Vitamin B1 thiazole derivative reduces transmembrane current through ionic channels formed by toxins from black widow spider venom and sea anemone in planar phospholipid membranes

The vitamin B1 (thiamine) structural analogue 3-decyloxycarbonylmethyl-4-methyl-5-(β-hydroxyethyl) thiazole chloride (DMHT) (0.1 mM) reversibly reduced transmembrane currents in CaCl2 and KCl solutions via ionic channels produced by latrotoxins (α-latrotoxin (α-LT) and α-latroinsectotoxin (α-LIT)) from black widow spider venom and sea anemone toxin (RTX) in the bilayer lipid membranes (BLMs). Introduction of DMHT from the cis-side of BLM bathed in 10 mM CaCl2 inhibited transmembrane current by 31.6 ± 3% and by 61.8 ± 3% from the trans-side of BLM for α-LT channels. Application of DMHT in the solution of 10 mM CaCl2 to the cis-side of BLM decreased the current through the α-LIT and RTX channels by 52 ± 4% and 50 ± 5%, respectively. Addition of Cd2+ (1 mM) to the cis- or trans-side of the membrane after the DMHT-induced depression of Ca2+-current across the α-LT channels caused its further decrease by 85 ± 5% that coincides favorably with the intensity of Cd2+ blocking in control experiments without DMHT. These data suggest that DMHT inhibiting is not specific for latrotoxin channels only and DMHT may exert its action on α-LT channels without considerable influence on the ionogenic groups of Ca2+-selective site inside the channel cavity. The binding kinetics of DMHT with the α-LT channel shows no cooperativity and allows to expect that the DMHT binding site of the toxin is formed by one ionogenic group as the slopes of inhibition rate determined in log–log coordinates are 1.25 on the trans-side and 0.68 on the cis-side. Similar pK of binding (5.4 on the trans-side and 5.7 on the cis-side) also suggest that DMHT may interact with the same high affinity site of α-LT channel on either side of the BLM. The comparative analysis of effective radii measured for α-LT, α-LIT and RTX channels on the cis-side (0.9 nm, 0.53 nm and 0.55 nm, correspondingly) and for α-LT channel on the trans-side (0.28 ± 0.18 nm) with the intensity of DMHT inhibitory action obtained on these channels allowed to conclude that the potency of DMHT inhibition increased on toxin pores of smaller lumen.