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

Tyramine β-monooxygenase (TβM) catalyzes the synthesis of the neurotransmitter, octopamine, in insects. Kinetic and isotope effect studies have been carried out to determine the kinetic mechanism of TβM for comparison with the homologous mammalian enzymes, dopamine β-monooxygenase and peptidylglycine α-hydroxylating monooxygenase. A new and distinctive feature of TβM is very strong substrate inhibition that is dependent on the level of the co-substrate,O₂, 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 TβM, with substrate inhibition ameliorated at very high ascorbate levels. The rate of ascorbate reduction of the E-Cu(II) form of TβM 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.