Modulation of protein synthesis in a cell-free system derived from rat brain by corticotropin (ACTH), magnesium,and spermine

Modulation of protein synthesis by fragments of the ACTH molecule has been studied in a cell-free system obtained from subcortical brain tissue of rats. Both the activity of the protein-synthesizing system and its sensitivity to ACTH-like peptides appeared to be highly dependent on the Mg²⁺ and spermine concentrations. At optimal Mg²⁺ concentrations (4 mM) the peptide sequences ACTH(1–24) and (11–24) were both inhibitory, the latter being the more active. The inhibitory effect was reduced or abolished at higher (suboptimal) Mg²⁺ concentrations. Spermine, like ACTH, inhibited protein synthesis at the optimal Mg²⁺ concentration. However, at lower Mg²⁺ concentrations spermine had a stimulatory effect and maximal activity was obtained at 0.75–1.0 mM Mg²⁺. In the presence of spermine (60 μM) and Mg²⁺ (0.75 mM), a half-maximal inhibition of protein synthesis was obtained with a peptide concentration of 5 μM. A structure-activity study showed that the peptides ACTH(7–16)-NH₂, (11–24), (5–18, ¹⁷Lys¹⁸Lys)-NH₂ and (15–24) were active in inhibiting protein synthesis, whereas the fragments ACTH(1-16)-NH₂ and (17–24) were inactive. The results are discussed in terms of an interaction between ACTH, Mg²⁺, and spermine, and intracellular processes involved in protein synthesis.