Acetoacetic Decarboxylase

Acetoacetic decarboxylase (acetoacetate carboxy lyase, EC 4.1.1.4), as extracted from acetone-powdered Clostridium acetobutylicum, contains some enzyme which is expressed and an equal amount which is latent and which may be measured only after its exposure by heating. Activation of the latent decarboxylase is an effectively irreversible, time- and temperature-dependent process which is kinetically first order. ΔHa for this activation is 25 kcal per mole. The expressed decarboxylase can be irreversibly inhibited by treatment with borohydride plus acetoacetate or 2-oxopropane sulfonate, without affecting the latent decarboxylase. The latent decarboxylase and the expressed decarboxylase sediment identically in a sucrose density gradient and are not separated by the procedures employed in the purification of the enzyme. The initially expressed decarboxylase and that which is exposed by heating, exhibit the same Km for acetoacetate and Ki for 2-oxopropane sulfonate. Monovalent anions protect against the irreversible inhibition of the enzyme by substrate plus BH4- but have no effect on the activation of the latent decarboxylase or on the thermal stability of the enzyme. Acetylacetone is similarly without effect on the process of heat activation but strikingly enhances the heat stability of the decarboxylase.