Ultracentrifugal studies on the dissociation of hepatic ribosomes

Abstract

Ribosomes extracted from guinea pig liver and purified were dissociated either by dialysis against low Mg2+ or Mg2+ -free buffer, or by the addition of chelating agents. In both types of dissociation, the initial products produced in the presence of 0·05 M -KCl were always a major 50 sandaminor 60 s component. The addition of a chelating agent resulted in the appearance of a new 32 s component, the disappearance of the 60 s component, and the gradual and continuous transformation of the 50 s peak to a 47 s peak. In the final stage, the mass ratio of the 47 s to the 32 s component was always 2·0 : 1. From these results and from molecular weight determinations of the ribosomes and their dissociation products reported in the following paper, it is concluded that hepatic ribosomes are composed of one large and one small subunit with molecular weights of two-thirds and one-third of the monomer respectively. The 60 s component is thought to be a mixture of the large subunit and dirners of the small subunit, while the main constituent of the 60 s component may be a large subunit having a more compact configuration than the bulk of such subunits. The sedimentation coefficients of the large and the small subunits were found to depend on the concentration of the chelating agent used, as well as on the ionic strength of the buffer. These effects might result from the swelling of ribosomal particles through removal of Mg2+. The small subunit was much more unstable than the large one. In the presence of BDTA, the former degraded gradually into smaller heterogeneous materials even in the cold; pancreatic RNase selectiely degraded the small subunit. However, the dimerization of the small subunit, as in the 50 s + 60 s mixture, markedly increased its stability.