Physiochemical properties of rat liver mitochondrial ribosomes

In the present study, the physiochemical properties of rat liver mitochondr ial ribosomes were examined and compared with Escherichia coli ribosomes, T he sedimentation and translational diffusion coefficients as well as the mo lecular weight and buoyant density of rat mitochondrial ribosomes were dete rmined. Sedimentation coefficients were established using the time-derivati ve algorithm (Philo, J, S, (2000) Anal, Biochem, 279, 151-163), The sedimen tation coefficients of the intact monosome, large subunit, and small subuni t were 55, 39, and 28 S, respectively. Mitochondrial ribosomes had a partic le composition of 75% protein and 25% RNA. The partial specific volume was 0.688 ml/g, as determined from the protein and RNA composition. The buoyant density of formaldehyde-fixed ribosomes in cesium chloride was 1.41 g/cm(3 ). The molecular masses of mitochondrial and E, coli ribosomes determined b y static lightscattering experiments were 3.57 +/- 0.14 MDa and 2.49 +/- 0. 06 MDa, respectively. The diffusion coefficient obtained from dynamic light scattering measurements was 1.10 +/- 0.01 x 10(-7) cm(2) s(-1) for mitocho ndrial ribosomes and 1.72 +/- 0.03 x 10(-7) cm(2) s(-1) for the 70 S E, col i monosome, The hydration factor determined from these hydrodynamic paramet ers were 4.6 g of water/g of ribosome and 1.3 g/g for mitochondrial and E, coli ribosomes, respectively. A calculated hydration factor of 3.3 g/g for mitochondrial ribosomes was also obtained utilizing a calculated molecular mass and the Svedberg equation. These measurements of solvation suggest tha t ribosomes are highly hydrated structures. They are also in agreement with current models depicting ribosomes as porous structures containing numerou s gaps and tunnels.