SNX-325, A NOVEL CALCIUM-ANTAGONIST FROM THE SPIDER SEGESTRIA-FLORENTINA

A novel selective calcium channel antagonist peptide, SNX-325, has bee n isolated from the venom of the spider Segestria florentina. The pept ide was isolated using as bioassays the displacement of radioiodinated omega-conopeptide SNX-230 (MVIIC) from rat brain synaptosomal membran es, as well as the inhibition of the barium current through cloned exp ressed calcium channels in oocytes. The primary sequence of SNX-325 is GSCIESGKSCTHSRSMKNGLCCPKSRCNCRQIQHRHDYLGKRKYSCRCS, which is a novel a mino acid sequence. Solid-phase synthesis resulted in a peptide that i s chromatographically identical with the native peptide and which has the same configuration of cysteine residues as the spider venom peptid e omega-Aga-IVa [Mintz, I. M., et al. (1992) Nature 355, 827-829]. At micromolar concentrations, SNX-325 is an inhibitor of most calcium, bu t not sodium or potassium, currents. At nanomolar concentrations, SNX- 325 is a selective blocker of the cloned expressed class B (N-type), b ut not class C (cardiac L), A, or E, calcium channels. SNX-325 is appr oximately equipotent with the N-channel selective omega-conopeptides ( GVIA and MVIIA as well as closely related synthetic derivatives) in bl ocking the potassium induced release of tritiated norepinephrine from hippocampal slices (IC(50)s, 0.1-0.5 nM) and in blocking the barium cu rrent through cloned expressed N-channels in oocytes (IC(50)s 3-30 nM) . By contrast, SNX-325 is 4-5 orders of magnitude less potent than is SNX-111 (synthetic MVIIA) at displacing radioiodinated SNX-111 from ra t brain synaptosomal membranes. SNX-325 will be a useful comparative t ool in further defining the function and pharmacology of the N- and po ssibly other types of high-voltage activated calcium channels.