Kinetic parameters for dimeric and tetrameric forms of bovine dopamine beta-monooxygenase and their relationship to non-Michaelis-Menten behavior.

Bovine dopamine beta-monooxygenase has been assayed over a 10,000-fold range in protein concentration, to approximate conditions where the enzyme was shown to be a dimer or tetramer. Michaelis-Menten kinetics are observed with k(cat) and k(cat)/Km for dissociated enzyme reduced 30% and 200-300% relative to tetramer. Addition of chloride ions to very dilute enzyme or the use of intermediate enzyme concentrations causes non-Michaelis-Menten behavior, attributed to an equilibration between dimer and tetramer. This is not expected to contribute to activity within the chromaffin vesicle, where enzyme and chloride ions are at high levels.