Protein Synthesis in a Cell‐free System from Rat Brain Sensitive to ACTH‐like Peptides

Protein synthesis was measured using a cell‐free system obtained from subcortical rat brain tissue. The concentrations of Mg2+ and K+ and the amount of tissue, during both the preparation and the final assay, were critical to the incorporation of amino acids as expressed per milligram protein. Even under optimal conditions mainly elongation of growing peptide chains was measured. Behaviorally active fragments of ACTH modulated the activity of the system in a biphasic manner; i.e., at a low concentration (10−8 M) of ACTH a stimulation of between 10 and 70% was found; a high concentration (10−4 M) was inhibitory (50 to 70%). Structure‐activity studies revealed that the stimulatory effect was confined to the N‐terminus of the peptide (1–24), whereas the C‐terminal sequence was responsible for the inhibition. The stimulation by ACTH1–24 was dependent on Ca2+ and Mg2+. Cyclic AMP (10−5 M) stimulated the amino acid incorporation too. When a similar cell‐free extract was prepared from brain tissue of hypophysectomized rats, the lower in vivo protein synthesis in these animals was preserved in the present cell‐free system. The data are discussed in terms of a possible direct intracellular effect of ACTH on brain protein synthesis.