Ribosomes from Rat Liver Mitochondria: II. PARTIAL CHARACTERIZATION

Abstract

A method for the isolation of ribosomes from rat liver mitochondria is reported. These ribosomes have a sedimentation coefficient of 55 S and an average diameter of 145 A in negatively stained preparations. Under the same conditions, 78 S rat liver microsomal ribosomes have an average diameter of about 190 A. The possibility that the 55 S ribosomes originate from the dissociation of contaminating microsomal ribosomes was discounted by showing that conditions used for mitochondrial purification and in the isolation and analysis of mitochondrial ribosomes were sufficient to maintain microsomal ribosomes as 78 S monomers and 114 S dimers. When intact mitochondria were pulse labeled with 14C-leucine in vitro, the 55 S particles acquired label whereas, under the same conditions, microsomal ribosomes did not. The 55 S particle probably represents the monomeric form of the mitochondrial ribosome because (a) it is isolated as the major ribosomal component, (b) its sedimentation coefficient is consistent with the size of native ribosome-like particles observed in mitochondria in situ, and (c) the 55 S ribosomes attain a specific radioactivity 50 times that of the total mitochondrial protein when mitochondria are pulse labeled with 14C-leucine for 5 min in vitro.