The Sensitivity of Rat Liver and Yeast Mitochondrial Ribosomes to Inhibitors of Protein Synthesis

Abstract

Amino acid incorporation by isolated intact rat liver mitochondria was severely inhibited by chloramphenicol, carbomycin, or sparsomycin, partially inhibited by emetine, and unaffected by erythromycin or lincomycin. By contrast, amino acid incorporation by inner membrane-matrix fractions prepared from rat liver mitochondria by digitonin was strongly inhibited by emetine, erythromycin, and lincomycin. These results suggest that the mitochondrial membrane acts as a permeability barrier to these drugs thus preventing their inhibitory effects on protein synthesis in intact mitochondria. Ribosomes with a sedimentation coefficient of 55 S were isolated from rat liver mitochondria. When incubated with supernatant factors from Escherichia coli, these ribosomes catalyze poly(U)-dependent polyphenylalanine synthesis at rates comparable to those obtained with ribosomes isolated from yeast mitochondria. Protein synthesis on ribosomes isolated from both rat liver and yeast mitochondria was inhibited to the same extent by carbomycin, chloramphenicol and erythromycin over a wide concentration range of drug. Hence, mitoribosomes from yeast or rat liver do not differ in their sensitivity to these inhibitors of protein synthesis. The binding of radioactive chloramphenicol to yeast mitoribosomes and E. coli ribosomes was blocked by erythromycin, carbomycin, or lincomycin. Carbomycin and lincomycin partially prevented the binding of radioactive chloramphenicol to rat liver mitochondrial ribosomes, while erythromycin only inhibited 10 to 20 % of the binding. These results suggest that significant differences may exist in the binding sites for these antibiotics on rat liver and yeast mitoribosomes.