Modified Semax Peptides Show Enhanced Stability Against Degradation

The peptide Semax (Met-Glu-His-Phe-Pro-Gly-Pro), a synthetic analogue of a fragment of adrenocorticotropic hormone (ACTH), is known for its neuroprotective and nootropic properties. However, like many therapeutic peptides, it is susceptible to rapid degradation by aminopeptidases, a class of enzymes that cleave amino acids from the N-terminus, thereby limiting its therapeutic half-life and efficacy. This study specifically investigated how N-terminal amino acid substitutions affect Semax's stability against degradation by N-aminopeptidases.

The study revealed that modifying the N-terminal amino acid significantly alters the rate of peptide degradation by N-aminopeptidases. Specifically, the [Ala1]Sem, [Gly1]Sem, and [Thr1]Sem analogues demonstrated notably greater stability to proteolysis compared to native Semax, indicating a slower breakdown rate. Their primary degradation product was consistently identified as His-Phe-Pro-Gly-Pro (Sem-5). In contrast, the [Trp1]Sem analogue produced both Glu-His-Phe-Pro-Gly-Pro (Sem-6) and Sem-5 as initial products in similar quantities, suggesting a different cleavage pattern. All investigated Semax analogues were found to effectively inhibit the proteolysis of native Semax, suggesting they can compete with the parent peptide for the enzyme's active site. This indicates that strategic N-terminal modifications can protect Semax from enzymatic breakdown, potentially prolonging its action in biological systems.