Mechanistic Studies on Dopamine β-Monooxygenase Catalysis: N-Dealkylation and Mechanism-Based Inhibition by Benzylic-Nitrogen-Containing Compounds. Evidence for a Single-Electron-Transfer Mechanism

Article abstract of DOI:10.1021/ja00247a033

Dopamine β-monooxygenase (DBM) readily catalyzes oxidative N-dealkylation of N-phenylethylenediamine (PEDA) and N-methyl-N-phenylethylenediamine (N-MePEDA) with the reaction characteriscics expected for a monooxygenase-catalyzed process.The products of this reaction have been quantitatively identified as aniline (or N-methylaniline for N-MePEDA) and 2-aminoacetaldehyde, the latter compound being successfully trapped by using NaBH4 reduction followed by N-succinimidyl p-nitriphenylacetate (SNPA) derivatization, and identified by HPLC and mass spectroscopy.In contrast, either analogues of PEDA, i.e. phenyl 2-aminoethyl ether (PAEE) and its p-hydroxy derivative (p-OHPAEE), as well as 2-phenoxycycloprpylamine are not substrates but are competitive inhibitors.Furthermore, 2-methyl-2-anilino-1-aminoethane (β-MePEDA) did not exhibit measurable substrate activity with DBM, in contrast to the excellent substrate activity of the sulfur analogue of β-MePEDA, 2-methyl-2-(phenylthio)-1-aminoethane (β-MePAES).DBM is inactivated during the N-dealkylation reaction in a time- and concentration-dependent manner, a phenomenon that has not, to our knowledge, been observed for any other oxygenase-catalyzed N-dealkylation reaction.Both PEDA and N-MePEDA, as well as β-MePEDA, inactivate DBM under turnover conditions.The inactivation exhibited pseudo-first-order saturable kinetics and expected protection by the DBM substrate, tyramine.No reappearance of enzyme activity was observed after extensive dialysis.Radioactive labeling experiments with ring-tritiated PEDA showed incorporation of nondialyzable radioactivity into DBM in the expected amount, consistent with covalent attachment of a reactive species derivd from PEDA to the DBM active site during enzyme inactivation.Although aniline, N-ethylaniline, N-(2-fluoroethyl)aniline, m- and p-anisidine, p-toluidine, and 5-hydroxyindole were found not to exhibit detectable DBM substrate activity, all of these inactivated the enzyme under turnover conditions.The isotope effect on partition ratio measured for dideuteriated PEDA was found to be a reflection of an isotope effect on V_max and not on k_inact.Our results provide a strong support for the conclusion that the initial nitrogen cation radical species is responsible for enzyme inactivation.Results with ring-deuteriated and ring-tritiated PEDA revealed that the amount of radioactivity incorporated into covalently inactivated DBM by ring-tritiated PEDA is in agreement with that expected for covalent attachment of the para carbon to the protein.An ¹⁸O labeling study was carried out to test for oxygen rebound into the aminoacetaldehyde product, and results demonstrated that the aldehyde oxygen of enzymatically produced 2-aminoacetaldehyde exchanges very rapidly with solvent water, in agreement with literature reports.On the basis ...