Modification of Substrate Inhibition of Synaptosomal Acetylcholinesterase by Cardiotoxins

Different types of cardiotoxin (I-V and n) were isolated and purified from the venom of the Taiwan cobra (Naja naja atra). The effects of these cardiotoxins were studied on membrane-bound acetylcholinesterase, which was isolated from a sheep's brain cortex. The results showed that cardiotoxins I-III, V, and n activated the enzyme by modification of substrate inhibition, but cardiotoxin IV's reaction was different. The inhibition and activation of acetylcholinesterase were linked to the functions of the hydrophobicity index, presence of a cationic cluster, and the accessible arginine residue. Our results indicate that Cardiotoxins have neither a cationic cluster nor an arginine residue in their surface area of loop I; therefore, in contrast to fasciculin, cardiotoxins are attached by loop II to the peripheral site of the enzyme. As a result, fasciculin seems to stabilize nonfunctional conformation, but cardiotoxins seem to stabilize the functional conformation of the enzyme. Based on our experimental and theoretical findings, similar secondary and tertiary structures of cardiotoxins and fasciculin seem to have an opposite function once they interact with acetylcholinesterase.

Evidence against tubulin oligomer dissociation to tubulin dimer at assembly temperatures

Chromatography of microtubule protein through 8% agarose at 4 and 31 degrees C demonstrated that tubulin oligomer was not dissociated to tubulin dimer and microtubule associated proteins by assembly temperatures. During chromatography, formation of microtubules was prevented by using 10 microM podophyllotoxin and/or 25 microM GTP. In the presence of 25 microM GTP, tubulin oligomer at 31 degrees C and microtubule protein retained the ability to polymerize. Evidence against dimer formation from tubulin oligomer was also obtained via a turbidity study of the initial events (the first 90 s) of microtubule assembly.

Evidence for two growth steps in microtubule polymerization

For microtubule assembly, the data reported here support an initial nucleation phase followed by a growth or elongation phase. The nucleation phase was not detected kinetically. Evidence for this step was given by the existence of the critical concentration and the dependence of the number of microtubules on oligomer concentration. Kinetic evidence indicated the existence of two consecutive steps in the growth phase of microtubules. The fast process increased and the slow one decreased with the concentration of microtubule protein. Similar kinetics were found upon recombination of tubulin oligomer and dimer which had been resolved by agarose chromatography. The fast process increased with oligomer and decreased with dimer concentration while the slow one depended positively on dimer concentration. Microtubules were formed when the oligomeric fraction only was employed. In contrast, under identical conditions, no microtubule formation was detected turbidimetrically or by electron microscopy from dimer alone. When dimer caused elongation of seed tubules, there was only one growth step with a rate constant of the same order of magnitude as the slow process for the other experiments.