O-(Ethenylsulfonyl)-L-tyrosine is a metabolite of L-Tyrosine (Levodopa EP Impurity B) which is one of the 22 proteinogenic amino acids that are used by cells to synthesize proteins. L-Tyrosine is biologically converted from L-phenylalanine and is in turn is converted to L-DOPA and further converted into the neurotransmitters: dopamine, norepinephrine, and epinephrine.