Structure-activity relationship for D-ring derivatives of grayanotoxin in the squid giant axon.
Grayanotoxin (GTX) binds specifically to the voltage-dependent sodium channel and induces a persistent increase in the membrane permeability to sodium ion. By studying the structure-activity relation of the GTX action, we attempt to elucidate the molecular moiety of the sodium channel facing around the carbon atoms C-15 beta, C-16 beta and C-14S in the D-ring of GTX in exerting the biological activity. A dose-response curve for each GTX analog was constructed using membrane depolarization as an index and assuming a one-to-one stoichiometry. Addition of alpha-OH, carbonyl and beta-OH groups to either C-15 or C-16 sequentially reduces the toxin potency, suggesting that the domain of the Na channel facing C-15 and C-16 contains a positive charge. Substitution of a hydroxymethyl group to the beta side of C-16 reduces GTX activity 10 times more than a similar substitution in the alpha side, indicating that this positive charge is located close to the beta side. Introduction of a hydrophilic hydroxy group into C-14S reduced GTX activity by a factor of 20, whereas introduction of an electronegative amino group totally eliminated it. We infer that hydrophobic bonds are a predominant factor on the alpha surface of the GTX molecule. In summary, 3 beta-OH, 5 beta-OH and 6 beta-OH of the GTX molecule make contact with the Na channel by hydrogen bonding and with most of remainder by hydrophobic bond in binding to the Na channel.(ABSTRACT TRUNCATED AT 250 WORDS)
Department of Physiology, School of Medicine, Hiroshima University, Japan.
M
M
Yakehiro
S
S
Yamamoto
N
N
Baba
S
S
Nakajima
J
J
Iwasa
I
I
Seyama
D000818:Animals; D001369:Axons / Q000187:drug effects*; D049832:Decapodiformes; D004224:Diterpenes / Q000737:chemistry / Q000494:pharmacology*; D004563:Electrochemistry; D066298:In Vitro Techniques; D009467:Neuromuscular Depolarizing Agents / Q000737:chemistry / Q000494:pharmacology*; D010100:Oxygen / Q000737:chemistry; D015222:Sodium Channels / Q000187:drug effects; D013329:Structure-Activity Relationship
D004224:Diterpenes; D009467:Neuromuscular Depolarizing Agents; D015222:Sodium Channels; D010100:Oxygen; C001311:grayanotoxin I