The binding of saxitoxin to axolemma of mammalian brain. Cooperative competition between saxitoxin and sodium ion.
Saturable, high affinity binding of tritium-labeled saxitoxin ([3H]STX) to axolemma-enriched membranes from white matter of bovine brain was identified. The apparent [3H]STX equilibrium dissociation constant (Kd*) was strongly affected by the cationic environment:choline ion had little effect; cesium ion increased the mammalian axolemma Kd* in a simple competitive manner. In contrast, sodium ion more dramatically increased the Kd*--this effect was highly cooperative between 75 and 200 mM sodium (Hill coefficient of 2.85). The cooperativity is most pronounced at the normally expected [sodium] external to the axon in the mammalian central nervous system. This sodium-specific cooperative modification of the STX binding site (the hypothetical "ion selectivity filter" of the axonal Na+ gate) may be indicative of some as yet undefined regulatory mechanism of the Na+ gate in mammalian myelinated axons.
D000818:Animals; D001369:Axons / Q000378:metabolism*; D001667:Binding, Competitive; D001921:Brain / Q000378:metabolism*; D002417:Cattle; D002462:Cell Membrane / Q000378:metabolism; D002586:Cesium / Q000494:pharmacology; D002794:Choline / Q000494:pharmacology; D007700:Kinetics; D011955:Receptors, Drug / Q000378:metabolism; D012530:Saxitoxin / Q000378:metabolism*; D012964:Sodium / Q000494:pharmacology*
D011955:Receptors, Drug; D002586:Cesium; D012530:Saxitoxin; D012964:Sodium; D002794:Choline