BREVETOXIN-B - CHEMICAL MODIFICATIONS, SYNAPTOSOME BINDING, TOXICITY,AND AN UNEXPECTED CONFORMATIONAL EFFECT

Five naturally occurring brevetoxins and seven synthetically modified brevetoxins were examined for their affinity for site 5 of the voltage -gated sodium channel and their toxicity to mosquito fish, Gambusia af finis. All but three of the toxins studied still retained some affinit y for their receptor site (IC50's in the range of 1-100 nM). Compound 7, having all three carbon-carbon double bonds reduced, is almost 3 or ders of magnitude less strongly bound than 4, which has only two carbo n-carbon bonds reduced. This large effect resulting from H-ring reduct ion was unexpected, due to the similarity of this region of the molecu le to the corresponding region of brevetoxin-A, which has a fully satu rated eight-membered G-ring and is the most strongly bound toxin of th ose studied. Conformational analysis revealed that the unsaturated H-r ing of brevetoxin B favors the boat-chair conformation as does the sat urated G-ring of brevetoxin A. Upon reduction, the H-ring of brevetoxi n B shifts to a crown conformation. This subtle change in conformation al preference induces a significant change in the gross shape of the m olecule, which we believe is responsible for the loss of binding affin ity and toxicity.