Phenylalanine:tyrosine ratio detection

Since L-tyrosine is the main precursor of melanins and a substrate of tyrosinase, the L-DOPA/L-tyrosine ratio could more accurately reflect tyrosinase activity than l-DOPA alone. In 1997, we developed two reversed-phase HPLC techniques, one with electrochemical detection to measure simultaneously l-DOPA, norepinephrine (NE), epinephrine (E), dopamine (DA), and DOPAC (3,4-dihydroxyphenyl acetic acid) (all compounds easily oxidizable between +0.15 and +0.50 V), and one with fluorimetric detection to measure L-tyrosine (and phenylalanine) on the same blood sample. 

The aromatic amino acids also have fluorescence emissions when excited by light in the UV range. Table Bl.3.3 gives the excitation wavelength, fluorescence emission wavelength, and quantum yield (Q) for tryptophan, tyrosine, and phenylalanine. The quantum yield is the ratio of photons emitted to photons absorbed. Typically, phenylalanine fluorescence is not detected in the presence of tyrosine and tryptophan due to low Q. Furthermore, tyrosine fluorescence is nearly completely quenched if the tyrosine residue is ionized or near an amino group, a carboxyl group, or a tryptophan residue (Teale, 1960 Freifelder, 1982). Therefore, tryptophan fluorescence is what is customarily measured. 

---