Molecular weights of hepatic ribosomes and their subunits

Abstract

The molecular weights of undissociated ribosomes from guinea pig liver and their dissociation products have been determined by an equilibrium sedimentation method. The molecular weight of the monomer form of the hepatic ribosomes (77 s component) was found to be 5·0 6 ± 0·2 × 10 6 ; the 50 s + 60 s mixture and the 47 s component were approximately two-thirds the size of this monomer (3·05 ± 0·06 sx 10 6 and 3·2 ± 0·3 × 10 6 respectively). Determination of the molecular weight of the 32 s component was not successful because of extreme instability. Over a certain range of EDTA or pyrophosphate concentrations the hepatic ribosomes dissociate into 47 s and 32 s components in a ratio of 2·0 : 1, implying that the 32 s component is probably one-third the size of the undissociated ribosome monomer. These findings support the hypothesis that the hepatic ribosome monomers are composed of one large and one small subunit, similar to the E. coli ribosomes. It is also suggested that, in the 50 s + 60 s mixture, the small subunit may exist as a dimer, thus overlapping the sedimentation peak of the large subunit.