L-Phenylalanine, an essential amino acid, is a precursor of the amino acid tyrosine and to a whole series of neurotransmitters and hormones. Phenylalanine is first converted to tyrosine, which becomes L-dopa, which then converts to norepinephrine, a key neurotransmitter and adrenal hormone. L-Phenylalanine is also a direct precursor to PEA (Phenylethylamine), another important neurotransmitter.