Abstract
Tyrosine hydroxylase can be measured by release of tritiated water from labeled tyrosine, and the assay method has now been modified to allow recovery of 3H 2O from the reaction mixture in a much more rapid and less tedious manner than previously possible. In the new method, the tyrosine hydroxylase reaction is stopped with sodium carbonate, pH 11.6. At this pH the tritium in 3H 2O, but not other 3H species, is extracted into an organic scintillant containing 25% isoamyl alcohol, toluene, 2,5-diphenyloxazole, and p-bis-[2-(5-phenyloxazolyl)]benzene. The selective extraction occurs by means of exchange of tritium in 3H 2O with the hydroxyl proton of isoamyl alcohol. It is the [ 3H]isoamyl alcohol that is then extracted into the scintillant and quantified by liquid scintillation spectrometry. Although the organic extraction method is somewhat less sensitive than the more frequently used ion-exchange method for isolating the 3H 2O formed in the tyrosine hydroxylase reaction, it is much more rapid, as well as cost effective, since the enzyme reaction, extraction, and counting are carried out within the same vial.
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