The effect of sulfhydryl reagents on the activity and stability of yeast ribosomes

By titration with N-ethylmaleimide (MalNet) and p-chloromercuribenzoate (pCMB) of native yeast ribosomes, 40-43 SH groups were found, which represent about one third of the total SH groups present in these ribosomes. The reaction with pCMB proceeded fast, and that with MalNet was slow. The SH-blocking reaction led to inhibition of the ribosome activity. The activity of the MalNet-treated ribosomes in the polymerization reaction decreased concomitantly with the decrease of their activity in the enzymic and non-enzymic binding of phenylalanyl-tRNA and N-acetylphenylalanyl-tRNA. pCMB completely inactivated the ribosomes which seems to indicate that this reagent modified some SH groups which were not accessible to MalNet. The ribosome stability was affected by the blocking of the SH groups; in the buffered 0.5 M-KCl solution a significant portion of protein was detached from the modified ribosomes. Thus it appears that the SH groups of ribosomal proteins play some role in maintaining the ribosome structure.