title: Preparation of a pure monoiodo derivative of the bee venom neurotoxin apamin and its binding properties to rat brain synaptosomes.
abstract: The preparation and purification of an active monoiodo derivative of apamin is described. Radiolabeled monoiodoapamin (2000 Ci/mmol) binds specifically to rat brain synaptosomes at 0 degrees C and pH 7.5 with a second order rate constant of association (ka = 2.6 x 10(7) M-1 s-1) and a first order rate constant of dissociation (kd = 3.8 x 10(-4) s-1). The maximal binding capacity is 12.5 fmol/mg of protein and the dissociation constant is 15-25 pM for the monoiodo derivative and 10 pM for the native toxin. The apamin receptor is destroyed by proteases suggesting that it is of a proteic nature. Neurotensin and its COOH-terminal partial sequences are the only molecules unrelated to apamin that are able to displace monoiodoapamin from its receptor at low concentrations. Half-displacement occurs at 170 nM neurotensin. This property is due to the presence in the COOH-terminal sequence of neurotensin of two contiguous arginine residues, a structure analogous to that of the apamin active site. The binding of monoiodoapamin to its receptor is sensitive to cations. Increasing K+ or Rb+ concentrations from 10 microM to 5 mM selectively enhances the binding by a factor of 1.8. Increasing the concentration of any cation from 1 to 100 mM completely inhibits iodoapamin binding. Both effects are due to a cation-induced modulation of the affinity of monoidoapamin for its receptor without any change of the maximal toxin binding capacity of synaptosomes. Guanidinium and molecules containing a guanidinium group are better inhibitors of iodoapamin binding than other inorganic cations or positively charged organic molecules.
mesh_terms: D000818:Animals; D001030:Apamin / Q000031:analogs & derivatives / Q000138:chemical synthesis* / Q000378:metabolism* / Q000494:pharmacology; D001514:Bee Venoms / Q000138:chemical synthesis* / Q000378:metabolism*; D001681:Biological Assay; D001921:Brain / Q000378:metabolism*; D003106:Colon / Q000187:drug effects; D006168:Guinea Pigs; D007457:Iodine Radioisotopes; D007700:Kinetics; D008297:Male; D009126:Muscle Relaxation / Q000187:drug effects; D009496:Neurotensin / Q000494:pharmacology; D015221:Potassium Channels; D051381:Rats; D011919:Rats, Inbred Strains; D011956:Receptors, Cell Surface / Q000378:metabolism*; D013574:Synaptosomes / Q000378:metabolism*
chemical_list: D001514:Bee Venoms; D007457:Iodine Radioisotopes; D015221:Potassium Channels; D011956:Receptors, Cell Surface; C033799:apamin receptor; D001030:Apamin; D009496:Neurotensin; C033900:monoiodoapamin