Binding of beta-scorpion toxin: a physicochemical study.
The binding to rat brain synaptosomes of a beta-scorpion toxin, i.e., toxin II of Centruroides suffusus suffusus (Css II), was studied as a function of pH, temperature, and concentration of some monovalent and divalent cations. At 10 degrees C and pH 6.0, the specific binding of 125I-labeled Css II corresponds to a single class of noninteracting high-affinity binding sites (KD = 0.18 nM) with a capacity (4.2 pmol/mg of protein) that is almost identical with that generally accepted for saxitoxin. The equilibrium dissociation constant of beta-scorpion toxin is pH independent, but the maximum binding capacity is reduced with increasing pH. Li+, guanidinium, Ca2+, Mg2+, and Mn2+ modified the apparent KD of the 125I-labeled Css II toxin. The equilibrium dissociation constant varies markedly with the temperature. The van't Hoff plot of the data is curvilinear, corresponding to a standard free-energy change associated with an entropy-driven process. The association rate constant also varies considerably with the temperature whereas the Arrhenius plot is linear between 1 and 30 degrees C. The energy of activation determined from these data is 17.6 kcal/mol. These results support the hypothesis that a cluster of nonpolar amino acid residues present on one face of the molecule is involved in the toxin-receptor interaction.
D000818:Animals; D001665:Binding Sites; D001921:Brain / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008433:Mathematics; D051381:Rats; D011950:Receptors, Cholinergic / Q000378:metabolism; D012604:Scorpion Venoms / Q000378:metabolism*; D015222:Sodium Channels; D013574:Synaptosomes / Q000378:metabolism*; D013696:Temperature
D011950:Receptors, Cholinergic; D012604:Scorpion Venoms; D015222:Sodium Channels; C029574:scorpion toxin receptor