L-Tyrosine is a free-form amino acid which rapidly crosses the blood-brain barrier. It is a precursor for the neurotransmitters dopamine, norepinephrine and epinephrine, and to enkephalins, special peptides that subdue messages of physical discomfort. Tyrosine is also a constituent of amino sugars (glycosaminoglycans), special compunds which have important roles throughout the body. L-Tyrosine is also a precursor for the protective skin pigment melanin.