Molecular and catalytic properties of mitochondrial (ketogenic) 3-hydroxy-3-methylglutaryl coenzyme A synthase of liver.

Mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase has been purified to homogeneity from avian liver. The enzyme in dilute phosphate buffer, pH 7.0, has an S20,w of 5.7 S and a molecular weight of 105,000 determined by sedimentation equilibrium; the presence of 0.1 M KCl causes dissociation to a form one-half that size, i.e. about 57,000 daltons. Since the subunit molecular weight of the synthase determined by the dodecyl sulfate acrylamide gel method is 53,000, it appears that the native enzyme is a dimer composed of weight-homogeneous subunits. A number of molecular and catalytic properties allow the mitochondrial and cytoplasmic 3-hydroxy-3-methylglutaryl-CoA synthases to be distinguished. The pI of the homogeneous mitochondrial enzyme is 7.2. This value, while identical to that of the single isoelectric-focusing species of broken mitochondrial preparations, differs from those of the multiple 3-hydroxy-3-methylglutaryl-CoA synthases found in the cytoplasmic fraction which exhibit pI values of 4.8 and 6.7. Rabbit antibodies against the purified mitochondrial synthase are capable of precipitating the mitochondrial, but not the cytoplasmic, synthase(s) of avian liver. Finally, the synthase differ kinetically in their responses to divalent magnesium ion, the mitochondrial enzyme being inhibited and the cytoplasmic enzyme(s) activated. It is proposed that the mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase of liver functions in ketogenesis while its cytoplasmic counterpart participates in cholesterogenesis (Clinken-Beard, K. D., Sugiyama, T., Reed, W. D. and Lane, M.D. (1975) J. Biol. Chem. 250, 3124-3134).