ISOTOPE STUDIES ON MECHANISM OF ACTION OF ADRENAL TYROSINE HYDROXYLASE

The position of 18O in the dopa formed from either tyrosine-4-18O and 16O2 or from nonisotopie tyrosine and 18O2 by tyrosine hydroxylase has been determined. The methodology included the enzymatic conversion of dopa to 3-methoxytyramine and the periodate oxidation of this material to the quinone and methanol. The 18O content of the methanol was determined by mass spectrometry. Based on this assay, the dopa formed from tyrosine-4-18O contained only small amounts (~10%) of 18O in the 3-position while dopa formed from nonisotopic tyrosine and 18O2 contained approximately 90% of the label in the 3-position. Hydroxylation of phenylalanine-4-3H with tyrosine hydroxylase resulted in migration of the tritium from the para-position since the final product, dopa, contained 42% of the original radioactivity. The mechanistic significance of these findings is discussed. © 1968.