Butyryl-CoA:acetoacetate CoA-transferase from a lysine-fermenting Clostridium

on the assay method, and is considerably higher than that of other CoA transferases. The molecular weight of the enzyme is between 90,000 and 108,000; it contains two pairs of subunits having molecular weights of about 23,000 and 25,000. The enzyme is active from pH 4 to above pH 9 and has a broad pH activity optimum from about pH 6.5 to 8.0. Butyrate, acetoacetate, and their CoA thioesters are the preferred substrates but the salts of a number of C,-C,; monocarboxylic acids are also utilized at lower rates. Apparent K, and relative V,,, values are reported. When acetoacetate or acetoacetyl-CoA serves as one substrate, the reaction rate is strongly stimulated by lower concentrations of monovalent or divalent cations and, after an optimal concentration is reached, progressively inhibited by higher concentrations. The activity is insensitive to the nature of the accompanying anion. The optimal concentration of divalent cations falls in the range of 1.2 to 12 mM and that of monovalent cations from 80 to 250 mM. Since rate stimulation by cations does not occur with substrates other than acetoacetate or acetoacetyl-CoA, it is probable that cations are required to facilitate the enolization of acetoacetyl at some stage of the reaction. The role of