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 Mg 2+ and spermine concentrations. At optimal Mg 2+ 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) Mg 2+ concentrations. Spermine, like ACTH, inhibited protein synthesis at the optimal Mg 2+ concentration. However, at lower Mg 2+ concentrations spermine had a stimulatory effect and maximal activity was obtained at 0.75–1.0 mM Mg 2+ . In the presence of spermine (60 μM) and Mg 2+ (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)‐NH 2 , (11–24), (5–18, 17 Lys 18 Lys)‐NH 2 and (15–24) were active in inhibiting protein synthesis, whereas the fragments ACTH(1‐16)‐NH 2 and (17–24) were inactive. The results are discussed in terms of an interaction between ACTH, Mg 2+ , and spermine, and intracellular processes involved in protein synthesis.