Rat Liver Adenosine Triphosphate:Adenosine Monophosphate Phosphotransferase Activity: II. SUBCELLULAR LOCALIZATION OF ADENYLATE KINASE ISOZYMES

Abstract

Abstract Adenylate kinase activity (ATP:AMP-phosphotransferase, EC 2.7.4.3) was studied in subcellular extracts of rat liver tissue. Essentially all of the activity, over 150 units per g of tissue, wet weight (1.7 units per mg of protein), was released into the cytosol when the tissue was homogenized in 0.025 m sucrose. Isotonic (0.25 m sucrose) homogenization and separation of subcellular fractions by both differential centrifugation and linear sucrose density gradient centrifugation revealed 8 units per g (0.4 units per mg) in the nuclear fraction, 84 units per g (2 units per mg) in the heavy mitochondrial fraction, 22 units per g (0.6 unit per mg) in the light mitochondrial fraction, and 30 units per g (0.5 unit per mg) in the cytosol fraction. Rat liver adenylate kinase activity was separated into four electrophoretically distinct isozymes. Isozyme I, at 3 units per g of tissue, wet weight, was observed only in the nuclear fractions. Isozyme II was found in the cytosol at 19 units per g. Isozyme III, at 110 units per g, was the predominant adenylate kinase isozyme in the rat liver; it was localized in the outer compartment of the mitochondria. Since adenylate kinase III is easily released from mitochondria, mitochondrial damage can be observed by monitoring this isozyme. Also, because of this release, the true subcellular distribution of rat liver adenylate kinase activity is probably better represented by the corresponding values for each isozyme, not the values obtained by normal subcellular extraction procedures.