Structure-activity relationships and physico-chemical properties of synthetic lipopeptide inhibitors of PKC.

Four synthetic lipopeptides, (K-pm 19,31), (K-pm 19,21,31), (K-pm 19,28,31) and (K-pm 19,21,28,31) with the lysine-palmitoyl (K-pm) residue as a lipophilic moiety, based on the pseudosubstrate sequence 19RFARKGALRQKNV31 (R19-V31), were found to be potent protein kinase C (PKC) inhibitors. However, the lipopeptides (K-pm 19,21,31), (K-pm 19,28,31) and (K-pm 19,21,28,31) were also found to act as protein kinase cAMP-dependent (PKA) inhibitors. Peptide (K-pm 19,31), the least water soluble, is marginally selective towards PKC, unlike the other palmitoyl derivatives studied here. Since the non-palmitoylated analogues (K 19,31), (K-ac 19,31), (K 19,21,31) and (K-ac 19,21,31) were inhibitors of PKC but not of PKA, the palmitoyl moiety must play a role in the specificity of protein kinase inhibition. In vitro, the lipophilic peptides showed greater stability to protease-mediated hydrolysis than the pseudosubstrate peptide depending upon the number of lipophilic (K-pm) residues. CD studies showed that in comparison with the peptide analogues, the remarkable resistance of the pseudosubstrate (R19-V31) to adopt an alpha-helix conformation in TFE, known to be strongly alpha-helix inducing, rules out this structure as the peptide binding conformation to PKC. By contrast, in aqueous media all the peptides show an extended conformation that correlates well with their inhibitory activity. This is in compliance with the crystallographic observation that an extended structure has been observed for the (5-24) PKI peptide inhibitor bound to PKA.