Amino acid incorporation into the protein of mitochondria and mitochondrial fragments from beef heart

Mitochondrial protein synthesis: figuring the fundamentals, complexities and complications, of mammalian mitochondrial translation

Mitochondrial protein synthesis is essential for all mammals, being responsible for providing key components of the oxidative phosphorylation complexes. Although only thirteen different polypeptides are made, the molecular details of this deceptively simple process remain incomplete. Central to this process is a non-canonical ribosome, the mitoribosome, which has evolved to address its unique mandate. In this review, we integrate the current understanding of the molecular aspects of mitochondrial translation with recent advances in structural biology. We identify numerous key questions that we will need to answer if we are to increase our knowledge of the molecular mechanisms underlying mitochondrial protein synthesis.

THE ISOLATION AND PROPERTIES OF CARDIAC RIBOSOMES AND POLYSOMES

1. A method is described by which good yields of ribosomes and polysomes free of contamination by submitochondrial fragments can be prepared from rat cardiac muscle. These preparations are capable of incorporation of amino acids into protein in vitro. 2. The ribosome preparation consists of 32% of monomeric ribosomes and 68% of ribosomal aggregates or polysomes. The polysome preparation has a decreased monomeric content. Dimers, trimers, tetramers, pentamers and larger components can be differentiated. 3. The polysome aggregate structure is degraded to monomeric ribosomes on incubation with small amounts of ribonuclease or by preparation in the absence of Mg(2+) ions. The degradation in the absence of Mg(2+) ions was not reversible and drastically decreased the incorporation of amino acids in vitro. 4. The cardiac ribosomes contained two major RNA species sedimenting at 19s and 28s in a 1:2.4 ratio. 5. The RNA/protein ratio of cardiac ribosomes and polysomes was consistently lower than that of similar preparations from liver. The concentrations of Na(+) and K(+) ions present during preparation had a great effect on the RNA/protein ratio. 6. Optimum conditions for the incorporation of amino acids into protein in vitro are reported. Cardiac ribosomes have a lower rate of incorporation of amino acids in vitro than liver ribosomes. 7. Heart cell sap is less active than liver cell sap: evidence is presented that a factor, present in liver cell sap and concerned with stimulating the synthesis of the peptide chain, is lacking in heart cell sap. 8. Pulse-labelling of perfused hearts followed by examination of the subcellular structures showed that the ribosomal fraction was the most active in the incorporation of amino acids in vitro.

Mitochondrial lesions in reversible erythropoietic depression due to chloramphenicol

The mechanism underlying the reversible depression of erythropoiesis by chloramphenicol has been investigated in rabbits in which hemolytic anemia had been induced by phenylhydrazine so that the compensatory erythroid hyperplasia would provide a situation where abnormalities in the bone marrow cells reflected predominantly those of erythroid precursors. Maintenance of chloramphenicol in the serum of these animals at concentrations in the order of 15 mug/ml resulted in erythropoietic depression after several days. The onset of this depression corresponded to the development of a cellular respiratory defect in the erythroid precursors which was associated with an abnormality in the composition of the mitochondrial respiratory pathway. The abnormality took the form of a selective depletion of cytochromes a + a(3) and b which can be explained by an inhibitory effect of the antibiotic on their formation by the mitochondrial protein-synthesizing system. The relationship between the mitochondrial lesion and the depression of proliferative activity was further indicated by the correlation between the restoration of the cytochrome deficit and the recovery of erythropoiesis after chloramphenicol administration was ceased. The features of the reversible depression of erythropoiesis corresponded closely to those in man, so that a specific action of chloramphenicol on mitochondrial formation provides a reasonable explanation for this important manifestation of chloramphenicol toxicity.