L-phenylalanine is an essential amino acid which can be converted to L-tyrosine by a complex biochemical process which takes place in the liver. an essential amino acid which, once in the body, can also convert to another amino acid, tyrosine, which participates in synthesizing two key neurotransmitters that promote alertness, dopamine and norepinephrine. Through interaction with the central nervous system, it can elevate mood, decrease pain, aid memory and learning, and suppress the appetite.

Phenylalanine reduces cravings for alcohol and has been used to relieve inflammation, low mood, PMS, migraine and other chronic pain. The DL form is a combination of the basic protein building block and that which acts as a painkiller. Should not be taken by pregnant women or persons who suffer from panic attacks, diabetes, high blood pressure, phenylketonuria (PKU). or preexisting pigmented melanoma (a type of skin cancer). L-tyrosine can be converted by neurons in the brain to dopamine and norepinephrine (noradrenaline), hormones which are depleted by stress, overwork and certain drugs. By replenishing norepinephrine in the brain, mental energy levels are enhanced, some forms of depression are alleviated, and a feeling of contentment often occurs. Because of the liver conversion necessary for L-phenylalanine to have these effects, L-tyrosine is often faster acting. In addition, the conversion step from L-tyrosine to norepinephrine may be enhanced if the cofactors (vitamins B6 and C) are included.

L-phenylalanine is a protein amino acid. It is classified as an essential amino acid because the body requires a dietary source of the amino acid to meet its physiological demands. L-phenylalanine is found in proteins of all life forms. Dietary sources of the amino acid are principally derived from animal and vegetable proteins. Vegetables and juices contain small amounts of the free amino acid. The free amino acid is also found in fermented foods such as yogurt and miso. The alternative sweetener aspartame is a dipeptide of L-phenylalanine, as is the methyl ester, and L-aspartic acid.

In addition to being involved in protein synthesis, L-phenylalanine is the precursor of L-tyrosine. The conversion of L-phenylalanine to L-tyrosine is via the enzyme L-phenylalanine hydroxylase. It is this enzyme that is virtually absent in those with the inborn error of metabolism phenylketonuria (PKU). L-tyrosine produced from L-phenylalanine is a precursor in the synthesis of the neurotransmitters norepinephrine and dopamine, among other reactions. L-phenylalanine is marketed as a nutritional supplement and used by some for its putative antidepressant activity.

L-phenylalanine is also known as beta-phenylalanine, alpha-aminohydrocinnamic acid, (S)-2-amino-3- phenylpropanoic acid and alpha-amino-beta-phenylpropionic acid. It is abbreviated as either Phe or by its one-letter abbreviation F. The molecular formula of L-phenylalanine is C9H11NO2, and its molecular weight is 165.19 daltons.

L-Phenylalanine aids in the production of dopamine and epinephrine (which effect transmission of impulses) are the result of this amino acid, suggesting that memory, alertness, and learning may be improved.