Spectroscopic properties and stability of the neurotoxic complex Vipoxin and its components

The neurotoxin Vipoxin from the venom of Vipera ammodytes meridionalis is a complex between a toxic basic phospholipase A(2) (PLA(2)) and a non-toxic acidic protein inhibitor (Inh). Tryptophan fluorescence parameters are determined for the complex and for its components. Iodide, caesium and acrylamide are not efficient quenchers of the Vipoxin indole emission. Increased accessibilities of tryptophans to ionic and neutral quenchers are found after the dissociation of the complex. Trp 20 and Trp 31 became more 'exposed' in the separated individual proteins. The indole rings of the complex are located in a positively charged environment. Inspection of the Vipoxin X-ray model showed that the three tryptophyl side chains are located in the interface region between the enzyme and the inhibitor and are completely 'exposed' in the separated components of the complex. In Vipoxin an efficient 'interchain' energy transfer between tyrosyl and tryptophyl residues from different polypeptide chains occurs. Static quenching with acrylamide is also detected in PLA(2) and Inh. The free energy changes Delta G(D) for the unfolding reactions of Vipoxin, PLA(2) and Inh are determined by circular dichroism spectroscopy. The complex formation between the toxic PLA(2) and the inhibitor increases Delta G(D)(H2O) to 23.5 kJ mol(-1). (C) 1998 Elsevier Science B.V. All rights reserved.