Protein synthesis in mitochondria. Requirements for the incorporation of radioactive amino acids into mitochondrial protein

Abstract

Keller (1951) observed that after the injection of radioactive leucine into the rat the most radioactive protein in the subcellular components of the liver was in the microsome fraction.After this Siekevitz (1952) found that the most efficient method in vitro for obtaining incorporation of radioactive alanine into protein was to incubate a mixture of micro- somes (precipitated at pH 5) and mitochondria in a medium that allowed the mitochondria to carry out oxidative phosphorylation.He suggested that the main function of the mitochondria was to provide energy for the microsomal system and this view was strengthened by the observation of Zamecnik & Keller (1954) that the mitochondria could be replaced by cell sap and an adenosine triphosphate-generating system.Because of low rates of labelling of the mitochondria observed in this early work, the high microsomal contamination in the mitochondrial preparations used, and the high proportion of radioactivity lost from the protein by treatment with ninhydrin (Rutman, Cantarow & Paschkis, 1954) or mercaptoethanol (Peterson & Greenberg, 1952), the mitochondria were ignored for several years as an active site for protein synthesis, and their function was regarded primarily as an energy donor for the microsomal system (see, for example, Brachet, 1958).More recently, however, evidence has appeared that mitochondria are capable of independent protein synthesis.McLean, Cohn, Brandt & Simpson (1958) found that, in contrast with rat liver, skeletal muscle showed equivalent rates of incorporation of radioactive leucine into mito- chondrial and microsomal protein in vivo.Equiva- lent rates of labelling of mitochondria and micro- somes in vivo have also been observed in ascites cells (E.M. Martin, personal communication).The incorporation of radioactive amino acids into the