The kinetic properties of human placental choline acetyltransferase.

1. Michaelis constants for human placental choline acetyltransferase were shown to be dependent on the concentration of the second substrate present. The primary plots indicate a sequential rather than a Ping Pong mechanism and are of the same type with 300mm- and 500mm-sodium chloride. 2. Similar results have been obtained with rabbit brain choline acetyltransferase. 3. Product inhibition of the forward reaction has been studied. CoA inhibits competitively with respect to acetyl-CoA and non-competitively with respect to choline. Acetylcholine inhibits competitively with respect to choline and non-competitively with respect to acetyl-CoA. No inhibition is given by acetylcholine when the enzyme is saturated with choline. 4. It is concluded that human placental choline acetyltransferase has an ordered mechanism of the Theorell-Chance type.