Assay of tyrosine hydroxylase based on high-performance liquid chromatography separation and quantification of L-dopa and L-tyrosine

An assay of L-tyrosine (Tyr) hydroxylating activity operating in lincomycin biosynthesis is described. The assay development consisted of HPLC procedure development, assessing the effect of reaction mixture components on non-enzymatic Dopa and Tyr oxidation, and sample stability evaluation. The HPLC procedure with isocratic elution and fluorescence detection was developed and validated. The method showed a wide linear range of Dopa determination of 0.125-25 micromol/L with lower limit of quantification (LLOQ) of 0.125 micromol/L, RSD of 7.2% and accuracy of 101.7%. The studied linear range of Tyr was 15.625 mmol/L to 500 mmol/L with LLOQ of 15.625 mmol/L, RSD of 1.1%, and accuracy of 98.1%. Recoveries for Dopa and Tyr were 100.66 +/- 0.89% and 94.76 +/- 0.94%, respectively. The inter- and intra-day accuracies and precisions were all within 10%. Samples of the reaction mixture were stable for at least 24 h at room temperature (RT) and 28 days at -20 degrees C. The method was tested for the enzyme activity monitoring in purified as well as crude preparations and enabled micro preparation of the enzyme product during confirmation of its identity. The influence of pH and ascorbic acid content in reaction mixture was studied with respect to non-enzymatic Tyr oxidation.