Fumarate is the cause of the apparent ping-pong kinetics of dopamine beta-hydroxylase.

The kinetic mechanism of dopamine beta-hydroxylase (dopamine beta-monooxygenase EC 1.14.17.1) was studied either in the absence or the presence of the nonessential activator fumarate. In the absence of fumarate, intersecting initial velocity patterns were obtained, consistent with a sequential mechanism. In the presence of saturating concentrations of fumarate, initial velocity patterns became parallel. Other activating anions, such as acetate and chloride, could replicate the effects of fumarate. Since previous initial rate studies of dopamine beta-hydroxylase have been performed in the presence of saturating concentrations of fumarate, the present results may explain why parallel initial velocity patterns, apparently consistent with a ping-pong mechanism, have been so far observed. As a plausible mechanism of the anion effect it is proposed that activating anions induce saturation of the enzyme with oxygen.