Tyrosine hydroxylase activity in rat brain synaptosomes: direct measurement using high performance liquid...

By use of high performance liquid chromatography with electrochemical detection to measure dopamine production, tyrosine hydroxylase (EC 1.14.16.2) activity has been measured in rat brain synaptosomes from striatum and forebrain. Normal specific activities three- to fivefold higher than previously reported in the literature for radiochemical methods of assay were found. It is suggested that synaptosomes contain a significant amount of endogenous substrate for tyrosine hydroxylase, which causes dilution of the added labelled tyrosine and hence underestimation of the activity of this enzyme when radiochemical methods are used.