L-arginine is a nonessential amino acid that may play an important role in the treatment of cardiovascular disease due to its antiatherogenic, anti-ischemic, antiplatelet, and antithrombotic properties. It has been promoted as a growth stimulant and as a treatment for erectile dysfunction in men. L-arginine is a nonessential amino acid that may play an important role in the treatment of heart disease due to its block arterial plaque buildup, blood clots, platelet clumping, and to increase blood flow through the coronary artery. L-arginine is commonly sold as a health supplement claiming to improve vascular health and treat erectile dysfunction in men. L-arginine, which is promoted as a human growth stimulant, has also been used in bodybuilding. In the 1800s, it was first isolated from animal horn.

Droxidopa (Northera, Chelsea Therapeutics) is a synthetic catecholamino acid precursor of norepinephrine indicated for the treatment of orthostatic dizziness or lightheadedness in adult patients with symptomatic neurogenic orthostatic hypotension (NOH) caused by primary autonomic failure, dopamine beta-hydroxylase deficiency, and non-diabetic autonomic neuropathy. Droxidopa was approved as oral therapy in February 2014 under the FDA’s accelerated approval program. Droxidopa is directly metabolized to norepinephrine by dopadecarboxylase. The specific mechanism of action of the drug is not known completely, but it is supposed to exert the pharmacological effects through norepinephrine and not through the parent molecule or other metabolites. It increases blood flow to the brain by stimulating peripheral arterial and venous vasoconstriction.

L-threonine is an essential amino acid. Threonine is a precursor of glycine. The biochemical studies on rats proved that glycine is synthesized from threonine (through threonine dehydrogenase pathway). Threonine dehydrogenase is the key enzyme in mammals like pigs, cat, and rats for degradation of 80% threonine. In adult humans, degradation of 7–11% of threonine is done by threonine dehydrogenase. The human L-threonine 3-dehydrogenase gene (GeneID: 157739, UniProtKB: Q8IZJ6 (TDH_HUMAN)) is an expressed pseudogene having lost the splice acceptor site preceding exon 6 and codon arginine-214 (CGA) is mutated to a stop codon (TGA). A few trials demonstrated that oral L-threonine may alleviate clinical signs of amyotrophic lateral sclerosis and spasticity in humans. L-Threonine has recently been brought into agricultural industry for balancing the livestock feed.

Leucine is an α-amino acid used in the biosynthesis of proteins. Leucine is an essential hydrophobic amino acid. It is used in the Leucine may be used some people as a supplement to build muscle. Leucine is also found in fish, meat, and poultry. In the pharmaceutical industry, L-leucine is used for parenteral and enteral nutrition and feeding, and is also used as a flavoring product and tablet lubricant in manufacturing. Leucine is an mTOR activator. It is a dietary amino acid with the capacity to directly stimulate muscle protein synthesis. As a dietary supplement, leucine has been found to slow the degradation of muscle tissue by increasing the synthesis of muscle proteins in aged rats. Long-term leucine supplementation does not increase muscle mass or strength in healthy elderly men. Leucine potently activates the mammalian target of rapamycin kinase that regulates cell growth. Infusion of leucine into the rat brain has been shown to decrease food intake and body weight via activation of the mTOR pathway.

Ornithine is an amino acid produced in the urea cycle by the splitting off of urea from arginine. It is a central part of the urea cycle, which allows for the disposal of excess nitrogen. Ornithine is also a precursor of citrulline and arginine. Arginine stimulates the pituitary release of growth hormone. Burns or other injuries affect the state of arginine in tissues throughout the body. As de novo synthesis of arginine during these conditions is usually not sufficient for normal immune function, nor for normal protein synthesis, ornithine may have immunomodulatory and wound-healing activities under these conditions (by virtue of its metabolism to arginine).