Regulatory Action of Synthetic Cationic Peptides Containing Residues of Glutamic Acid on Functional Activity of Components of the Adenylyl Cyclase Signal System

One of the most important stages of hormonal signal transduction in cells through the hormone-sensitive adenylyl cyclase signal system (ACS) is functional coupling of receptor of the serpentine type to heterotrimeric GTP-binding protein (G-protein). The main role in realization of such coupling is played by spiralized regions of the receptor cytoplasmic loops proximal in relation to membrane, most of them carrying positive charge. To study molecular mechanisms of interaction of the receptor with G-protein, we compared effects of synthetic cationic peptides containing residues of glutamic acid on the process of regulation of ACS by hormones (biogenic amines) and non-hormonal agents in smooth muscles of the freshwater bivalve mollusc Anodonta cygnea and skeletal muscles of rat. All peptides had the clearly expressed ability to form -helices. Peptides H-(Leu-His-Glu-Lys)₄-Leu-NH₂ (I), H-(Leu-His-Glu-Lys)₃-Lys-His-Glu-Lys-Leu-NH₂ (II), H-(Leu-Lys-Glu-Lys)₄-Leu-NH₂ (III), and H-(Ile-His-Glu-Lys)₄-Ala-NH₂ (IV) at concentrations of 10^–6–10^–3 M reduced dose-dependently the value of stimulating effects of serotonin (in mollusc muscles) and isoproterenol (in rat muscles) on the adenylyl cyclase (AC) and protein kinase A (PKA) activities. Values of concentration of these peptide causing a 50% decrease of the hormone-stimulating effect (IC₅₀) vary from 150 to 750 µM. According to the degree of this inhibitory action on stimulating effects of hormones, they may be arranged in the following series: III II > IV I. The peptides I–IV were more effective than the peptide H-(Glu-Lys)₈-Ala-NH₂ (V) with the charge close to zero, but much less effective than the studied earlier cationic peptides containing only positively charged amino acid residues. The inhibitory effect of the peptides I-IV on stimulation of AC by non-hormonal agents, NaF, GppNH]p, and forskolin, was essentially less pronounced and was marked only at 10^–4–10^–3 M concentrations. Thus, the inclusion of negatively charged amino acid residues in the primary structure of polycationic peptides leads to a decrease in their ability to inhibit hormonal stimulation of AC and PKA, which indicates importance both of the total positive charge of peptides and of distribution of the charged amino acids in the formed helices for realization of the uncoupling action on the ACS components—the receptor and G-protein.