Mechanism of the insect enzyme, tyramine β-monooxygenase, reveals differences from the mammalian enzyme, dopamine β-monooxygenase

Tyramine beta-monooxygenase (T beta M) catalyzes the synthesis of the neurotransmitter, octopamine, in insects. Kinetic and isotope effect studies have been carried out to determine the kinetic mechanism of T beta M for comparison with the homologous mammalian enzymes, dopamine beta-monooxygenase and peptidylglycine alpha-hydroxylating monooxygenase. A new and distinctive feature of T beta M is very strong substrate inhibition that is dependent on the level of the co-substrate, O-2, and reductant as well as substrate deuteration. This has led to a model in which tyramine can bind to either the Cu(I) or Cu(II) forms of T beta M, with substrate inhibition ameliorated at very high ascorbate levels. The rate of ascorbate reduction of the E-Cu(II) form of T beta M is also reduced at high tyramine, leading us to propose the existence of a binding site for ascorbate to this class of enzymes. These findings may be relevant to the control of octopamine production in insect cells.