Tyrosine Metabolism Disorders By Matt Demczko, MD, Mitochondrial Medicine, Children's Hospital of Philadelphia Reviewed / Revised Oct 2021 | Modified Sep 2022 View Patient Education More Information Topic Resources Phenylalanine and Tyrosine... Tyrosine is an amino acid that is a precursor of several neurotransmitters (eg, dopamine, norepinephrine, epinephrine), hormones (eg, thyroxine), and melanin; deficiencies of enzymes involved in its metabolism lead to a variety of syndromes.