D-Tryptophan is used in the liver and kidneys to produce d-kynurenine. It can be used as a reagent for chemical synthesis.

DL-3-Tyrosine has a hypotensive effect on blood pressure and is converted to dopamine in rats. It can also be used as a reagent for chemical synthesis.

Proline and its derivatives are often used as asymmetric catalysts in organic reactions. The CBS reduction and proline catalysed aldol condensation are prominent examples. L-Proline is an osmoprotectant, and therefore is used in many pharmaceutical, biotechnological applications.

Methionine functions as a reagent and with metals in non-biological applications, L-methionine in particular has been used with metals to form photosensitizers and ligands for transition metal catalyzed bond forming reactions. [11C]L-Methionine has also been used in Positron Emission Tomography of brain tumors for diagnostic purposes.

Methionine is an alpha amino acid used in protein synthesis. In human beings methionine is a vital amino acid not synthesized in the human body and must therefore be ingested. Methionine in the human body is used to improve skin elasticity and tone, tissue growth and repair, and keratin systems such as hair and nails. As a detoxifying agent methionine chelates heavy metals, while the sulfur in it protects cells. L-Methionine is an enantiomerically active amino acid used in the body.

N-gamma-Hydroxy-L-arginine is an intermediate in the biosynthesis of nitric oxide from L-arginine. Arginase is inhibited by N-gamma-Hydroxy-L-arginine, as is the cell proliferation of MDA-MB-468 cancer cells.