Substitution Of A Single Residue In Stichodactyla Helianthus Peptide, Shk-Dap(22), Reveals A Novel Pharmacological Profile

ShK, a peptide isolated from Stichodactyla helianthus venom, blocks the voltage-gated potassium channels, K(v)1.1 and K(v)1.3, with similar high affinity. ShK-Dap(22), a synthetic derivative in which a diaminopropionic acid residue has been substituted at position Lys(22), has been reported to be a selective K(v)1.3 inhibitor and to block this channel with equivalent potency as ShK [Kalman et al. (1998) J. Biol. Chem. 273, 32697-32707]. In this study, a large body of evidence is presented which indicates that the potencies of wild-type ShK peptide for both K(v)1.3 and K(v)1.1 channels have been previously underestimated. Therefore, the affinity of ShK-Dap(22) for both channels appears to be ca. 10(2)-10(4)-fold weaker than ShK. ShK-Dap(22) does display ca. 20-fold selectivity for human K(v)1.3 vs K(v)1.1 when measured by the whole-cell voltage clamp method but not in equilibrium binding assays. ShK-Dap(22) has low affinity for K(v)1.2 channels, but heteromultimeric K(v)1.1-K(v)1.2 channels form a receptor with ca. 200-fold higher affinity for ShK-Dap(22) than K(v)1.1 homomultimers. In fact, K(v)1.1-K(v)1.2 channels bind ShK-Dap(22) with only ca. 10-fold less potency than ShK and reveal a novel pharmacology not predicted from the homomultimers of K(v)1.1 or K(v)1.2. The concentrations of ShK-Dap(22) needed to inhibit human T cell activation were ca. 10(3)-fold higher than those of ShK, in good correlation with the relative affinities of these peptides for inhibiting K(v)1.3 channels. All of these data, taken together, suggest that ShK-Dap(22) will not have the same in vivo immunosuppressant efficacy of other K(v)1.3 blockers, such as margatoxin or ShK. Moreover, ShK-Dap(22) may have undesired side effects due to its interaction with heteromultimeric K(v)1.1-K(v)1.2 channels, such as those present in brain and/or peripheral tissues.