The Intracellular Location of Pyruvate Carboxylase in Rat Liver

Abstract

Because of reported extramitochondrial location of pyruvate carboxylase in rat liver concluded from extraction studies and the importance of these findings for the concept of the regulation of gluconeogenesis, the intracellular location of this enzyme in rat liver was reinvestigated. The following results were obtained. Reported differences in extraction properties between pyruvate carboxylase and glutamate dehydrogenase in rat liver are due to secondary binding of the latter enzyme to nuclei, cell debris, mitochondria and microsomes. Rat‐liver pyruvate carboxylase is located exclusively in mitochondria, a location in another subcellular component is excluded. Rat liver mitochondria are heterogeneous with respect to their enzyme content. Rate‐zonal centrifugation reveals asymmetric distribution of mitochondrial matrix enzymes: Ornithine aminotransferase is shifted towards slower‐sedimenting mitochondria, pyruvate carboxylase, malate dehydrogenase and aspartate aminotransferase in faster‐sedimenting mitochondria. Glutamate dehydrogenase as well as monoamine oxidase (outer membrane) and adenylate kinase (inter‐membrane space) are located in an intermediate position. Electronmicrographs show that the mitochondria retain their intact outer membrane and most of them are in condensed conformation. The biological significance of the observed heterogeneities with respect to the regulation of gluconeogenesis is discussed.