Biological Activity of Corticotrophin Peptides W Ith Homo-arginine, Lysine or Ornithine Substituted for Arginine in Position

The steroidogenic and lipolytic activities of corticotrophin-(l-24)-tetracosapeptide and [Lys17.18]corticotrophin-(l-18)-octadecapeptide amide were compared with those of the corresponding analogues variously substituted in position 8 with homo-arginine, ornithine or lysine. Peptides substituted with homo-arginine showed a surprisingly high degree of biological activity; in all the tests performed, the loss of activity due to the substitution did not exceed a factor of 3 to 10, while peptides sub­ stituted with ornithine or lysine in position 8 only showed residual biological activity or none at all, depending on the test system used. The results reported demonstrated that the presence of a guanidino group (arginine or homo-arginine side chain) in position 8 of the ACTH peptide is essential for the preservation of a high level of biological activity; the length of the aliphatic chain carrying the guanidino group is of minor importance.