The different effects of N-ethylmaleimide and iodoacetamide on the activity of rat liver 60S subunits for peptide bond elongation.

The activity of 60S subunits of rat liver ribosomes in poly(U)-dependent polyphenylalanine synthesis was inhibited by incubation with N-ethylmaleimide. However, when 60S subunits were incubated with iodoacetamide, their activity decreased only slightly. Furthermore, iodoacetamide-pretreated 60S subunits became insensitive to N-ethylmaleimide. Similar results were obtained for the activity of EF-2-dependent GTPase of 60S subunits. As a whole, the labeling patterns of ribosomal proteins on two-dimensional gel electrophoresis were similar for 60S subunits labeled with both 14C-labeled sulfhydryl reagents, although the extent of labeling of some proteins was somewhat different. These results indicate that the SH groups in the 60S subunits are not directly involved in the activities of the subunits described above.