A re-examination of the electron microscopic appearance of pyruvate carboxylase from chicken liver.

Electron microscopic studies of chicken liver pyruvate carboxylase conducted under a variety of conditions show that this enzyme has an overall rhombic appearance and is comprised of four nonspherical subunits. The square planar tetramers originally identified by Valentine et al. (Valentine, R.C., Wrigley, N.G., Scrutton, M.C., Irias, J.J., and Utter, M.F. (1966) Biochemistry 5, 3111-3116) as pyruvate carboxylase have been shown to represent a minor protein contaminant found in many impure preparations of this enzyme. The contaminating protein has been separated from pyruvate carboxylase and further purified. It does not contain biotin and its constituent polypeptides are smaller than those of pyruvate carboxylase. This protein, whose function is not yet identified, shows a strong tendency to aggregate and is highly visible under many conditions of electron microscopy. Several lines of evidence support the thesis that the nonsquare tetramers are pyruvate carboxylase. When essentially homogeneous material is examined with a variety of different negative stains, numbers of intact molecules represent 20 to 70% of the visible protein. These tetramers, like pyruvate carboxylase, are very cold-labile and are protected from dissociation under these conditions by acetyl-CoA, a specific activator of this enzyme. Also, the structures form complexes with avidin and antibiotin antibody and thus, like pyruvate carboxylase, contain biotin.